• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

car brake physics!

Share this page

NutmegCT

NutmegCT

Great Pumpkin
Bronze
Offline
Gentlemen - could you help me settle an argument.

Imagine a car hydraulic brake system. Single line system - not left/right cross over.

Left side of car has 1" wheel cylinders on front. Right side of car has 1 1/4" wheel cylinders on front. Rear of car has all 1" wheel cylinders.

Push pedal.

Will one side get more "force" than the other? Will one side "pull" more? Will car (for example) pull left when brakes are applied hard?

Thanks.
Tom
 
Assuming all brakes are adjusted with no slack (these are drum brakes.....right?), I would say the bigger right-front cylinder would apply slightly more force than the smaller left-front cylinder. It will also move slightly less at first until any "slack" is taken up in the other three brakes.This is due to the hydraulic "leverage" caused by the bigger cylinder. For light braking, you might not notice any difference: with heavy, "panic" braking, pulling to the right might be more noticeable.
 
I disagree about the "slightly" part. The force applied to the shoe is proportional to the square of the piston diameter, so the shoe with the 1.25" piston will get 56% more force than the other one. That also means that it will overcome the return springs first, so that side will apply first. So it will start out pulling to the right, and keep pulling to the right until you get far enough into a panic stop to lock the RF wheel (when it will probably spin out to the left).
 
Thanks gentlemen. The Mercedes guys I asked are split 50/50. Some say, all else being equal, cylinder size difference won't affect the pull as the fluid pressure is equal; others say cylinder difference will *definitely* cause imbalance in the "pull". And 50% of the latter say pull to the left, 50% pull to the right.

And Randall's factor of *square* of the diameter explains why the car veers to the right, and eventually locks the wheel.

Present situation: four cylinders with four different "stamping" numbers. Could be size in mm, or could just be a casting number. Regardless, until I remove the cylinders and measure the bores, I don't know if they're all the same inside.

Solution to the problem will likely be four matching cylinders

Thanks!
Tom
(currently preparing for a *long* trip in an old car ...)
 
I think it will have a short initial pull left then quickly right and stay pulling in that direction. The reason I say this as the left should engage first, pull then when the right hits it overcomes the left. Same effect as with an old/new brake hose. Whomever says size doesn't matter is mislead as they make different sizes for a reason...:whistle:
 
NutmegCT said:
Some say, all else being equal, cylinder size difference won't affect the pull as the fluid pressure is equal;
Click to expand...
That much is clearly wrong. The force exerted by a hydraulic cylinder is the pressure of the fluid times the area of the piston. The area of the piston is 3.1415... times the radius squared (or if you prefer, .785398... times the diameter squared). So, for example, if the pressure is 100 psi and the cylinder is 1" diameter, the force will be 78 lbf. If the pressure is 100 psi and the cylinder is 1.25", the force will be 123 lbf. Quite a difference.

And 50% of the latter say pull to the left,
Click to expand...
The theory there is that it takes time for the fluid to flow into the cylinder, so it would take longer to fill the larger cylinder. There is something to be said for that, but I think the return springs will more than compensate. The pressure does not rise instantly, it takes time for the pedal and master cylinder to move (and the fluid to move through the common part of the lines). So, again for example, if the springs take 10 lbf to overcome, the pressure in the 1" cylinder has to reach 12 psi before the piston even starts to move; while the 1.25" cylinder will start to move at only 8 psi.

Of course, there can be lots of other reasons for them to pull. The front drum braked TR3 I owned for awhile always seemed to pull one way or the other and it had the correct cylinders all round (I checked). The amazing part was that it rarely pulled the same way twice! Sometimes it pulled left and sometimes it pulled right. Never did work that one out. Instead I robbed the OD (which was the only reason I bought it in the first place) and sold the car.

Note that a size difference front/rear may be intentional, to account for the weight distribution of the car and possibly any construction differences in the brakes.

If it still pulls, check out the front suspension mounts to the frame. I had a TR3A that would pull to the left, and eventually discovered that the inner lower pivot for the RF wheel had torn loose from the frame. The pin was still attached at the other end, so it would spring back straight when there was no force on it, but the pin would bend under heavy braking and make the wheel turn to the left. Three different shops failed to spot the problem. That was around 1975, so it wasn't even that old a car at the time and not particularly rusty.

The pull wasn't too bad under normal braking; but got progressively worse as I braked harder. I finally decided that I had to stop driving it that way after someone pulled out in front of me, and I was unable to keep the car from jumping the median into what would have been oncoming traffic if anyone was there.
 
The theory there is that it takes time for the fluid to flow into the cylinder, so it would take longer to fill the larger cylinder.




But if the brakes are properly adjusted, there should be very little motion required to begin forcing the brake pad onto the drum. In any case, until ALL the brake shoes are touching the drums (or disks) the increase in hydraulic pressure is limited to the minimal pressure generated by the return springs.... So physics still wins and the larger cylinder is going to generate more force on the pad, and pull to the right.
Either way, assuming that this is a real car we are talking about you should repair this unless you are planning on racing in Nascar....
 
These explanations are excellent. I've shared details and rationale with "the doubters".

I'll be installing four correct front brake cylinders (matching 1 1/8" bores), plus matching shoes and drums. Also replacing the steel brake lines and hoses.

They're manual adjusting (eccentric cam) drum brakes - on the ol' Mercedes.

Thank you all!
Tom
 
Great discussion and very good points. I've been a bit too busy to reply until now.
I think we all agree that the wheel cylinder should be changed or the car will "pull" to the right. Never any doubt about that.

The confusion is the fact that the pressure is equal thought this single master-cylinder system. While that is true, the same amount of pressure can cause the right front cylinder to exert more force (and less motion) due to the increased mechanical advantage created by the larger bore.

But, as we all know, on cars things can get complicated.

For example, if try to decide if the car will "pull" to the right or left, we have to look forces exerted by both right side cylinders versus both left side cylinders. If we used Randall's example of 100 psi in the system then the right side forces look like this:

78 + 123 = 201

And left side forces look like this

78 + 78 = 156

Here, we see a nominal difference of about 23% from side to side.

But a car is a dynamic device and we know that the front brakes do more work.
In an ordinary, moderate stop (which I imagine Tom is contemplating), the front brakes probably create 60% of the braking action. In a harder, panic stop, those number can easily jump to 80% or more. So the forces on the right side are greater, but the mathematics isn't really clean here.

Note that in my initial comment, I said the brakes should be adjusted with no slack. This would (mostly) reduce the effect of the bigger cylinder overpowered the return spring sooner.

Also (and this was what I was really thinking of in my initial comment) virtually all drum brakes, including the ones on Tom's car, are self-energising ("slop" in the brake shoes allows shoes to rotate slightly as brake is applied.......as they rotate, the design wedges shoes out against the drum). This self energising effect adds its own force to the brake shoes, and may dampen the differences created by the two differing wheel cylinders.

By the way, my race car is supposed to have 7/8" rear wheel cylinders but one failed when I was at a race a few years ago. My only spare was a 3/4" cylinder which I installed so I could finish the weekend. Admittedly, this was on the rear, where the effect is less. But I didn't really notice much difference and left it in for a few events.

Anyway, good discussion and I'm glad to hear Tom will be getting equal-sized brake cylinders before his journey.
 
I agree Nial that Tom will be equalizing his cylinders. Understanding hydraulics is a science all of it's own. I'm not a hydraulics engineer, but from the practical side, I know that hydraulic cylinders with different internal measurements have different results with the same pressures. On farm equipment, we use a lot of hydraulics and in rare cases, when matching up a pair of cylinders that work in tandem, the same spec cylinder by a different manufacture can have different results, such as lead and lag. PJ
 
Also, differing cylinders on the rear don't cause a pull to one side or the other as much as causing one rear wheel to lock up sooner than the other. In normal driving, one would probably never notice the effect. Differing braking on the front wheels definitely causes a torque on the steering wheel, what we percieve as "pull" even in light braking.
 
aeronca65t said:
) virtually all drum brakes, including the ones on Tom's car, are self-energising ("slop" in the brake shoes allows shoes to rotate slightly as brake is applied.......as they rotate, the design wedges shoes out against the drum). This self energising effect adds its own force to the brake shoes, and may dampen the differences created by the two differing wheel cylinders.

.
Click to expand...

I think it's important to point out for those who don't know, Nial is referring to "floating" adjusters drum brakes, not "fixed position" like on a Spridget or air cooled VW.
 
You always get some amount of boost (although it might be for the other direction of travel). But since the effect is proportional to the force from the cylinder, it doesn't change the answer.
 
TR3driver said:
You always get some amount of boost (although it might be for the other direction of travel). But since the effect is proportional to the force from the cylinder, it doesn't change the answer.
Click to expand...


It's a little know fact that a brake system's ability to stop is directly proportional to the size of the object in it's path. The larger the object, the less it's ability to stop. This deficiency is also proportional to the square of the value of the vehicle. :square:
 
As a follow-up, here's what I found when I compared the front brakes. Different size shoes, and different size & configuration for cylinders left and right. All are supposed to be 1 1/8".

View attachment 28927
View attachment 28929


Correct cylinders (and rebuild kits) are on the way! (Internet sure makes it less frustrating to find 50+ year old brake parts.)

Thanks.

Tom
 
NutmegCT said:
Gentlemen - could you help me settle an argument.


Thanks.
Tom
Click to expand...
No.No we can't. I have never seen a hydraulic brake discussion end with any concrete answer. Weather it be the classic discussion of what will cause a pull, or what type of brake fluid to use, or what type of lines are suitable.
It has been my experience that these discussions carry the same amount of personal belief as a discussion about religion, and usually don't end well. I am delighted to see that this thread has remained civil thus far. I hope it remains that way. I would offer my opinion, but it's already been offered by others in this thread , and I made a personal rule not long after joining this forum that I would never get technically involved in another brake discussion online. I'm probably breaking that rule just writing this.
 
Back
Top