How did you set up Brake Bias?

[SteveB]

Hi

I have looked back in searches to find out how bias has been set by builders. There are lots of references to the calculation of master cylinder and caliper piston size but not much to guide you to the percentage split that is best for a GT40.
Other suggestions include using the brake testing equipment used for UK MOT tests.
There is detail on the 'Trial & error' method to get the result. I understand that race set up requires fine tuning to get the ballance just right for the conditons.

So how did you do it? What brake effort split resulted?

Help with this will be appreciated

Steve

 

[Dave P]

Steve:
If you contact your local AP racing distributor. AP racing has a sheet that you can fill out and the AP engineers will calculate the sizes you would need with AP products. Why not draw on decades of race winning expirence.

 

[Russ Noble]

Hi

I have looked back in searches to find out how bias has been set by builders. There are lots of references to the calculation of master cylinder and caliper piston size but not much to guide you to the percentage split that is best for a GT40.
Other suggestions include using the brake testing equipment used for UK MOT tests.
There is detail on the 'Trial & error' method to get the result. I understand that race set up requires fine tuning to get the ballance just right for the conditons.

So how did you do it? What brake effort split resulted?

Help with this will be appreciated

Steve

Steve, If the MOT brake testing is a simple rolling road setup then the results will be way off if it can't reproduce the forward weight transfer that occurs in real life.

FWIW, my setup works well and was virtually bang on from day one. Equal sized master cylinders and the same sized discs and calipers all round. The brake split on my car is near enough to 50/50 in the dry. Thats with 10" and 14" rims.

It is easy to set up the bias, you just keep screwing more onto the fronts until they start to lock up and then back it off about half a turn. The actual setting will depend on the grip you have available which is a function of (amongst other things) tyre sizes/type and road surface.

For wet settings where you get much less weight transfer to the front you will probably have to back the fronts off about three turns from your dry settings, if you want max effort in the wet. Do this the same methodology as the dry, just keep screwing the fronts off until they stop locking. Simple really.

On a road car it would be foolhardy trying to drive at 10 tenths in the wet, so you're probably not going to worry about the wet settings, just drive accordingly knowing your fronts are going to lock early.

 

[Mike Drew]

At the risk of stating the obvious, the objective is to get the front brakes to lock up just slightly before the rears. Different cars will have their proportioning set up differently, depending on the innumerable variables of master cylinders, calipers, rotors, pads, etc. etc. etc. Front-engined cars often have a proportioning valve set into the rear brake line to restrict pressure to the rears; mid-engined cars often need the reduction in the front circuit. And GT40s originally came with dual, independent master cylinders whose bias was adjusted at the pedal with a bellcrank assembly.

Regardless of how your car is plumbed, keep adjusting it until your rear brakes lock first. That tells you that you have gone too far; back up several steps and you're there. :thumbsup:

 

[Eric]

Gentleman,

Be sure that the front locks up just before rear does.

If the rears lock first you will slide right through the next
red light and only way out will be to release the brake pedal mid stream and make a quick right or left to avoid a potential crash.

I speak from experience.

Thanks, Eric

 

[Dave Bilyk]

HI Steve,

I started by assuming a 45%Front 55%Rear static weight distribution and a CG height of 16" (), and a weight of 1200kg if I remember correctly. Then I worked out the balance bar ratio required to give simultaneous lock at front and rear. Assuming I had done it right, this meant that an adjustment of the balance bar to the front would ensure the fronts locked first in the dry, with the option of winding it back for the wet.
As Eric says you need to test in controlled conditions to make absolutely sure your rears dont lock first, and I think there has also to be a safety margin in case rear wheel lock can be induced by say, a bumpy road.
Some people recommend testing on grass as a first step.
I programmed the various cylinder dimensions and pad areas etc into Mathcad to do this, and experimented with disc sizes, and cylinder and pad sizes from AP, Wilwood and Hispec until I achieved a theoretical balance. I compared with another brake balance spreadsheet from somewhere and got the same result.
The method seems to work, but I have only tested on cold brakes so far.
If will dig it out and have a look.

Dave

 

[Paul W]

Did mine in a similar way to what Mike said a few posts up. It was very obvious when I had too much rear bias - braking heavily from say 100 mph. The car gets all unstable - a few turns back and I found the ideal setup for a dry road at least.

 

[Cliff Beer]

Once you've done the calculations and got the sizing proper, then go out somewhere uncrowded and stomp on the brakes while an observer watches on from a safe distance. Turn bias knob accordingly....

 

[Russ Noble]

Gentleman,

Be sure that the front locks up just before rear does.

If the rears lock first you will slide right through the next
red light and only way out will be to release the brake pedal mid stream and make a quick right or left to avoid a potential crash.

I speak from experience.

Thanks, Eric

You must have had miles too much on the back to start with Eric!

Steve, think twice before you set it up with front brakes locking first.

Three very good reasons. Firstly when your fronts are locked you lose all steering effect. Secondly, the fronts contribute most to forward weight transfer, when they lock you lose the grip and the majority of the weight transfer benefit and hence a lot of the retardation ( Same as if you don't have enough on the front as in Erics case which also causes instability). Thirdly, a dry setup with fronts locking is just that much more diabolical in the wet.

You want your rears locking just slightly before the fronts, this gives you warning that you are almost on the limit and are about to lock all fours. You still have steering, and you still have maximum retardation. Neither of which you will have if the fronts lock first.

So keep screwing brake onto the front until they lock first then just take the adjustment back a little. Then you've got the best you can get.

All IMHO, of course

 

[Clayton]

Hi Steve
I'm with Dave, fill out the AP form and have a chat with them.
I did the same and the results came out pretty close to the 2 spread sheets that I found on the web (can't remember where).
Its not finished yet, so cant comment on the results just yet.

 

[Russ Noble]

An interesting spreadsheet Clayton.

I haven't seen the AP one, but the one you have shown doesn't seem to make any allowance for the difference in width of front and rear tyres as is typical in a GT40. I think this will skew the results somewhat as there is a lot more grip available on the back, than if skinny front tyres had been used there.

 

[Clayton]

Yeah I thought the same thing, but when I got the AP sheet, it didn't have tyre widths as well..........
The AP sheet is only a data request form and not a calculator like the other two.
If you want a copy let me know.

 

[Bob Putnam]

Tire widths don't have a significant effect unless the tires are undersized enough to change their coefficient of friction. It's the axle weight that counts.

 

[Russ Noble]

Bob,

What's your definition of "significant'" and "undersized enough"? Got any tech?

Hell, I wish I had known before, that width didn't significantly affect grip, maybe I could have got away with 205x15s all round. That would have saved me a real bundle!

 

[Bob Putnam]

Unfortunately, tire companies don't release adhesion details. (I've tried.) You'd need a 5 dimensional graph anyway, since the numbers would be dependent on size, load, camber, temperature and probably other factors too. My guess is that the difference between a tire with a real load/load rating of 50% and a tire with a real load/load rating of 35% is less than 5%. Tire compound is likely to be much more significant.

So, when figuring brake balance, 5% difference in adhesion only results in about a 1% change in front/rear proportioning.

 

[Trevor Booth]

The width of the tyre has no bearing on calculations of braking.
Friction is an emperical value with no units and is independant of contact area.

If you look at the charts that Clayton lodged you will see that the deceleration is stated at 0.8 g .This is based on the co-efficient of friction between the road and the tyre.
Typical g value on normal roads is 0.5 to 0.7 and in the wet it can be down to 0.1 - 0.2. It has to be a good road surface to get 0.7.
The 0.8 is more akin to a race track that has "rubbered up". It can be higher dependant on the track material

Where they tyres come into play is then being able to transmit the braking force to the road way. The contact patch needs to be sufficiently large enough to cope with the heat generated, The size of the contact patch required is dependant on the compound of the tread. For any given compound if the contact patch is too small the contact patch overheats and a "structural failure" occurs at the interface. If the contact patch is too large the tread may not get hot enough to get maximum grip. All racing tyres have an optimum temperature at which they develop maximum grip. ie improve the CoF between road and tyre. This same principle applies in cornering.

The same principle applies to brake pads, too small a pad will over heat (fade) too large a pad may not get hot enough. Typical pads have a higher CoF Hot than they do Cold.

You may have at times noted a letter code on the edge of brake pads which could be EF, or FF what this means is that the CoF cold is E and the CoF hot is F, the value for E I thing (without looking it up) is 0.5 and for F 0.6. The higher the CoF the greater the braking force generated at the disc face. So you can see that if the pads are too cold you may be not getting max braking (at the disc)

Any chart used for calculating brake force is a great starting point followed by physical tests.. A generalised chart cannot deal with all the variables.

There are different schools of thought on wet setup as opposed to dry, it is not uncommon on heavy cars ie racing cars with roofs on them,to not change bias from wet to dry, you are going slower, brake pedal effort is less, weight transfer is less.

Lighter cars can be a different story, although with modern super soft wet weather tyres it may not be necessary.

It is all a case by case situation wet or dry and the car and driver package on the day.

Serious track work requires a cockpit adjustable bias so it can be fine tuned as conditions change.

Now look at which should lock first front or rear.

When wheels lock they lose directional stability, that is why you cant steer with front wheels locked- you just go straight ahead.

Now if your rear wheels lock first they have lost directional stability and the car will spin or in the least be very unstable.

Try this, disconnect your front brakes and do a hard application of the brakes at say 150Km/h to lock the back wheels then simulate turning into a corner, if you already have not spun!!

Dont try this on a public street , a disused airfield would be more appropriate -)

 

[Trevor Booth]

posted twice for some reason

 

[Ron Earp]

Steve, think twice before you set it up with front brakes locking first.

Three very good reasons. Firstly when your fronts are locked you lose all steering effect. Secondly, the fronts contribute most to forward weight transfer, when they lock you lose the grip and the majority of the weight transfer benefit and hence a lot of the retardation ( Same as if you don't have enough on the front as in Erics case which also causes instability). Thirdly, a dry setup with fronts locking is just that much more diabolical in the wet.

You want your rears locking just slightly before the fronts, this gives you warning that you are almost on the limit

Good discussion and it makes me wonder if I must be doing it wrong (260Z, 50/50 weight distribution exactly as scaled with 180 lbs driver). Here is my thinking:

I set my car up so the fronts lock first. The reasoning is that the fronts do most of the braking work and I want to take every last advantage of the front braking effectiveness. My job as a driver is to drive the car, use the brakes and use 99.99% of all the fronts have to offer for the given conditions. Quite naturally if I overstep that limit I have to compromise, but again, you as the driver have to make sure that doesn't happen.

If I set my car up for the rears locking first I'd be giving up some front brake effectiveness. Not only that, depending on conditions I could be giving up a lot of front brake effectiveness. And, trail braking might become nigh impossible as it'd be really easy to lock the rears up whilst they were unloaded in a turn if the bias is toward the rear. Trail braking is useful from time to time.

So, more discussion on locking fronts and rears?

 

[Russ Noble]

Trevor a good post, and plenty of food for thought there.

Where they tyres come into play is then being able to transmit the braking force to the road way. The contact patch needs to be sufficiently large enough to cope with the heat generated, The size of the contact patch required is dependant on the compound of the tread. For any given compound if the contact patch is too small the contact patch overheats and a "structural failure" occurs at the interface. If the contact patch is too large the tread may not get hot enough to get maximum grip. All racing tyres have an optimum temperature at which they develop maximum grip. ie improve the CoF between road and tyre. This same principle applies in cornering.

So what you're saying there Trevor is the tyres need to be sized suitably to cope with the braking forces generated? That figures.

There are different schools of thought on wet setup as opposed to dry, it is not uncommon on heavy cars ie racing cars with roofs on them,to not change bias from wet to dry, you are going slower, brake pedal effort is less, weight transfer is less.

Nevertheless, if you are still driving on the (reduced) limit but the weight transfer is less, won't this mean the fronts will be set up to do too much of the work thus causing premature front end lock up?

Now look at which should lock first front or rear.

When wheels lock they lose directional stability, that is why you cant steer with front wheels locked- you just go straight ahead.

Now if your rear wheels lock first they have lost directional stability and the car will spin or in the least be very unstable.

Try this, disconnect your front brakes and do a hard application of the brakes at say 150Km/h to lock the back wheels then simulate turning into a corner, if you already have not spun!!

Fair enough, but what happens if you disconnect the rears and try and turn in with the front wheels locked?

(I did that, literally, at a race meeting at Teretonga way back in the late sixties. In a FF type single seater when we had issues with both rear calipers, so disconnected them. First time at Teretonga, hadn't done any laps, came into one slow corner on the first lap of qualifying miles too fast. Wound up with a bent car and six weeks in hospital.)

At least when you're spinning you're converting forward momentum into rotational momentum with the chance of losing a lot of energy/speed before you hit something. Sledging straight off may not do that and a high speed head on in a rear engined car is not much fun for the driver

Its all too complicated for me, I just fitted the biggest discs I could inside the 15" rims, decided on 4 pot calipers, got suitably sized master cylinders. Everything the same front and rear, and have wound up with a 50/50 split in the dry. Had I had to move the balance bar too far away from the central position, I would have simply changed one of the master cylinders. Either smaller or larger for one of them, depending on which way I wanted the pedal pressure to go. And suitable master cylinders are cheap.


Ron,

It's probably horses for courses, I prefer a car that is 'slightly twitchy' under braking, you can make it use up a bit more road in the braking area and that can often psych out any drivers who might be thinking of doing a demon run up the inside.......

But I bet in racing conditions if we were in identical cars there would be nothing between your setup and mine. The differences we are talking are miniscule. I think it's just driver preference.

Trail braking works for me, its the way I like to set a car up into the corner, it's probably a matter of picking the right line to suit the handling/braking characteristics of the slightly different setups.

An interesting thread with some really good information, but after noting that he had seen many of the calculations, Steve's original question was regarding the percentage split for a GT40. "How did you do it? What brake effort split resulted." Maybe Steve is looking for some real life results. He's got mine. Who else is going to weigh in with their setup?

 

[Trevor Booth]

Russ,
size your tyres for cornering/handling is the norm, wheel lockup is just one of the limits of the vehicle. Most cars are overbraked to some degree, demonstrated by the fact that you CAN lock the wheels. Being overbraked is better than being underbraked, however if your brakes are too large they may not get to operating temp and have less braking force than a setup which is slightly smaller and reaches operating temp.

What also affects the size of the contact patch is camber change , mass transfer, spring rates,fuel load, aero dynamic downforce, gradient of roll axis etc etc

In the wet with dry bias setup it is not necessarily so that premature front brake lockup will occur , it is all relative to the balance of the car in the first instance.

The late Peter Brock changed nothing between dry to wet and he wasnt the only one, but some drivers changed every conceivable thing that could be changed!

If the rear is disconnected and the front wheels are locked - sure you will go straight ahead.

If you are a trail braker, then locking rear wheels as you dive down the inside of someone would not be a good thing and if you have a setup which produces a bit of rear end wander "twitchy" you need more room to pass!!

Driver ability to control the braking within the limitation of the vehicle is the ART.

In terms of bias it should be set so you get MAXIMUM braking front and rear.

Now maximum braking does not mean lock up, it means when lockup is impending.

I.E. Too much front bias and you are not getting maximum at the rear and vice versa.

Physical trials at the same speed and with the same pedal effort then adjusting bias so that front locks up and then winding back till rears lockup and then split the difference will be very close. Fine adjustments to suit driver preference are then easily made.

I might suggest that the difference between front bias setup as opposed to rear bias setup is a little more than miniscule.

Driving on the limit with a front bias setup produces a more easily controlled vehicle and quicker.

If you can get the difference down to say 1-2% you will be getting maximum braking.

once you have got some miles under your belt, the handling how you want it start playing with bias. Also get some temp crayon or temp tabs and see what temp your brakes are running.

In relation to the original question and starting from scratch I would do the calcs working back from the road/tyre CoF at 0.5g. It will be a good starting point.