brake bias set up

[bucko]

ok i need some help i got nova rallycross and i seam to be haveing difficulty in setting up the brake bias the type i got is the one with 2 master cylinders for front and rear with a swivel bar that is all adjustable ,then is linked to the brake pedal ,( i hope i make sense ) on setting up what arm into the brake master cylinder needs to be longer or shorter to stop the rear brakes from locking up , we tried both in oppsite ways and it seams to happen all the time , so can anyone help me with a starting point .

[Tim Falce]

You need the bias bar wound more towards the front cylinder for more bias to the front and less rear. The other problem could be master cylinder size. You generally want a smaller cylinder on the front brakes so you get more pressure to the front. Other factors to take into account are brake calliper and/or drum size and how much pedal travel you have or want.

[graham bahr]

Originally Posted by falcemob

You need the bias bar wound more towards the front cylinder for more bias to the front and less rear. The other problem could be master cylinder size. You generally want a smaller cylinder on the front brakes so you get more pressure to the front. Other factors to take into account are brake calliper and/or drum size and how much pedal travel you have or want.

spot on falcemob,

both my own car and andy's escort are on mk2 escort bias pedal boxes but also have a seperate bias limiting valve in the rear circuit as even with a large cylinder for the rear circuit there is still too much effort to the rear brakes on its own.

[bucko]

so on the bias adjustment swivel bar do i move the rear brakes away from the centre and the front towards the centre or the other way around to get the front brakes on 1st,

[Tim Falce]

You want the centre bit nearer the front cylinder, I.E. more thread showing on the front cylinder side. Take a look at this page for more info.

[graham bahr]

or just adjust it one way and push the brake pedal!

you will see which cylinder is being bias'd once the inital slack has been taken out of the system, and if you get it wrong go the other way

[bucko]

thanks for your help

[Goran Malmberg]

The basic braking balance should not be set using the balance bar. The adjusting window is to narrow. The caliper piston area should match the weight distribution of the car under braking so that the balance bar could be left in the middle possition while the front brakes barley locks upp before the rears. If caliper piston size is not easely matched, master cylinder size may be altered.
Then you have full balance bar adjustability left for track tuning.
Goran Malmberg

[Lukin]

I agree with Goran. The whole bias spectrum I have seen is only 4-5%. You need to make sure your close to where you want to be as a datum point.

[Anuauto]

One other cause of the original problem described in the first post can be the two yokes being set too close together on the balance bar so that one binds against the centre part of the pedal when it is pushed and stops the piston going into the master cylinder cleanly. No changes of master cylinder size or fitting of rear limiting valve will cure that (as I know from bitter experience.....).

[Tim Falce]

I did that once and it caused the front brakes to stick on and I had to constantly hook my foot under the pedal to release them.

[Goran Malmberg]

This misstake is easely done as the distance between the pushrods joint become less when the angle of the balance bar change.

The brake system has a few troublesome spot of security. Another one is front caliper braided hoses, that is getting twisting motions from steering if mounted wrong. Then banjo end connections might destruct.
Especially on rallye cars.
Goran Malmberg

[dtype38]

Hi Lukin, and welcome. A good topic that has had me scratching my head more than once

Goran has the right starting place. If you are using the standard manufacturers callipers and discs on your car the correct bias will already be accounted for there. If you have aftermarket ones, you need to check that you've kept to approximately the same ratio of disc diameters and piston areas or you might have a struggle getting enough adjustment on the bias bar/master cylinders.

Anyway, assuming that's right, checking the master cylinder sizes and ratio is next. For this you need an idea of how much movement you get on the front master cylinder. Roughly speaking 25-30mm travel at the cylinder from off to full pedal pressure shouldl give you good pedal leverage without too much travel. If that gives you too much pedal travel try a slightly larger front master cylinder. With that sorted, the rear master cylinder is "usually" one size BIGGER than the front. This will give you less rear pressure and you should notice that the rear cylinder hardly moves compared to the front when you press the brakes. Then set up the start postions with the front piston further out than the rear to give it room to move (see the link Falcemob put up) and remember to have a gap either side of the balance tube to allow for the change in angle.

Get all that right and as Lukin says, you get to do a bit of fine adjustment with the balance bar!

However... don't think that brake bias is the only reason you can suffer from locking rears. If its a completely standard car, then it is likely, but if you've adjusted the suspension, dampers or ride height, then the fault could lie there as well. I had exactly your problem on my own car and spent ages fruitlessly trying to sort it out with every possible combination possible of master cylinder and balance bar position. In the end I started looking at the weight transfer I was getting under braking. It transpired that under heavy braking the car was pitching forward so hard I was virtually lifting the rear wheels off the ground! I could probably have locked them up by holding onto them with my hands . Anyway, I lifted the front ride height a little, dropped the rear a little and hey presto! Problem solved.

Err.. ok that solved the brake problem, but I started losing stearage over 110mph! Oops! So I dropped the front back down a bit and added a bit more spring poundage at the front end to control the dive. Both problems solved.

Now where did I put that text book on anti-dive geometry?

[graham bahr]

interesting, its worth noting that the faster you stop the more the weight transfer and the less braking you can do on the rears,i have done may races with no rear brakes whatso ever, my new car has rear brakes that work and for the whole race rather than just the first three laps, the only difference i notice is that i now wear out rear pads faster! i did one race in a borrowed FWD car that wanted to swap ends every time to hit the middle pedal, greasing the rear linings so put paid to that and from then on it stopped beautifully

[Goran Malmberg]

As for both braking and acceleration, more static rear weight is better.
But, the acceleration traction may be better off with a slight high CGH while the braking do better with lower CGH, not to move so much weight to the front end. I leave cornering discussion out of this right now.
Goran Malmberg

[dtype38]

Couldn't agree more Goran. I rebuilt my car to significantly lower the COG and it stops beautifully. Cant get traction off the line for toffee! Although my team mate tells me that's just because I'm a rubbish driver

[bucko]

blimey i thought this was going to be simple how wrong i was !!!!
looks like i got some work ahead of me before the super prix at croft next weekend .

[Tim Falce]

Originally Posted by bucko

blimey i thought this was going to be simple how wrong i was !!!!
looks like i got some work ahead of me before the super prix at croft next weekend .

It took me four years to get mine all sorted out to how I want it, but that was mainly using races as my test bed. Don't expect to get it right overnight unless you are extremely lucky early on.

Originally Posted by dtype38

Couldn't agree more Goran. I rebuilt my car to significantly lower the COG and it stops beautifully. Cant get traction off the line for toffee! Although my team mate tells me that's just because I'm a rubbish driver

You're certainly not a rubbish driver Just quite slow reacting to the lights, I learnt my lesson when I was on the start line, the green light came on and the rest of the field disappeared to the first corner while I was still changing a CD

[bucko]

thanks for all your help had a good look today with my mate found that the centre spindle was fowling and both front and rear rods where the same distance , think now i know where we are heading and the physics involved , and now got the fronts on 1st then the rears later and a lot less pressure on the rears ,and a fully floating spindle that not fowling , thanks to all who had some imput on this thread . thanks