Changes to brakes & resulting bias

[Motorhole]

I'm in the middle of a 5-stud conversion and will be using E36 m3 front brake calipers and E36 318ti rear calipers. Currently planning to stay with the stock E30 master cylinder.

My question is, will this affect my front/rear bias significantly? If so, how? Is there a calculation I can use to work this out?

Cheers!

[GeoffBob]

You'll find all of the equations you are looking for in this document here. Ignore the fact that this document describes how to size dual master cylinders and all the other gumpf that goes with it, and focus only on those equations that apply to the actual discs and callipers in order to calculate brake torque. In order to retain your bias as is (and thus keep your current tandem master-cylinder) you need to make sure that you achieve the same percentage increase (or decrease for that matter) in brake torque at both the front and rear wheels. So long as you make identical percentage changes your bias will not shift. In other words, if you double the surface area of your front calliper piston (as one example) then you would need to double the surface area of the rear calliper piston too. Be sure to look at those equations that determine your brake torque since this is not only dependent on the size of your callipers, but also on the radius of your brake disc and where the calliper is located on the disc. FYI, I am not familiar with the 5-stud upgrade and I have no idea how the callipers and discs compare to the 4-stud variety.

If you would prefer, post up all the details of your new/proposed brake setup (disc sizes, calliper piston sizes, number of pistons, pad details etc) and I'll do a comparison to see if your bias will be affected. You are right to be concerned about your bias BTW. All too many well meaning brake upgrades have turned out to be downgrades as a result of bias problems.

[ross_jsy]

Geoff, you never cease to be a source of incredible knowledge. Been trying to find out how to size some m/c's up for my new pedal box and haven't had a clue how to do it. I shall read through all that later

[Motorhole]

Thanks Geoff! I have no problems running through the calcs myself as long as I can get hold of the data I need. The thing I am still left wondering is how the calcs would vary for a rear drums-disc upgrade. I would expect the original braking torque has to be worked out very differently for a cylinder/shoe arrangement? Easy to work out coefficient of friction and shoe surface area but do cylinders work the same way as pistons in a caliper?

[Rav335uk]

I think Geoff looks like this VVVVVVVVVVVVVVVVV

[Motorhole]

That looks a bit like our supercomputing cluster at work!

[GeoffBob]

I think Geoff looks like this VVVVVVVVVVVVVVVVV

That's very kind of you Rav, but I suspect I'm a bit more like this

[GeoffBob]

Geoff, you never cease to be a source of incredible knowledge. Been trying to find out how to size some m/c's up for my new pedal box and haven't had a clue how to do it. I shall read through all that later

You're welcome Ross. I wrote that up a while ago for zoner Theo325 which is why you'll see I make reference to having the weight of a Nissan SR20DET up front. You'll have to substitute your own values as per your own cars weight as you see fit to get those equations to work for you.

For a more detailed analysis of the dual M/C setup see Theo's thread here. Somewhere around page 4 I posted some graphs that include the influence of a prop-valve inserted in the rear brake line. This is quite a useful device to have whose function is actually quite separate from the bias bar, so is worth looking at. The analysis of the brake performance with the prop-valve included is not as easy as without, but I can offer help if required.

[GeoffBob]

Thanks Geoff! I have no problems running through the calcs myself as long as I can get hold of the data I need. The thing I am still left wondering is how the calcs would vary for a rear drums-disc upgrade. I would expect the original braking torque has to be worked out very differently for a cylinder/shoe arrangement? Easy to work out coefficient of friction and shoe surface area but do cylinders work the same way as pistons in a caliper?

You're welcome. Sorry, I wasn't aware you had upgraded from drums. I have no idea whether the E30 with drums employed the same tandem M/C as the E30 with all round discs, but I would assume that to replace your M/C with one from a 325is, say, would not be a major obstacle if it is indeed different. The basis for my track car was a 325is and had front and rear discs, and although long since upgraded with other 4-stud alternatives (VW front, Z1 rear) I still have the original brake components laying around somewhere, and more importantly, I have the original caliper and disc data. What I am saying is, we can easily tell what was originally on a 325is so we can thus tell how your 5-stud componets would compare, provided you know the data for your new discs and callipers. If your new components effectively perform the same (or are equally scaled up, front and rear) compared to a 325is, then we know you can happily use the tandem M/C from a 325is. Easy

[rix313]

Really REALLY interesting read that Geoff

[Rav335uk]

I think Geoff looks like this VVVVVVVVVVVVVVVVV

That's very kind of you Rav, but I suspect I'm a bit more like this

Well no matter Geoff, keep the info coming

[randomspeedfreak]

geoff, how and why does the positioning of the brake caliper, on the disc, affect the brake torque?

[GeoffBob]

geoff, how and why does the positioning of the brake caliper, on the disc, affect the brake torque?

The position of the calliper on the disc determines the line along which the force due to the pad acts to create the brake torque. This force, multiplied by the mean distance from the hub axis to the centre of the pad, is effectively the brake torque. Hence, moving that mean distance outwards (by fitting a bigger radius disc for example) results in a greater brake torque, even though the actual pad may apply the same force to a smaller radius disc. Likewise, if you had the freedom to move the position of the pad across the disc (like, say, the stylus on an old LP, assuming the disc were large enough) you'd get more brake torque running the pad further outwards on the disc. Of course, pads are made to fit discs, so you have no freedom to move the callipers around, but the point still stands that the brake torque is the product of both a force (due to the hydraulic pressure acting on the pad) and a dimension (due to the position of the calliper).

HTH
Geoff

[GeoffBob]

Really REALLY interesting read that Geoff

Pleased you liked it Rich. Really old one that too. originally posted a looooong time ago

[randomspeedfreak]

geoff, how and why does the positioning of the brake caliper, on the disc, affect the brake torque?

The position of the calliper on the disc determines the line along which the force due to the pad acts to create the brake torque. This force, multiplied by the mean distance from the hub axis to the centre of the pad, is effectively the brake torque. Hence, moving that mean distance outwards (by fitting a bigger radius disc for example) results in a greater brake torque, even though the actual pad may apply the same force to a smaller radius disc. Likewise, if you had the freedom to move the position of the pad across the disc (like, say, the stylus on an old LP, assuming the disc were large enough) you'd get more brake torque running the pad further outwards on the disc. Of course, pads are made to fit discs, so you have no freedom to move the callipers around, but the point still stands that the brake torque is the product of both a force (due to the hydraulic pressure acting on the pad) and a dimension (due to the position of the calliper).

HTH
Geoff

ah, i thought you originally ment the placement of the caliper on the disc as you look the car from side on, i.e. 2 o clock, 10 o clock etc

cheers,
Matt

[Motorhole]

Ok, going by part numbers, it appears facelift E30's with rear discs and rear drums both use the same master cylinder. They also use the same brake pressure regulator. If I can find default brake bias for a rear drum model I can then work it out using the specs of my "upgrade" and compare the values.

One remaining question is does the brake pressure regulator influence the brake bias calculations? I should add my car doesn't have ABS.

[German-Whips]

By having the pad on further away it's basically the same as having a pad closer to the hub but applying a greater force.

It's just like a moment force.....Moment= Force x Distance

Increase either one and you get a greater moment, so by simply increasing the distance you can acheive a greater braking force.

Just like when you can't undo a bolt or a nut, you get a breaker bar or a longer handled bar, and the whole job becomes alot easier, as the torque is also increased.

[Motorhole]

You've just given a physics lesson to a physicist mate but while we're on breaker bars, my 2.5m scaffolding pole is invincible

Anyway, back to brakes...

[GeoffBob]

One remaining question is does the brake pressure regulator influence the brake bias calculations? I should add my car doesn't have ABS.

Sorry, I almost missed this.

Yes, your pressure regulator will influence your bias. The device that sits in-line with your rear brake line (I forget its exact name) functions not to regulate (cap) your rear brake pressure, but in fact to control the rate of rise of rear calliper pressure as a function of rear line pressure, once a certain critical line pressure has been exceeded. It basically helps to stop your rear wheels from locking up as you brake and the cars weight shifts forwards, thus reducing traction at the rear tyres. Now, should you happen to upgrade your brakes such that they now exert a greater braking torque for the same line pressure then you won't have to press the brake pedal as hard in order to brake (and thus decelerate) as hard as before. This means the "pressure regulator" (or whatever its called) will only kick in much later. You'll have to push the pedal much harder, thus braking much harder, before the "regulator" has any effect on the rear line pressure to the rear callipers. Hence, there is an increased chance of locking the rear wheels.

I assume this is a track car since it is here in the track section, or at least a car that sees double duty? My suggestion is remove the "pressure regulator" from your brake circuit and replace it with an adjustable "brake proportioning valve". I have a Tilton unit on my car, and I think Theo325 has one by Wilwood? These work the same as what you have, but are adjustable. Besides, the old ones have a habit of failing so you probably need something new by now anyway.

HTH
Geoff

[Motorhole]

Thanks Geoff, that makes sense. Yes, it is primarily a track car I am building (well...attempting to build). If I'm doing this I guess it also makes sense to route the main front-rear brakline through the car! Thanks for your help, lots of fun work for me to do then!

[Motorhole]

This brings me to another thing too actually! My plan at the moment is to run the stock E30 master cylinder (perhaps a new) one with a clio servo to make space for the M52 engine. If I'm re-doing the front-rear brake lines, I have already re-done the hard lines on the rear trailing arms, I might aswell do the lot! Why don't I just get an adjustable pedal box?

E.g. this one http://www.demon-tweeks.co.uk/Motorspor ... /1723/6316

I accept I would loose servo assistance. But otherwise, what would the advantages/disadvantages be in running a pedal box like this? Or are pedal boxes in this price range crap?

[GeoffBob]

Indeed, that is the ideal way to do it on a track car. You do lose your servo assistance but that but that's not actually a bad thing since the response of a servo is uncomfortably non linear. Even on a well setup road car you always get that cm or so of travel where little or nothing happens and then suddenly the pedal goes hard. Non-servo assisted brakes are extremely linear, meaning that the travel of the pedal is directly proportional to how hard you press down with your foot. My pedal is hard right from the top, there is no travel before the brakes take. Also, the force you feel with your foot is directly proportional to the brake torque. To regain some of the lost mechanical advantage due to the deleted servo you simply work with a longer pedal ratio (compared to normal pedals) and callipers with bigger pistons.

I wouldn't suggest floor mounted pedals though, unless that's what you really want. Those are best kept for single-seaters where you sit flat on the floor with your legs almost straight out in front of you. There are lots of other options to choose from and a few threads in this section where they are discussed. I'd suggest starting here which includes pics of my own, Theo’s and Martin Pallot’s pedal boxes. You can locate your fluid reservoirs inside or outside depending on where you have more room. Both mine and Theo’s exhaust manifolds are on the left of our engines so we both had room on the outside of the firewall for the fluid reservoirs. With an M52 you won’t have that luxury so on the inside may be your best option.

[Motorhole]

Hmmm looking into it a little more it may not work for me. Having no fabrication skills and my garage power running off a 3.2kW genny, I can't fit the generic ones! And the bespoke E30 items are well outside my budget. I do wonder if something like this:

http://www.compbrake.co.uk/Pedal-Boxes/ ... od_24.html

Uses the stock pedal, fitting inside the car where the pushrod goes through the bulkhead? But then I can imagine a very heavy pedal.

Otherwise it's back to Plan A, stock MC, clio servo and running the front-rear line through the car with a bias valve fitted!