Yet another tyre pressure question

[Bewls]

Within a few hours of picking up my first Caterham a few weeks back, I fitted a new set of tyres, a set of 185 60 R13 Yokohama A048s in medium compound.

I asked the garage to put about 19PSI in them but they thought this seems a bit low, but claimed they put 19 in anyway. The car felt a bit "loose" even coming from Eagle F1s so I pulled into a petrol station to use their air pump. They pressures seems just over 20, but all were slightly different, so I let some out of all of them to what I thought was about 17-18PSI, but it was hard to tell due to the poor scale. After this the car seemed much better and after a hundred or so miles of running the tyres in they gripped really really well.

 

Yesterday, I checked them on my digital tyre pressure gauge at home and they were all 11-12 !

I then pumped them all up to 17PSI all round and headed of towards snake pass. But now the car seems "pointy" and a lot more twitchy. Also if I was not smooth when changing up to second gear both rears wheels would start spinning, even in a straight line ❗

 

The car is VERY stiffly set up at the moment, I would guess that this doesn't help, but has anyone got any advice on what PSI I should be running?

 

The car in question is a 155bhp live axle cross flow with a Quaife ATB LSD

 

Oh, and all the above tyre pressure were measured cold.

 

Thanks,

Joe

 

 

Edited by - joseph on 24 May 2005 13:28:45

[Terry Field]

Joe, I'm running 021's in a 1700 XFlow (145bhp) and 18psi is just about right for road use.

 

Terry

 

1700XF - Team Lotus with flares.

[Shaun_E]

When I moved from A021Rs to A048Rs the car became much more of a handful. The sidewalls of the A048R are quite stiff and make the ride much bumpier. I got so fed up with the car trying to spit me off the road that I had the bump steer measured and dialled out and at the same time gave the car a bit of toe-in. The toe-in has transformed the car on the road and I haven't noticed any big downside on track. You could get this checked - Caterhams are often set up with the wheels parallel or even with a bit of toe-out to give excellent turn in on track but this can translate to tramlining and handling issues on bumpy roads

I put about 17psi cold in my 48s and this seems to work fine on road and track giving me about 20-21psi when hot. Some people have mentioned that they are very sensitive to pressure so you could try reducing the pressure slightly (0.5 psi at a time) but if you go too low the tyres could move around on the wheels giving you a different set of problems.

My car is also set up quite stiffly with a greeen (18mm) ARB on the front. I plan to move to an adjustable ARB and a Freestyle spring and damper package as soon as possible.

 

 

 

Yellow SL #32

 

Edited by - Shaun_E on 24 May 2005 14:23:46

[Bewls]

Other than tyre wear, what are the problems of running to low?

When I was running 12psi the car seemed much better than it is now, I know it's not a good thing for them to be that low, but why? They never got THAT hot when the pressure was that low.

 

Joe

[Shaun_E]

With really low tyre pressures (and I don't know how low) the tyre could flex rather a lot and "squirm" on the wheel. This would make the handling a bit vague - the front might not turn in very positively and the back could move around a bit - would feel a bit like lots of roll on the suspension I guess. In the extreme I suppose the tyre could even come off the rim although I doubt that would happen. On a hot dry day or on track they would almost certainly overheat.

Most people seem happy with around 17psi cold but as I said, you could try lowering a bit at a time until you find a reasonable compromise. I guess you need to look at the rest of the setup on the car. I would definitely get your toe-in checked as that could make all the difference.

 

Yellow SL #32

[Normans_Ghost]

I had my 48's at 18psi and were fine once I'd sorted the front suspension and REMOVED the rear ARB.

 

I think nearly everone runs at 18psi.

 

Joe, 12 is too low. You'll get the tyre too hot on long runs and the side walls will "melt" - end of tyre.

 

However, are you sure the gauges are correct?

 

Norman Verona, 1989 BDR 220bhp, Reg: B16BDR, Mem No 2166, the full story here

[Bewls]

Norman, I know 12psi was FAR to low which is why I changed to 17psi, its just it seems alot more "skippy". I checked the tyre pressure with someone else’s gauge and it confirms that it was correct.

 

I left am exam yesterday morning, feeling stressed, looked outside, blue sky, so completely spare of the moment decision me and a friend drove down to Nottingham and picked up my car to take it for a spin. 11 hours later in the small hours of this morning we got back to Nottingham and tucked it up in the garage.

 

I now have a big pile of receipts for super unleaded! Worth every penny though.

 

I gave you a call yesterday, I was going to see if you wanted to take it for a spin over snake to see what you thought, but I couldn’t get hold of you. I'm sure there will be another time though

 

Car was great all day though, took some friends out for spin, they all thought it was great

 

My next technical question is how do you stop the face cramp from grinning like this 😬 all day?

 

Joe