When brake engineers start the work of designing a new brake system for a car, the first thing they do is create something called the “Ideal Brake Curve.” If this sounds like jargonese to you, fear not, your resident suspension engineer is here to help.
The Ideal Brake Curve is a graph that brake engineers create before they know anything about the brake system of a car. It is the first step in designing the new system. It shows us the stopping power of the front and rear axles under ideal conditions. In other words, if we had a perfect brake system that could use all the available traction of the front and rear axles at all times and under all possible road conditions, what would the stopping power of each axle be? It does not tell us what the actual brake system can do; that comes later.
At the point when we create the Ideal Brake Curve, we know nothing about the actual brakes. We don’t know how big the rotors will be or what type of calipers we will use or how big the brake booster is going to be. We know nothing at all about the brake hardware because that hasn’t been designed yet. The Ideal Brake Curve only tells us what the potential stopping power of the front and rear axles could be. It is then up to the brake system engineer to design a system that makes the best use of this stopping power.
The other thing the Ideal Brake Curve tells us is what the front and rear axle stopping forces could be on a variety of different surfaces, such as ice, wet, or dry pavement. Here, allow me to explain on my new YouTube channel, which you should absolutely subscribe to (please):
A Typical Ideal Brake Curve
Let’s look at a typical ideal brake curve:
The front axle brake force is on the horizontal axis of this graph and the rear axle brake force is on the vertical axis. The diagonal lines represent different surfaces based on their friction coefficients. Notice that the scale is a bit different on each axis; each marker on the X-axis is 2000 Newtons, while each on the vertical axis is just 1000 — the front axle will generate more brake force than the rear, so this should come as no surprise.
Friction Refresher
Before we go any further, we need a quick refresher on friction coefficients. Any time two objects sit against one another — imagine a block of wood sitting on a table, or a tire sitting on the road — it takes a certain amount of force to make the objects slide relative to one other. A block of wood on a glass table may be fairly easy to slide but a tire on the road will be much harder. The amount of force it takes to make the objects slide relative to one another depends on the force pushing the objects together and something called the friction coefficient.
Let’s take an example. Let’s assume have a block of wood sitting on a table, and the block of wood weighs 10 lbs. If it takes 5 lbs of force to slide the block across the table, then the friction coefficient between the table and the block would be 5 / 10 = 0.5. If on the other hand, we polish both the table and the block, and suppose it now takes only 2 lbs of force to slide the block then the friction coefficient would now be 2 / 10 = 0.2. The relationship between force, friction coefficient and weight (well, normal force) is Force = friction coefficient x normal force. For a car sitting with all 4 tires on dry pavement, the friction coefficient will be close to 1, meaning that if the car weighs 4000 lbs, it will take 400 x 1 = 4000 lbs of force to make the car slide across the road. For ice, on the other hand, the friction coefficient will be closer to 0.2.
Let’s get back to our graph and learn how to read it. The first thing we need to know is what surface we are driving on. If we assume. for instance, that we are driving on dry asphalt, then we could also assume our friction coefficient is about 1. We would then go the diagonal line marked 1 on our graph and follow it up until we reach the curve:
From there we can draw vertical and horizontal lines to our axes to find out what the maximum front and rear brake forces would be that our vehicle could achieve:
We see here that on dry pavement, our tires should be able to generate about 13,500 Newtons (N) force on the front axle and just under 6000 N force on the rear axle before they would lose grip. Of course, we would need a brake system that is powerful enough to generate this much braking force, but at least we know what our tires are capable of.
If on the other hand we assume we are on ice with a friction coefficient of 0.2, we would go to the line marked 0.2 and follow it to our curve:
Now, instead of being able to generate 13,500 N of braking force, the front tires are only able to generate a little over 2000 N of stopping force. Similarly, the rear tires can only generate a bit under 2000 N of force instead of the 6000 N we had on dry pavement. This makes perfect sense since ice is much more slippery than pavement so we would naturally expect to get much less stopping power on it versus pavement.
The Curvature of the Line
You probably noticed that the ideal brake curve is not a straight line, but has a lot of curvature to it. The reason for this is weight transfer. As we brake harder and harder, more and more weight transfers from the rear axle to the front. This is why when you hit the brakes in your car, the front of the car goes down and the rear comes up. The front has effectively gotten heavier, and the rear has effectively gotten lighter. If we had a super sticky road, super sticky tires, and massive brakes, we could imagine a case where we could brake so hard that there would be enough weight transfer from the rear to the front that the rear wheels would come off the ground. This is pretty common in bicycles and motorcycles since they are so much shorter and taller than cars, so if you ride a bike, you know all about doing “stoppies.”
But, if you look at the ideal brake curve, you can picture the curve continuing to the right until it curves all the way round and comes back down to the horizontal axis. This is the point where the rear wheels have lifted off the ground and are no longer able to provide any more stopping power.
How To Come Up With The Ideal Brake Curve
Let’s look into what it takes to generate one of these ideal brake curves. There are four basic parameters we need to know:
- Vehicle weight
- Front to rear weight distribution
- The height of the center of gravity
- The wheelbase length
The weight and weight distribution tell us how much weight is on the front and rear axles right from the start. The height of the center of gravity along with the wheelbase tell us how much weight transfer there will be when we hit the brakes. Once we know these numbers, we can easily calculate how much force is on the front and rear tires as we brake harder and harder. From there we can calculate the actual stopping power for all the different friction coefficients we want to look at, in this case everything between 0 and 1.
We can look at the effect of each of these four parameters by changing them one by one and seeing how the curve changes. The example above used a vehicle that had a mass of 2000 Kg, a front weight distribution of 50%, a CG height of 550 mm and a wheelbase of 2,800 mm. Here is that curve again as a reminder:
Weight Effect
If we now change the mass of the car down from 2000 Kg to 1500 Kg, the curve would look like this:
Notice that the shape of the curve is basically the same, just a bit smaller since it would take less braking force to stop the lighter car.
How Weight Distribution Effects The Ideal Brake Curve
What if instead of reducing the weight, we changed the weight distribution? Let’s change it from 50% front weight to 60% — so a bit nose-heavier:
Now we see that the shape of the curve has really changed. It is much flatter than before, and this makes sense. We have a lot more weight on the front axle, so we need to get a lot more braking force from the front. The lower rear weight means we cannot get as much stopping power out of the rears as we did before.
CG Height Effect
We could also change the height of the center of gravity. If go back to our original example and change the CG height from 550 mm to 700 mm we would get this curve:
It’s not easy to see but the curve is more horizontal and more rounded than it was before. This makes sense because the higher CG height means we get more weight transfer the harder we brake. Of course, this effect gets worse the harder we brake so the curve has more “bend” to it.
The Wheelbase Effect
Lastly, we can see what would happen if we changed the wheelbase. Let’s shorten the wheelbase from 2,800 mm to 2,500 mm:
The effect is even more subtle here than with a CG height change, but you still see that the curve has more “bend” to it but it’s not as much as the CG height change we did before. This is because just like when we changed the CG height, a shorter wheelbase means more weight transfer the harder we brake. The effect of both would really be the same but they look different here because changing from 550 to 700 mm CG height is a bigger percentage change than going from 2,800 to 2,500 mm wheelbase.
I want to take a moment here to remind everyone that everything we have been talking about here has nothing to do with the actual brakes in the car. This is all theoretical and only shows what the stopping power of the front and rear axles would be under ideal conditions and with a perfect brake system. As I mentioned before, these curves are created very early in the design process when we know absolutely nothing about the real brakes yet. We don’t know how big the brake rotors will be or what types of calipers we will use, nor do we know anything about the brake booster. These curves only tell us what the tires are capable of — the maximum braking fore that we can possibly get from each axle under ideal conditions. It is now the task of the brake systems engineer to design a brake system that uses as much of the tires’ capabilities as possible to get the best stopping power.
Some Real World Examples
To bring this discussion some perspective, let’s look at two real world examples. We’ll use a 2022 Porsche 911 and a 2022 Honda Accord as comparators. We will unfortunately have to make some assumptions about the heights of the centers of gravity for these vehicles, but all the other parameters are published and easily available. Here is the ideal brake curve for the Porsche. Notice that I have assumed a CG height of 450 mm:
You can see immediately that this curve is much more upright than the ones we’ve seen so far. This is because the 911 has so much weight on the rear axle that we can use the rear brakes much more than on any other car. Also, the CG height is quite low so there is much less weight transfer happening. This reduces the amount of “bend” in the curve considerably.
Conversely, here is the curve for the Honda. I’ve assumed a CG height of 550 mm of this vehicle:
You can see how the huge difference in weight distribution means that the curve for the Honda is much more horizontal. There is much more weight on the front axle here, so we need to get much more stopping power out of the front brakes. Similarly, there is very little weight on the rear axle so we can’t use those brakes nearly as much.
Looking at these curves also makes it clear why when you look at the brakes of a Honda Accord, you will see that the rear brakes have smaller calipers and thinner rotors than the fronts. That is pretty typical of front heavy front-wheel-drive cars. The Porsche, on the other hand, has rear brakes that are as big, if not bigger, than the fronts. With all that weight in the back, the Porsche needs to use those rear brakes much more.
Now that we know what the Ideal Brake Curve we have the information we need to design a brake system that makes the best use of the stopping power each axle has. More on this in a follow-up article.
This gets particularly complicated when you have to design a pickup truck that has certain characteristics when driven unloaded, and unpredictable weight distribution and center of gravity when loaded. The same with a 12-passenger van, which could have more than a ton of weight added, in random locations in its interior.
Saw this much too late. But, how does one adjust for brake modulating when ones brake pedal is poorly placed. Many cars I’ve driven are basically setup for pure automatic style driving with right foot solely doing 1 pedals work at a time. On most manuals, they (mostly) seem to make adjustments for the right foot handling brake and accelerator at the same time. But on autos, it always seems like the ergonomics lessons were tossed out at build time and pedals are placed in really odd positions. For instance, on my sons 2015 Civic, the height disparity between accelerator and brake is quite substantial, like about 3 inches. Brake modulation is great. Some quality ergonomics should go along with that.
Could someone explain why the rear brakes on my 2014 328xdrive wear faster than the front? I’m a mild-mannered drift-up-to-the-stop-sign type of driver.
What I’ve read specifically related to the F-chassis 3-series is two-fold. First, the rear brakes are used for stability control, which intervenes more than you’d think it would. This causes faster rear brake wear (Ford is similarly aggressive with rear brake use, there are multiple forum posts about Fiesta/Focus owners experiencing disproportionately fast rear brake wear). Second, BMW uses earlier application of the rear brakes compared to the front to reduce brake dive, which makes the car stay flatter and feel ‘sportier’ than its counterparts.
Interesting. Thanks.
I am excited to see articles like this. It puts me in a mind of when Mike Kojima used to write for SCC.
That said, your editors (Talking to you, JT and DT) doing you a disservice. “Effect” is rarely a verb. After reading the paragraph, it should not be one here, either.
“How Weight Distribution Effects The Ideal Brake Curve”
Keep up the great work.
Next up: Please explain why brake modulation is so important. I am on some forums for old Land Rovers. People talk about swapping the front drums for disk brakes, and then someone chimes in that “the drums have all the stopping power you need — you can lock ’em up!” Can you explain why modulation makes for better, more precise braking? On a similar note, I have been leery about doing that swap myself, for fear of upsetting the balance between front and rear that this article has so eloquently explained.
I’m not sure what you mean by “modulation”. If you have non-ABS brakes then you will need to modulate them, i.e., vary the brake pedal pressure with your foot, to prevent lockup when braking hard. You certainly don’t need to modulate the brakes if you have ABS. Just jam your foot down and let the ABS do the work. If you try to modulate the pedal like you would in a non-ABS car you’ll just confuse the system. Let it do the work, it’s a LOT faster than your foot.
As far as your disc swap goes, I say go for it. I did the same thing on a 1968 Cougar and it was like night and day. Just make sure you include the proportioning valve (if it has one) that comes with the disc setup. Also make sure there is nothing different about the rear drums. The wheel cylinders might be different so just check to make sure.
What I meant by modulation is that, all things being equal, stronger brakes give you the ability to be more precise in braking exactly as much as you need. When I switched brakes on my mountain bike from old-style calipers to linear-pull, I suddenly found my self riding faster because I could brake more precisely — even though the old calipers were still strong enough to flip me over the handlebars or skid the rear tire. So just how do you explain in layman’s terms WHY the disc brakes on your Cougar were so much better than the originals?
I suspect the upgrade to my Cougar had a similar impact as your bike brakes did. The stopping power was greatly increased but more importantly, the stopping confidence increased. The pedal effort was much lower so you felt like there was a lot more braking capability left over. With the lower effort it was also easier to modulate the brakes. You weren’t doing a Hercules workout every time you hit the pedal. Small changes in pedal force resulted in big changes in braking force. It was much nicer.
I am looking forward to Huibert explaining why drums at the rear are fine for many cars (often better, since they will require less maintenance).
Just totally winging it here, since most cars will be doing most of its braking with the fronts, the rears don’t need to be as good.
And as far as I recall, while drum brakes are actually pretty good when cold, they have problems with heat dissipation and lose braking performance under heavy loads.
Drum brakes have an inherent problem that they expand as they heat up. As the drum expands, the brake shoes have to move outward to maintain contact with the drum. The result is that your foot will slowly sink down as the brake fluid follows the moving brake shoes. The other problem is that the surface of the drum that transfers heat to the atmosphere is not the same surface that the heat is put into. In a disc, the radiating surface is the same surface the pads ride on. This means the heat doesn’t have to travel through the material to dissipate. It can cool faster and resist fade better because of this. The advantage of drums is that the effective radius is large, and the shoes are partially self-actuating meaning as you apply the brakes, the friction of the shoes against the drum tries to pull the shoes harder into the drum. You don’t need as much (or any) brake boost with drums.
I’ve noticed on my older (2004) Cadillac SRX the rear brake pads always wear out before the fronts.
Someone told me braking is biased toward the rears to provide anti-dive, and with ABS there’s no issue with that as a problem during limit braking. Wondered it you’ve seen this?
I’m not familiar with the SRX but my first thought is that maybe they undersized the rears to save cost and then put higher friction pads on to compensate. The higher friction then causes the pads to wear faster. I doubt they did this for anti-dive reasons. The difference would probably be minimal, but I could be wrong. Any GM brake engineers out there?
My Mazda3 was like that—3 sets of rears for 1 front, but in that case, I think they used organic pads in the rear and ceramic in the front. Maybe something similar?
I’ve noticed that as well. I think by the early 2000s most manufacturers were using long life front pads,combined with soft rear shoes.
I dont think it was a deliberate strategy. Front discs were all power assisted so why not use hard wearing pads i guess?
Meanwhile the rears did double duty as E brakes.They had to grip perfectly with very little mechanical assistance so it kinda makes sense they were soft and grippy.
Although the SRX has an additional set of drum brakes (similar to the friction band in an automatic transmission) as the rear parking brake system, so that theory probably doesn’t fit here.
Not adjusted properly? Rubbing, not releasing completely, engaging early? Check for corrosion on the guide pins, make sure they move easy. Are you using OEM Delco pads and discs front and rear?
I’ve had a lot of GM cars and it’s always been the other way around.
Real world example. My 1964 Corvair has 4 wheel drum brakes. The front and rear brake drum size are similar indicating it would have a quite vertical brake curve. This is because the engine is hung out behind the rear axle like a Porsche 911 so the rear axle has a lot of traction to contribute to stopping the car.
Despite being drum brakes, when properly adjusted, the brake force and lack of front end dive is remarkable for a 1960s car. Since the braking is shared more equally across four drums, it is more resistant to fading than a comparable front engine car such as a Mustang from the same era.
I was just thinking to myself, having DD’d many older and even rear engined cars, when this level of analysis entered the equation. At what point did this analysis begin/get figured out, and then at what point did OEMs start specing out very specific brake setups rather than parts bin fishing. With drums, their stopping power is pretty much 1000% dependent on their state of wear and adjustment; doesn’t seem like an engineering department would put a ton of time into something that will immediately wear out of spec and/or be adjusted out by some schmuck with a flathead. I always kept my brakes tuned with a nice biased grip and a firm pedal, but it was rare to encounter any other car enthusiasts (let alone JQPublics) who even knew how to adjust brakes at all.
Drum brakes can be very effective. I used to own a 1932 Packard with mechanical drum brakes all round and the car would stop quite well. Of course, they were 16″ diameter, so they were huge! The problem with drums is fade resistance. They just don’t dissipate heat well so they fade quickly. As far as your Corvair goes, the weight distribution might have been just right to allow the same brake size front and rear. I would check to make sure the wheel cylinders are the same front and rear though. There might be a difference in the cylinder bores that would make the fronts stronger than the rears.
Nice! My grandfather had a ’28 limo when he ostensibly worked as an iceman during prohibition. ’32 has the nicer looking front.
To me the 32’s are the prettiest and i was lucky to fall into one 25 years ago. The sweep of the open front fender and the V-grill are a gorgeous combination to me.
Many small FWD cars have drums on the back even today, and this article has explained to me why. For a small, cheap, car with most of it’s weight in the front, the rears can only provide a relatively small braking force, so it makes sense to save money and fit drums.
On the other hand, having owned many cars with rear drums, I’ll take disc brakes any day, just for ease of maintenance. I hope I never have to replace another set of pads 🙁
That was fun. I actually understood most of it immediately! (had one brief brainfart)
Also thanks a ton -sorry 9806.65N- for your super simple explanation of friction coefficients. Somehow i’ve lived for decades without bothering to learn how they’re calculated!
Am I the only one wondering why the article mentioned the ideal brake curve of a Honda Accord (repeatedly) when the image at the top of the article clearly shows a Honda Civic Type R?
I don’t think it’s a Honda Civic Type R; I think it’s just a tree in the forest.
However that does raise an interesting question: most people seem to think that increasing horsepower requires bigger brakes, but from what I think I understand from this essay horsepower is irrelevant to the calculation….
Is that correct?
Irrelevant in terms of direct relationship, yes. You can put a massive engine on banana peels tied to a plywood circle with rubber bands, and you can put drag slicks on all four corners of a Honda. They’re totally discrete. Stopping power starts with the tires, then the brakes, then suspension tuning and other far less critical factors. If your brakes out stop your tires, you lose traction and get a lesson in understeer vs. oversteer. If your tires out stop your brakes, you just don’t stop as fast. If you add a +5 horsepower sticker, the previous two sentences are in no way affected.
More horsepower by itself doesn’t require bigger brakes, no. But the other changes and use cases that usually accompany higher horsepower (stickier tires, track driving, etc.) mean that bigger brakes are a good idea. Bigger brakes not only increase stopping power that can be used by the stickier tires, they will also be able to handle more aggressive use by resisting brake fade better.
In simple terms, yes. Conventional braking systems turn kinetic energy into thermal energy, and if you have a given mass moving at a given speed, it doesn’t matter how much horsepower you used to get you there. If you significantly increase horsepower you will increase top speed, which will push your maximum energy-conversion requirements out.
In practical terms the main context that I can think of in which significant power increases will make better brakes worthwhile is a case like track driving or perhaps extended very fast road driving with lots of braking into corners, where a lot more power gives you higher corner entry speeds and more kinetic energy to get rid of when you’re braking for corners. You need tires that can cope with this too, though.
I am an engineer and can think things through from first principles, but I’ve never designed braking systems so there’s a lot that I don’t know. So…fair warning.
Thanks, theotherotter. You’ve said it well.
I always assumed it was because most manufacturers would pick a braking system that was “good enough”, but as cheap as possible. I guess there’s often room for improvement, especially in cheaper cars, and/or when you move to larger/better tyres.
How is the brake bias chosen in an actual production vehicle? What part of the curve do you aim for?
Or can it be actively proportioned based on brake fluid pressure or something? Seems like actual braking torque would vary too much to have a dynamic bias.
I’m sure modern ABS can go a long way towards having good braking balance in all conditions.
The brake force distribution in a production vehicle is chosen exactly this same way. The ideal brake curve is created based on the basic vehicle parameters and then a brake system is modeled until the performance falls under the ideal curve. Components that match the model are then sourced from suppliers.
Modern ABS systems include a function called EBD, Electronic Brakeforce Distribution, which does the job of a proportioning valve and walks the ideal brake line.
I remember some pickup in the 80s that had a brake bias proportioning valve that was adjusted for cargo weight by how weighed down the rear end was. I always wondered how well that worked. I guess that it was no weight, no brakes.
What about race cars with downforce at speed that lose grip as they slow down? Seems complicated.
That brought back a memory, my 1973 Renault 12 sedan had a rear proportioning valve that was adjusted by a lever that was linked to the ride height of the rear suspension.
Pre-ABS, you want the front tires to lock up before the rears do because you’re less likely to spin into a wall that way.
I had an 03 miata without ABS that was very conservative with it’s brake force distribution and it was way too easy to lock up the front brakes. This was compounded by the fact that I was running staggered tires (wider in the back). An adjustable proportioning valve helped get SIGNIFICANTLY more braking performance out of it. The key was to keep dialing in more rear braking force and then backing off before there was a chance the rear end would lock up.
Yes there have been several vehicles with rear ride height sensing valves to adjust the rear brake force. Toyota had them in the 80’s on their pickups for one.
I’ll have to poke around for some actual brake curves. I’m not understanding how the actual forces generated could vary that much relative to each other without going crazy with accelerometers and computers. Seems like a Urus with more piston area than most cars on the each front caliper and baby rears would be relying heavily on electronics to not lock up the fronts in any conditions that are not paved and dry.
Thanks, this is very informative. It’s great to know that painting my calipers red WILL actually make my brakes perform better.
I’m glad to hear you’ve fully taken my post to heart. Go for it!
Well I did read it very carefully, but I’m known to have comprehension issues. Also, it’s kind of irrelevant, because I just discovered I only have drum brakes. Inboard at that…
I guess I’ll have to wait for your deep dive into leaf springs for ideas towards my next effort on vehicle dynamics tuning.
I think you can still get the same performance boost by painting the drums and the backing plates red. I’m not a brake expert though.
Don’t care, I just want mechanical brakes. Hydraulic fluid is horrible and the less fluids you have to have in a car the better.
Mmm, yeah. I’ve owned a car with mechanical brakes. I’ll take the hydraulics, thank you very much.
Mechanical brakes are perfectly adequate for the cars I want as I don’t need something that weighs over 2000lbs. Now if wanted something that weighs more than 2000lbs I’d want more braking force, though I’d still prefer air brakes over hydraulic ones.
I’ll take working Mechanical brakes over leaking hydraulic brakes any day. If I snip 2 hydraulic lines on a modern car you’ll have no hydraulic brakes whatsoever. With cable brakes on a car I can snip 3 brake cables and I’ll still have 1 working brake.
Good to know that if I have a car with mechanical brakes, my enemies can snip 50% more of my brake lines than they’d have to otherwise and even then I’d still be good to go!
Foiled again!
If I’d been driving a car with those fussy newfangled hydraulic brakes I’d be so screwed!
Snipping is for the example, but the point still stands. Dual circuit hydraulic brake failure is much more likely than having 4 brake cables snap.
Wanna drive my 1917 Stephens with mechanical brakes on the rear axle only? Trust me, you don’t.
I’d give it a shot. However basically everything from 1917 will have sketchy brakes if it has brakes in the first place.
Cars like the 30’s and 40’s air cooled VWs and the first gen Ford Popular are more my Jam.
Mechanical brakes are perfectly adequate for the cars I want. Now if wanted something that weighs more than 2000lbs I’d want more braking force, though I’d still prefer air brakes over hydraulic ones.
In this case, username very much does *not* check out 😉
Lol. Honestly the LM002 was the scariest vehicle I have ever driven, and for a while it was my favorite (hence the name).
You got 6000lbs of Italian reliability with a clutch so heavy my butt would come off the seat when I depressed it on the fattest rubber they could stick it on being steered by a tiny steering wheel that turns the wheels via the wonder that is power steering and the machine is stopped by what seem like adequate power brakes, however both get their power function from the engine, due to the previously mentioned Italian reliability the engine regularly quits. When that happens you have no power steering and no power brakes, and due to the sheer mass and the sheer size of the tires along with the tiny steering wheel you cannot turn the wheels and you cannot stop it without the engine running.
You’re stuck in a $400K irreplaceable Italian freight train made of fiberglass that’ll go wherever the wheels are pointing until you run out of kinetic energy which there is plenty of at the speed limit. So I learned to drive with my right hand on the key whenever possible to I can turn it to try to get it started and avoid disaster. The last time I drove it there was a crowd of photographers and parents for a prom in front of me when the engine quit and I was so quick to restart it noone noticed it quit but after I got it to its home I refused to ever drive it again. That Machine is the main reason why I distrust anything that NEEDS power steering and or power brakes.
You could convert it to an electric power steering pump and an electric vacuum pump that run independently of the engine if the darn thing won’t keep running all the time.
If I were the owner and I was going to daily drive it I would.
Really looking forward to the follow up article!
As a Honda owner, I’ve noticed the difference in size between front & rear brakes. Taking what I’ve just learned from this article, can I conclude that the brakes are that size because that’s all the car can effectively use? Which would mean (if the previous sentence is correct) that increasing the rear brakes (rotor size or stronger calipers) would only increase the likelihood of locked rear wheels in a hard braking scenario?
You are correct that smaller rear brakes is all your car can use because of the forward weight distribution which makes the rear end comparatively light. Putting larger brakes on your Honda would be a waste of money and time and it could severely disrupt your brake balance unless you also upgraded your front brakes to match.
Would larger rear brakes on a fwd car just lock up the rear tires causing it to skid/lose control?
Yes, because most FWD cars have little weight in the rear. If you have rear brakes that are bigger than they need to be, you’re just wasting money and weight.
If it doesn’t have ABS, then yes. If it does have ABS, then no.
In the United States, every car built during and after 2012 is federally mandated to include ABS. So if the FWD car is 2012 or newer, ABS will keep the rear tires from locking up even with oversized rear brakes.
So oversized rear brakes in a FWD car with ABS may not be very useful, but they also will not cause you to crash either.
Seems interesting, but I had to stop at the first figure to post this. Can you please look into technical publications (like Nature or IEEE journals) and their style guides? These graphs and their font sizes are unreadable small. The graph needs to be generated at the size that it will actually go into the publication.
Thanks
This is true, but very difficult to do, when everyone has different sizes of screen (and probably contrast), from phone to big TV. Resolution is also a factor. All those are easy in print publication but hard to predict in electronic publication.
My gotcha is the typo “Weight Distribution Effects The Ideal Brake Curve”. It does not, it AFFECTS the curve. It does have an effect, though. As a verb, “affect” means “influence” and “effect” means “makes happen”. (As a noun, “affect” is a psych term, and “effect” means “the result”.)
The graphics need to be linked to full(er) sized versions.
They are full size versions, just open in a new tab.
Thanks for the comment jb996. I made those graphs myself and they are indeed hard to read, especially on a phone. There’s room to make the font bigger without losing data, which I will do next time.
So this would at one corner of the envelope, right?
https://youtu.be/bTwxHCHaoMo
Holy crap! That is amazing. Is there anything Mini’s can’t do?
Classic! It’s been years since I last watched this.
An appetizer.
Wow this article sure made me hungry for more.
I have Subscribed to the YouTube channel because clicks are good for you and I love to learn the why and wherefore of vehicles.
On YT the audience is potentially huge, the Autopians should give you a good early bite.
More is on its way…
Everybody please go check out his youtube channel. Link at the top. I’m going through the roll centers videos right now. Great stuff!!!!
Where’s the line at mu = 1.1 & 1.2 for the Cup 2R option on the 911?!?! 🙂
Did you make 13k a month by designing braking systems? If not, then it’s time to stop.
Fascinating stuff.
When designing a truck, I assume multiple curves need to be taken into account for empty vs loaded, different distributions of the weight, etc?
Also curious how different tire designs affect the friction coefficient; something like a mud tire is going to be quite different than an R compound track tire. What kind of variation is typically seen?
For any vehicle with a very wide operating envelope you would design for worst case. So, yes, you would generate many different envelopes to represent the different loading cases and see which one ends up being worst.
On some older Mopar products there was a load sensor attached to a brake proportioning valve. Usually a rod with a lever, whever it was heavily loaded, it would add more rear brake bias. Unloaded, it would take away rear brake bias. Only problems would stem when it would either fail, and get stuck closed, or when you lowered the vehicle and it would assume “loaded” all the time.
The early Ford Taurus’ also had an adjustable proportioning valve tied to the rear suspension. It had the same issue with the rod rusting off and letting the valve think the car was unloaded all the time. At least that failsafe was less rear braking rather than full rear braking which would lead to early rear lockup.
This is why the engineer in me cringes when I see what people do to their cars with aftermarket stuff. All this work goes out the door.
Working in construction has made me fear this less and less over time. At the price points I work at everything (I mean *everything*) has been cost engineered to just barely hold up to it’s engineered specifications.
So if you were to go out and spend money on say, some better fasteners, or actual higher spec compontents than the OEM’s bean counters were willing to splash for, there could be gains to be made.
Telling me the spec tires on a 911 I can’t afford are idealized for the application is one thing, but the rubber on that there Kia Rio can probably see performance gains with compatible products available in the market.
You’d be surprised at the effort that goes into even a Kia Rio tires. Yes, they are cost driven but the engineers still spend an inordinate amount of time making sure even those cheap tires work as best as possible and to make sure those tires and the ABS system work well together. Yes, you can make improvements with aftermarket tires but don’t underestimate the work that goes into making even cheap cars work as best they can for the money.
I thought only Porsche had spent any meaningful amount of effort on tires besides just being cheap.
All the OEMs have ride and handling teams who care about this stuff, even the ones that sell cheap cars.
I know a guy who habitually upgrades the fasteners on his cars with stainless steel. He doesn’t understand that they aren’t as strong, or that they can actually make corrosion worse. He just “knows” his shiny new fasteners are “better” than the OEM ones that passed durability testing after someone like me engineered the system.
I’ve also done some engine dyno work on aftermarket “upgrades”, and found most of them were worse, and the ones that didn’t make the engine worse didn’t last as long as the parts we originally specified.
Sure, we work to a price, but we also work to durability targets that the aftermarket can’t afford to test to.
That said, I do a lot of mods to my car that I couldn’t recommend to anyone else with my professional engineer hat on >gestures to MX5 turbo that only caught fire once, and had brakes made from two other makes of car on it<
Oh god please do not mess with any of the fasteners on a vehicle. Literally all of them have had a torque calculation, durability test, and a lot of money spent in assembly to make sure the residual torque is the same as bolt on torque.
Also galvanic corrosion can be an issue with SS being more noble than regular steel, it’s likely the part it’s connected to will corrode instead of the fastener. While neither is fun to deal with, the fasteners are a cheaper fix much of the time (replacing the part when easier/cheaper than replacing the bolt would be the same either way).
Yeah there are potential gains in the aftermarket. But you do have to consider that while automakers are always looking to pinch pennies, or even hundredths of pennies (trust me, I work in the industry) the aftermarket suppliers are also doing the same thing. Everyone wants to make the cheapest part possible that will meet their design and performance targets to maximize revenues. The thing about oem parts though is that those design and performance targets (specs) we’re set by the customer and are usually aligned with the vehicle in question to provide the performance and durability the average consumer would expect from the factory.
In a shop I worked in a guy brought in his lifted brodozer with four rear shocks per side, two of which essentially wouldn’t get to the top or bottom of their stroke. It also had 10 inch disks on the front, he was in for an alignment which was a joke given the condition of the thing. Worst was he owned a big 4×4 shop in town and this truck was advertising his work
Well I’m no engineer, but sometimes I cringe with people’s mods especially when it fails the “it was better before” test. At least one hopes manufacturers give something stable as a blank canvas from which to work and usually they do. Don’t forget a person’s performance targets for a vehicle might be different than the manufacturer’s and they may be willing to live with different compromises to hit their targets.
I totally disagree. “but the factory engineers knew so much” they say.
Yeah, no. Have you ever seen an engineer at a major automaker, who was able to do what they thought was actually the best?
No, they have to compromise and choose really crappy designs that barely pass muster so bean counters are happy. They get hand me down requirements with subpar solutions. They have to choose materials that are not as good because supply chains are available for those materials.
Bureaucracy means their choices are hamstrung. What comes out of the manufacturer, is not at all what an ideal engineer would produce.
Engineers that make the kind of stuff you want to put on your car get fired for not doing enough cost-cutting.
Yes I have. Many of them. I’ve worked on high end, cost no object, projects. Their easy. The real challenge and test of skills comes from working to a cost target and still making it work. Do you think Toyota has their durability reputation because they have engineers who cave into the bean counters? And yes, Toyota has just as many bean counters as any other OEM. They’re no different.
I’ve worked on big budget hypercar projects, and they are fun, but the work I’m most proud of is the clever stuff I’ve had to come up with on a low-budget project.
Sure, sometimes you have to fight for the right solution (not “the best” because all engineering is a compromise), but the testing budget is huge, and no one wants to repeat a fleet of 100,000 mile durability vehicles because a bracket is too thin and your cats fall off.
One of the risks of an ineffective solution caused by saving a few cents is paying millions of dollars for unplanned testing, or more millions for a recall. Accountants are good at maths, they want the total number to be smallest.