Caliper piston stroke difference.

[Kasrawy]

Hey all,
I have to say seeing as this is my first post here (I've been lurking for a while though) that this is one of the most helpful forums out there, thanks for your efforts everyone.
Now back to topic: I am in charge of the braking department in my team and we wanted to overcome the bias problem from the pitching that occurs when brakes are applied in a novel manner (no bar or valves), so we went and made a program that optimizes the master cylinder/caliper combination so it produces a noticeable output force bias right off the bat with no input force bias (sorta like how a bias bar works only no uneven pedal torque leading to a lighter pedal). This went good until we calculated the stroke which was a disaster; the caliper pistons in the rear moved almost 2 mm for each 1 mm in the front (since the front MC/caliper area is much larger to produce bias). My question is: will this actually affect performance or will I not be able to see it seeing as the actual pad rotor clearance in disc brakes is negligible to start with? I's like to add that we have a 2:1 distribution with one inboard rear brake mounted on the differential and 2 outboard brakes in the front.
Thanks in advance

[Kasrawy]

Hey all,
I have to say seeing as this is my first post here (I've been lurking for a while though) that this is one of the most helpful forums out there, thanks for your efforts everyone.
Now back to topic: I am in charge of the braking department in my team and we wanted to overcome the bias problem from the pitching that occurs when brakes are applied in a novel manner (no bar or valves), so we went and made a program that optimizes the master cylinder/caliper combination so it produces a noticeable output force bias right off the bat with no input force bias (sorta like how a bias bar works only no uneven pedal torque leading to a lighter pedal). This went good until we calculated the stroke which was a disaster; the caliper pistons in the rear moved almost 2 mm for each 1 mm in the front (since the front MC/caliper area is much larger to produce bias). My question is: will this actually affect performance or will I not be able to see it seeing as the actual pad rotor clearance in disc brakes is negligible to start with? I's like to add that we have a 2:1 distribution with one inboard rear brake mounted on the differential and 2 outboard brakes in the front.
Thanks in advance

[flavorPacket]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kasrawy:
will this actually affect performance or will I not be able to see it seeing as the actual pad rotor clearance in disc brakes is negligible to start with? </div></BLOCKQUOTE>

Piston rollback is anything but negligible. Any respectable brake engineer will characterize the motion of each piston in a caliper to understand the behavior you're concerned with. But there's a lot more than piston dynamics going on: pad compressibility, fluid compressibility, head loss, and M/C performance will all affect braking torque in the time domain you're worried about.

So I guess the short answer to your question is, yes, it can be done, if you understand the 4 or 5 most important dynamic properties of the system. Lots of road and race cars use different seal grooves front vs. rear or other mechanisms to achieve this kind of response.

[Kasrawy]

Thanks for the info. We are now working on equating the piston travel in both rear and front callipers. regarding your previous post though, I can test for pad and fluid compressibility and even for line expansion under pressure but how am I supposed to get the M/C performance from? And what does that mean exactly? Losses, force pressure curves (assuming they are curves and not straight lines with negligible deviation)?

[Andrew07]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">it produces a noticeable output force bias right off the bat with no input force bias (sorta like how a bias bar works only no uneven pedal torque leading to a lighter pedal). </div></BLOCKQUOTE>

With the balance bar the force input to each master cylinder should be the same (when set for nominal balance). The difference is the pressure needed to react that force, which you control by choosing the right MC sizes. If you guys are deciding to not employ a method of varying the balance, then how will you compensate for a rainy day or some other circumstance that raises or lowers the vehicles max deceleration and therefore changes the ideal brake balance?

[Kasrawy]

Andrew, we are aware of the function of a balance bar what I was referring to was a one that was unbalanced to begin with (albeit by a small amount) so we would have a small forwards/backwards input force bias even with the same M/C and calliper choices not a device that would manage the overall grip of the vehicle for varying traction conditions. Said bar could work as a balance bar as well. The only problem with inducing the forwards/backwards bias with M/C pressure differences alone (specially since we are working with a single inboard rear brake) was the piston travel as I mentioned above.
So I guess what I am asking is: which of these methods would be the best choice: using balanced input forces to the M/Cs and inducing bias by pressures (seeing as this produces a piston travel mismatch) or using an unbalanced balance-bar (this would increase pedal loads and flexing) or going with a proportioning valve to induce the bias? If you have any other suggestions please comment.

[Drew Price]

Have you computed how much the angularity of the balance bar effects bias at different loads?

Generally for small angles you can neglect it altogether, but depends on the setup.

Usually proportioning valves are not used because they are not exceptionally repeatable.

[flavorPacket]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kasrawy:
Thanks for the info. We are now working on equating the piston travel in both rear and front callipers. regarding your previous post though, I can test for pad and fluid compressibility and even for line expansion under pressure but how am I supposed to get the M/C performance from? </div></BLOCKQUOTE>

I don't want to make assumptions, but make sure you're looking at the pistons' transient responses. Simple doing a flow vs. area calculation will not give you the results you're looking for.

Regarding M/C characterization, again I mean the transient response of the cylinder. The seals and actuation linkage (trunnion bar or whatever) will introduce differences in the response Front vs. Rear.

Stuff like this why I'm happy I work on road cars now. All we have to do is tune the EBD...

[nowhere fast]

Perhaps I'm missing something, but isn't this just the basic principle of mechanical advantage / velocity ratio at work?

I.e. The rear moves twice as far but with half the force. The same should happen if you used the bias bar to achieve that bias (neglecting angle effects etc.).

[flavorPacket]

If you're talking about quasi steady state bias, that's correct. But if you're interested in looking at how the bias is changing during the first few milliseconds of apply, when the weight transfer is just starting to develop, there's much more to consider. Until you asked that question, I thought that's what Kasrawy was trying to ask, but now I'm not so sure.

[Kasrawy]

flavorPacket what I was asking about was in the steady state actuation phase not in the transient period of applying the brakes. Is the transient period that influential anyway? Should I be basing my calculations on the period before the pedal achieves maximum/design point travel?
Also about the bias bar, I wasn't thinking of altering its angle as much as the armature itself. I.E. placing one master cylinder further than the other with respect to the centre of the brake pedal. Wouldn't this produce input force difference without

[Drew Price]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by flavorPacket:
Until you asked that question, I thought that's what Kasrawy was trying to ask, but now I'm not so sure. </div></BLOCKQUOTE>


This. I thought this too.

[Kasrawy]

This is what I meant by the bias bar if anyone is still confused by my explanation:
http://www.stoptech.com/tech_i...tioning_valves.shtml (http://www.stoptech.com/tech_info/wp_proportioning_valves.shtml)
it's the second strategy listed in the above link

[flavorPacket]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kasrawy:
flavorPacket what I was asking about was in the steady state actuation phase not in the transient period of applying the brakes. Is the transient period that influential anyway? </div></BLOCKQUOTE>

It's not influential compared to the steady-state, that's for sure.

If you're looking at the steady-state bias, then why do you care about piston travel at all? In steady-state, the pistons are hard up against the pads and then you've just got a bunch of springs in series that you need to account for.

[Pico]

Correct me if I'm wrong. You're trying to create a front to rear brake bias (eg 60:40) while just relying on only MC diameters and piston diameters? Since your larger diameter MC is to the rear brake and there is only 1 of them you'll be displacing more fluid to the rear than to the front so you NEED a balance bar (maybe not an adjustable one) but one to solely compensate for the difference in travel. If not your back brake is going to always brake before your front, and the front will get whatever is left possible through the flex of whatever liks it to the pedal.

Is there something novel about the system? We've run this 2 front 1 rear caliper system for quite some time but never without a balance bar. Please explain.

Thanks,

[Kasrawy]

Hey Pico,
you are correct on both counts; the rear brake will always brake before the front and the system has been done before. What I was asking is if, in the steady state actuation phase, the difference in the rear caliper's pistons travel relative to those of the front caliper would pose a problem. If it did not then we were ready to discard the bias bar altogether if the quasi static's effect was not influential. Cutting that middle step (the bias bar) would allow for a torque free pedal which would allow us to decrease its weight. That was the new thing we thought we could implement. I disagree with one thing you said though, the rear MC doesn't have to be larger than the front one, you can go either way. The important thing is the total frontward area has to be larger in the end.
There is one thing I still don't understand though: if I could have the pedal exert two equal forces onto the two master cylinders and still get the desired bias then why do I need a bias bar again? Couldn't I just go with an equal (lengthwise) balance bar to the front and rear MCs or an a balance bar alter the stroke of one MC relative to the other in some way?
BTW thanks for all your help so far guys. I wanted everyone to know that, after a little simulation effort we found that the rear brakes engaging earlier than the front (by less than a second even) had very damaging effects on the handling of a vehicle. We would therefore advice against implementing systems with rear and front stroke differences.

[Gruntguru]

1. Static bias has nothing to do with stroke of MC's or WC's. It is all about pressure (which is itself determined by leverages and MC piston area) and WC piston area.
2. The rear WC stroke will be a function mainly of piston retraction.

I know you have referred to the rear WC travel but are you actually talking about the rear MC travel? If that is your question the answer is usually "no - a little more travel in one MC is not a huge problem". It is probably a good idea to adjust the MC pushrod lengths so the balance bar is square to the pushrods with the brakes applied. This will result in a slightly angled balance bar with the brakes released.

[Kasrawy]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Gruntguru:
1. Static bias has nothing to do with stroke of MC's or WC's. It is all about pressure (which is itself determined by leverages and MC piston area) and WC piston area.
</div></BLOCKQUOTE>

Could you clear up what you meant as static bias? Correct me if I'm wrong but isn't the pressure within an MC (or a caliper for that matter) a function of its area and the force imposed on it by the pedal? Seeing as the bias bar controls one of those two elements then I'd say its all connected.

Also in what way is the rear caliper's stroke a function of piston retraction?

[flavorPacket]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kasrawy:
Also in what way is the rear caliper's stroke a function of piston retraction? </div></BLOCKQUOTE>

The piston can only travel as far as the pad will let it, so that gives you the ending position of the piston. The starting position is determined by how far the piston moved back into the caliper bore from the last apply. This is primarily determined by the design of the seal and seal groove.

[Buckingham]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">if I could have the pedal exert two equal forces onto the two master cylinders and still get the desired bias then why do I need a bias bar again? </div></BLOCKQUOTE>

Because your driver will want the bias adjusted differently than how you designed the system, and if you give them this capability it will make your car faster.

[Kasrawy]

Ok, sorry for dragging this on for so long but I still don't have a clear answer. How do you guys deal with the rear brakes engaging before the front ones in single inboard mounted rear brake systems?
Also why does the bias bar tilt when the driver applies the brakes (as in this image http://www.stockcarproducts.com/images/wilped2.gif)?
Thank you for your helpful replies so far.

[whiltebeitel]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kasrawy:
Ok, sorry for dragging this on for so long but I still don't have a clear answer. How do you guys deal with the rear brakes engaging before the front ones in single inboard mounted rear brake systems?
Also why does the bias bar tilt when the driver applies the brakes (as in this image http://www.stockcarproducts.com/images/wilped2.gif)?
Thank you for your helpful replies so far. </div></BLOCKQUOTE>

If the rear brakes engage before the fronts, then the bias bar is turned so the master cylinder for the front brakes has a shorter moment arm, thus taking a greater proportion of the braking load. The bias bar rotates in that example because he front brakes are of larger piston diameter, and assuming the master cylinder sizes are the same, the front master cylinder would have to have more stroke to move the front pistons than the rear master cylinder whould to move the smaller rear pistons the same distance.

[Gruntguru]

If the rear brakes are engaging before the front, there is something wrong with your system. Engagement of brakes is about pressure not travel. If your car has 50:50 weight distribution, maximum braking will need 60% front or more. When the pedal leverage, piston sizes, rotor diameters etc are all set up to give you that 60% front bias, you will get close to 60% front bias at all ranges of braking - light medium and maximum.

During the transition while the caliper pistons are still travelling and the pads are still moving (front or back) towards the discs there is NO braking. Braking doesn't start (front or back) till ALL the pads are starting to press on the discs and pressure is building in the system. Forget travel - its pressure that counts.

[flavorPacket]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Gruntguru:
During the transition while the caliper pistons are still travelling and the pads are still moving (front or back) towards the discs there is NO braking. Braking doesn't start (front or back) till ALL the pads are starting to press on the discs and pressure is building in the system. Forget travel - its pressure that counts. </div></BLOCKQUOTE>

Your car doesn't slow down with 10 bar in the system?

[Kasrawy]

I'd like to thank you all for your very informative posts and would like to say that we- at the braking team in Cairo Uni- have a very firm understanding of the mechanics of the dual master cylinder braking system due to your help. We are now commencing with the detailed design of our system.
Thank you again for your efforts.