Poor Rear Brakes After Fitting 4 Pots To The Front

[stuart 31 3 Cars]

I've recently fitted some Hi Spec 4 pot calipers to the front but at a test day I went to on Saturday, I couldn't get enough rear bias and just kept locking the fronts (it was really slippery but thought I'd still be able to get the rears to lock if I wanted).

 

I'm using a 23mm master cylinder so travel is about the same as standard and feel is good, just can't get the rears to work well enuogh no matter how far I unscrew the bias valve.

 

The rear pads are Pagid blues but after a 10 min session the discs were almost cool enough to touch, whereas the fronts were normal.

Are they a touch too hard for the rear on a rally car perhaps? Or do I need to look at something else?

The rears were always fine when I had 307 calipers up front but they weighed about as much as the moon.

 

Thanks in advance

 

Stu

[Rob Thomson 6]

By upgrading the front brakes you've reduced the fluid pressure you need to achieve the same front braking force. Therefore less fluid pressure goes to the backs too, but because those haven't been upgraded you're buggered. Options are a bias pedal box, uprated rears (BX front calipers etc) or smaller fronts. You might also try a standard pad in the rears but I doubt you'll see much affect.

[Cameron 16]

What he said.

[B1ack_Mi16 67]

You might try to remove the bias valve totally and see if that helps.

 

If you already don't have a bias valve to reduce pressure for the rears you simply have got front calipers with too large diameter pistons (or of course rear ones with to small..)

[stuart 31 3 Cars]

Thanks for the quick replies, thought I might be looking at a pedal bias box...I do have a set of standard pads though so might try them to see if it helps for now.

 

Next rally is tarmac so hope it'll be ok as long as it's dryish, I was running on worn out drys on a very slippy track this weekend so the fronts probably lost adhesion before they should've done no matter what rear bias I had.

 

If I understand right, if I can put more pressure on the brakes without the fronts locking, then the rears should also get more pressure and be more effective.

[TT205 42]

Have you actually removed the compensator (1.6), compensators (1.9) ?

[stuart 31 3 Cars]

Yes, just have 23mm master cyl, bias valve and hyd handbrake with the 4 pots, 1.9 calipers at the back.

 

I might try bleeding the rears again to make sure I've not got any air somewhere, seemed to be easier than the 1st time I bled them with the handbrake fitted. It was just the fact that both sides were almost cold enough to touch that surprised me.

[Rob Thomson 6]

You'd have a very soft pedal if there's enough air in the system to render the rears inoperable, but it's worth checking to eliminate. Also worth checking that you don't have a pinched line, and that everything's connected correctly (i.e. bias valve isn't back to front or something daft...?)

 

Things should be better on a good surface though. More braking effort means more weight transfer to the front and that means the rears will lock more easily, everything else being equal.

[Dj_mini 7]

I had the same set up (now have a pedal box) and found the balance very good with the 23mm mc i recon you may still have air in the rear as others have said do you feel any difference when you adjust the bias valve?

[jerseypug 1]

i had a nightmare with my rear brakes after fitting a hydroalic handbrake. I had blead them many times and they were still crap, the hydroalic handbrake could not lock the rears at all. in the end i had to remove the pads from the rear, get the pistons right into the calipers, clamp them to stay in and tap the caliper as i blead it. i got loads of small air bubbles out and they worked really well after that. in the end i ended up getting a pedal box but thats another nightmare if it is not set up right. hope that helps

[oonip 4]

By upgrading the front brakes you've reduced the fluid pressure you need to achieve the same front braking force. Therefore less fluid pressure goes to the backs too, but because those haven't been upgraded you're buggered. Options are a bias pedal box, uprated rears (BX front calipers etc) or smaller fronts. You might also try a standard pad in the rears but I doubt you'll see much affect.

 

Unless im missing something, we dont know if you have increased the front braking power? I.E we dont know that the front piston area has increased? BUT if the front piston area has increased considerably then its a logical explanation that your over braking the front or under braking the rear depending how you look at it!

 

However it's a good point about the wieght transfer in the wet being far less and there for meaning the rears will be harder to lock.

[stuart 31 3 Cars]

Yeah the front pad braking power and piston area has definitely gone up, they pull the car up a lot better in general than with the 307 calipers but just couldn't seem to get enough rear bias for the wet.

I think in the dry it will be better, I'll bleed the rears again using the tapping method mentioned and fit some standard pads, I'll just have to manage for now as I can't afford to spend any more on the car yet.

 

Thanks again

 

Stu

[oonip 4]

ok cool, Remember that on the 4 pot you only need to take into account two of the pistons when calculating the piston area compared the single piston sliding caliper, the car shouldnt actually stop any better, but just feel a whole lot nicer on the pedal and offer a massive wieght saving!

 

Cheers

 

Nick

[Rob Thomson 6]

Another advantage of 4-pots is that the effective radius of the disc usually increases a bit too, which also increases the braking effort for a given pressure.