tyre size problem?

[Mike Biddle]

Since rebuilding the car in 2005 I have had problems with fronts locking up too easily under braking.

 

This occured both with the initial standard wide track and the current Freestyle ultrawide inboard set set up. Currently Avo shocks all round on the freestyle system.

 

Now these are the same front brakes that were on the car prior to rebuild (Hi Spec ultra light racing four on 260MM solid discs) and then I did not have the problem, in fact the braking was superb.

 

So, I suspect that I have too much tyre on the fronts, (CR 500, 205/55 R13) because before rebuild a narrower ACB10 (7.0/21.0-130 was utilized.

The Cr500 has a tread width of 186MM, and the ACB 10 is 175MM.

 

BUT that does not seem to be a huge difference, so am I correct? What other factors could be in play here? Problem occurs with both suspension types, so unlikely to be down to that.

 

The rears have also changed, they are now the ventilated disc from the front of the Caterham big brake kit with Hi Spec handbrake caliper.

 

Standard master cylinder, and Pagid pads all round.

 

[paul richards]

Too much tread at the front? - I think not, I'd suggest that if you had less grip at the front there would be more likelihood of locking up.

I would suggest that whatever you have changed you have altered the balance of the brakes from back to front. If you've not changed the front and have changed the back, my thought would be that the back brakes are not as effective as previously.

What have you changed at the rear? It could simply be that the pads need to be "grippier", or it could simply be that the rear brakes are not yet bedded in.

 

Paul Richards

Area Representative - L.A.D.S. (Lancashire and District Sevens)

LADS Website

Growing old is compulsory - Growing up is optional

[Mike Biddle]

Thanks Paul, rear changes as follows.

 

Initially, nothing, continued to run the standard set up then changed to bigger brakes at rear which significantly improved overal braking but did nothing for fronts locking up far too easily.

 

Several thousand miles on rear brakes now including a two week tour in and around the Alps where I can assure you they were given a right good bedding in!

 

Edited by - Mike Biddle on 20 Oct 2009 15:21:43

[Mike Molloy]

What change in front camber angles did you make between the x-ply ACBs and the radial CR500s?

[Jason Plato]

The rears have also changed, they are now the ventilated disc from the front of the Caterham big brake kit with Hi Spec handbrake caliper

 

rear brakes too cool.

 

I would try upgrading the pads on the rear to a more agressive compound or fitting non ventilated disks again.

 

 

here is my Duratec R .... C7 TOP

Taffia AO

[paul richards]

Dave

Sounds like good advice

 

Paul Richards

Area Representative - L.A.D.S. (Lancashire and District Sevens)

LADS Website

Growing old is compulsory - Growing up is optional

[Mavic82]

First, it sounds like you have changed the front geometry in some way, perhaps having more anti -dive or changed spring rates and therefore have less weight transfer to the front. The wider tyres within reason should help

 

Second it sounds like you are now way over braked on the rears. Brake bias valve should sort the balance out to ensure the brake threshold is acheived on the fronts only slightly before the rear to ensure stability on the road.

 

Incidently, why go for vented discs on the rear, I can't believe solid discs were ever being worked to very high temp?

 

Best of luck

Guy

[Mike Biddle]

Thanks guys.

 

Guy, don't think I'm overbraked on the rear yet, otherwise the rears would be locking up first. I do have a bias valve in the rear brake circuit for tuning that out if I can ever get to that situation.

 

Dave, possible I suppose. We have an infrared surface temp measuring device somewhere, I'll utilise that first before I go back to solid discs. I changed to ventilated because the standard rears always overheated, resulting in brake fade, and blue discs whereas the fronts, while getting resonably hot, never did overheat. I reasoned therefore that vented rears made more sense than vented fronts.

 

For the rears, I think the best course of action would be to fit a bigger, four pot caliper and the softer pagids, then see if the rears will lock up, and compensate with the bias valve.

 

That will maximise rear braking but will do nothing for the fronts, which really are locking up when they just should not. I used to be able to brake untill they squealed a bit, and keep em squealing throughout the braking period, they were very progressive and controlable.

 

Mike, 2.5 degrees of camber with the Cr500's as opposed to 1.5 degrees previously with the ACB 10's.

 

What about shocks, would a better quality set up there improve things? Nitrons maybe?

 

 

 

 

Edited by - Mike Biddle on 21 Oct 2009 07:11:25

[Richard Price]

Mike,

Is the bias always too far to the front, hot or cold? If so, then this is purely a balance issue!

It sounds very much at though you simply need more hydraulic pressure to the rears than you do to the front! You could experiment swapping to calipers using a greater piston area, but its all a bit hit and miss!

Anyone who starts to fit non standard braking parts risks upsetting the balance! Fitting standard facory parts is tried and tested, but with non standard parts you need to be able to adjust the bias.

Twin master cylinders and an adjustable bias bar is the answer. If you run out of adjustment, you simply replace one of the master cylinders for one with a different bore.

I've recently modified my own pedal box to do just this. It's suprising how much more stopping power you have when your balance is right. You really can work the rears very hard!

 

 

 

Just beware of winding it a touch too far to the rear 😳 😳

[Mike Biddle]

Yes Richard, thats what I've been trying to acheive.

 

I still cannot make the rears lock up first, and then adjust the bias valve in the rear circuit to tone them down a bit.

 

However, I don't beleive it is simply a balance problem with the fronts because they sometimes lock up under quite normal braking, as if there is naff all grip, and it's usually one or the other, never both . There is something else going on here.

 

Standard Caterham rears are sh*te, thats why I changed them, they overheat far too easily and then it's big time brake fade.

[Jason Plato]

(stand to be corrected) As I understand the hispec calipers do have an issue where the pistons are equal sized and hence dont draw the pad onto the disk in a taper fashion. If this is occurs with high co efficient pads could this be a cause ?.

 

I would remove the rear porportion / reduction value and try from basics again.

 

 

here is my Duratec R .... C7 TOP

Taffia AO

[millsn]

I suppose you've confirmed that the discs run true in the pads e.g wheel bearings are ok and discs aren't warped? If the disc ran closer at one point in its revolution that might make it lock up? Also, that your tyres are in good condition and not distorted themselves?