Which Size Wilwood?

[mbayley77 0]

I cant get my head around a problem I have got.

 

I have 307 hdi brake conversion up front with disc brakes at the rear from a 309. I am running them off an adjustable bias pedal box (with screw type balance bar). This is connected to two wilwood 5/8" cylinders

 

Trouble is I have adjusted the pedal box to full front braking and still the back locks up before the fronts really get going.

 

I have read many topics on this but just cant get my head around it.

 

Do I need a larger or a smaller master cylinder for the front brakes in order to get more breaking force applied to the big stoppers before the rear ones lock?

 

Oh and no servos obviously on this set up.

 

help me please

[Rippthrough 98]

I cant get my head around a problem I have got.

 

I have 307 hdi brake conversion up front with disc brakes at the rear from a 309. I am running them off an adjustable bias pedal box (with screw type balance bar). This is connected to two wilwood 5/8" cylinders

 

Trouble is I have adjusted the pedal box to full front braking and still the back locks up before the fronts really get going.

 

I have read many topics on this but just cant get my head around it.

 

Do I need a larger or a smaller master cylinder for the front brakes in order to get more breaking force applied to the big stoppers before the rear ones lock?

 

Oh and no servos obviously on this set up.

 

help me please

 

 

Smaller for more force.

[C_W 3 1 Cars]

I wouldn't have though the master cylinder would have any effect on the bias and what locks up first, more just the overall pressure / effort?

[Rippthrough 98]

I wouldn't have though the master cylinder would have any effect on the bias and what locks up first, more just the overall pressure / effort?

 

He's running twin cylinders from what I read though - one for front, one for rear.

[C_W 3 1 Cars]

Ah IC.

 

Not sure how that setup works BUT if you put a smaller one on the front wouldn't that mean it requires more travel for the same amount of pressure and therefore even more pressure will directed to the rears?? In that case wouldn't it need a smaller one for the back?

[mbayley77 0]

Yeah I am running twin master cylinders for this!

 

If i am remembering correctly from school (oh god how long a go was that!) that pressure = force / area. So by reducing the size of the master cylinder to the front brakes I am increasing the pressure in the system taking into account that the force will stay the same of course.

 

What size cylinder would i have to use up front then in order to get a reasonable amount of balance adjustment on my breaks without having a huge travelling through the bulk head kind of brake pedal?

[C_W 3 1 Cars]

I'm not familiar with these setups but if you reduce the cylinder size on the fronts this will mean more travel (but less effort) BUT isn't the rear master cylinder going to be fed the same (increased) travel?

[mbayley77 0]

not necessarily. As this is what the balance bar does.

 

Basically imagine the pushing force of the brake pedal as the pivot on a seesaw. then where the peaople sit is the connection to each mater cylinder. By adjusting the balance bar you adjust the pivot point on the seesaw. Hence by making one side longer than the other the force is not dissapated evenly (force x distance = work done).

 

By using the balance bar you adjust the way the force is applied toward the front or the back master cylinder - trouble is I cant move the balance bar any more to get a greater force on the front and hence by the sounds of it need a smaller cylinder on the front.

[C_W 3 1 Cars]

Im not sure, if the adjustment is on max front and still it needs more front bias, fitting a smaller front bias would require more movement on the front cylinder which will make the problem worse? im confused hehe

[mbayley77 0]

me too and without messing around with it in the dark and taking it out for some test braking situations then i am stumped!

[C_W 3 1 Cars]

If that is how it works I'd say the rears need a smaller cylinder? so that most of the effort/travel will be going to the fronts?

[M3Evo 0]

The smaller the MC, the more pressure / unit force will be generated.

 

Wouldn't have a clue where to start with the sizes though, perhaps give Rally Design a bell, they provided MCs for my Wilwood pedal box based on the caliper sizes I supplied them.

 

Out of interest, do the rears lock up only when you stab at the pedal or just any time you apply the brakes?

 

Drove an old astra once with a buggered pressure releif valves on which the rears locked if you stabbed the brake pedal.

 

Anyway, would've thought that with a smaller front MC (or preferably a larger rear so the pedal travel doesn't increase) that the reduced relative pressure to the rears might help the problem?

Edited March 7, 2006 by M3Evo

[mbayley77 0]

The rears lock up all the time. really interesting when yoiur in traffic with a big plume of smoke coming off the rear left as that one locks first hehe.

 

There are no pressure relief valves in my system.

 

So can anyone supply the caliper sizes for 307 hdi front brakes and 205 rear calipers please? Ill give rally design a bell then once i have all the info.

 

Just been out and had a fiddle with the balance bar and something seemed to have moved since i first set it up but i dont think its gonna cure the problem i am having.

 

This might clear some things up for some peoples.

 

Balance bar adjustment

[Rippthrough 98]

The rears lock up all the time. really interesting when yoiur in traffic with a big plume of smoke coming off the rear left as that one locks first hehe.

 

There are no pressure relief valves in my system.

 

So can anyone supply the caliper sizes for 307 hdi front brakes and 205 rear calipers please? Ill give rally design a bell then once i have all the info.

 

Just been out and had a fiddle with the balance bar and something seemed to have moved since i first set it up but i dont think its gonna cure the problem i am having.

 

This might clear some things up for some peoples.

 

Balance bar adjustment

 

 

I think the rears are 30mm, and the fronts should be 54?

 

Not sure on the rears though, that's just from a parts catalogue.

Edited March 8, 2006 by Rippthrough

[jonah 1]

Yes the rear caliper pistons are 30mm.

 

And yes, you would need a smaller bore front master cylinder to increase the pressure to the fronts. The balance bar (as I understand it) apportions the pedal force, not travel, between the two cylinders. The amount of travel of one m/c shouldn't affect the force it receives from the balance bar (unless it's so much that something in the mechanism runs out of movement).

 

But, it sounds to me like something is wrong with the front brakes (or the N/S rear that's locking). Everything in your system is biased towards the front - I've been looking at effects of brake bias in the pinned thread on this forum, and 307 HDi front brakes are about 3.7x stronger than 1.9 GTI rears (assuming same pad material). Plus if you've got the balance bar adjusted for even more front bias then this figure becomes even larger (perhaps double or more, I don't know how much adjustment it provides). There's no way that the rears should be locking first if everything was working properly!