Tyre pressure dilemma - CR500 no grip

[jradley]

Hi,

 

A bit of background info - car is a 1.8SS with 15" CR500's. Only had the car since late last year and throughout winter on cold/damp roads treated the throttle with utmost respect to prevent any end swapping and have never had any issue. Until now....

 

Now that I have more confidence and have better weather I have been testing the boundaries somewhat, only to find they are nowhere near where I thought they'd be. Even with an open diff, on roundabouts at slow speed or even just pulling out of side roads the lightest use of throttle and the back end wants to get home before the front...

 

I noticed the rear tyres were looking a bit soft so checked the pressures all round - both fronts were 22psi (no issues with grip at the front at all, though that would seem a little high perhaps ?) and the rears were only 12psi, which I think is really rather soft.

 

I understand the rationale that more air = less grip, but is it possible the rears were so soft the sidewalls would deform under cornering forces and cause them to lose grip more easily ? I've put 18psi in them now but am wondering if they will be even more skittish when I'm next out or will they be better now ? Any thoughts ?

 

Cheers,

 

John

 

[Pat the Plumber]

18 psi cold all round.

 

my R400

[naddy]

I run 18psi all-around and I understand this is a fairly common pressure to use.

 

I'm no suspension guru, but I think corner weights/rake/anti roll bar settings could also be factors. Do you have adjustable platforms?

 

Those who know more about this will probally comment but if you gave a bit more info that would be useful I expect Your problems may not be entirely TP related but 22 front and 12 rear sems like an awful lot of difference to me - let us know how it feels with them all equal...

[jradley]

Thanks for the info so far.

 

Naddy, I don't have adjustable paltforms and I wouldn't have a clue how any of the other stuff is set up. I should have perhaps mentioned it is DeDion though.

 

I am going to Dunsfold on May 30th so may get to try different pressures in a safe environment but still undecided if i'll put my spare set of 13" A538's on for that day or not, so may not necessarily get to try out the CR500's there.

 

Cheers,

 

John

[naddy]

Well I would certainly try the CR500s at 18 all 'round straight away. If you are still finding the car is trying to spin around when you pull away from a give-way then I'm not sure A539s are the answer.

 

Anyhow, I'd have thought you would just spin one wheel, in a relatively harmless maner, if you have an open diff

 

Is there much tread left on these CR500s?

[Tim Smith]

I have run CR500s for the last 7 years on road and track and like them a lot as a compromise tyre. 12psi is far too low, the sidewalls will be flexing lots, my car fels a bit wobbly at 16psi so 12 must be awful.

 

As stated previously 18psi cold all round is a much better starting point.

 

Tim

[Curly]

I use 16 at front 18 psi at rear - SV on 15"

 

Also how good is your pressure gauge - I have two that give very different results - once you get a happy pressure stick to the same gauge

[jradley]

Thanks everyone - weather depending I may get out on Thursday night and try with the new pressures, failing that not sure when I'll next get chance to blat.

 

Interesting point about the gauge - I don't really have anything to calibrate it against but it was consistent and seems to be about right.

 

Naddy, the logic of using the A539's at Dunsfold was primarily to keep the CR500's in useable condition for touring later this year and also they're cheaper and on scruffy track day wheels, so scuffing the living daylights out of them won't make me wince quite so much as stripping rubber of the CR500's and dinging up my nice wheels.

 

Cheers,

 

John

[Cookie Monster]

You may be really suprised by how any slight variance in pressure makes a big difference to the handling. 12psi is way too low & like the other I run at 18 which seems about right IME.

 

Phil.

[naddy]

ahhh, I understand now, a cunning plan! 😬

[The Village Idiot]

During colder weather road work I run my cr500's at 20 psi, when warmer/track run them at 18 psi cold.

 

12 psi they would have been not good - very slippery.

 

I believe CR500's are temperature sensitive and even on a normal road blat the grip levels change alot as you use the tyre and it heats up.

 

On track this is pronounced and the first lap or two the tyre will not offer the grip it can when fully warm

 

Dicks

[GraemeP]

How old are the tyres? If they have been through many heat cycles and are old they may have "gone off"??

 

I have a set of CR500s on mine at present (13"), and the car was flat floored at Freestyle in January. It is now much more predictable, but when cold it slides around a lot (18psi all round) - once there is a bit of heat they are better - but that is part of the reason I wanted a Caterham, something lively that gives an exciting drive without having to be doing gallactic speeds!

 

I have invested in a set of A038s which I will be switching over in a few weeks.