Tyre Sizes

[CaterBram]

On my 1990 Super 7 Sprint Crossflow, In the handbook it details 185/70 R13's.

 

I'm looking at new tyres which will be selected from the List 1A selection.

 

However fitted on the car at the moment it has 185/70 R13's on the back and 185/60 R13's on the front, has anyone any idea why it would be set up like this ?

 

Does it improve breaking / Cornering or is it just wrong.

 

 

 

Newbie Owner of Q469 WET Caterham 1.6 Crossflow in Ali except for the red bits.

 

Edited by - CaterBram Jnr on 7 Sep 2009 07:06:06

[James_Russell]

CaterBram Jnr,

do you have 15mm of rake (body ride height 15mm higher at back than at front)?

caterhams are supposed to be set up this way and the miss-matched tyres may be a short-cut used by a previous owner to acheive this?

-James

[CaterBram]

I'm not sure if I have or not, however with the car I've been given a "Race" set of 185/60's as well.

 

I'll have to check what the rake angle is with the current road tyres as it could be that with the different front / back profiles that i've actually got too much rake.

 

Newbie Owner of Q469 WET Caterham 1.6 Crossflow in Ali except for the red bits.

[James_Russell]

If the 70 profile tyres are on the rear at the moment, surely that means with the 60's all round you'd have too little rake?

By the way I may be placing far too much emphasis on the whole "larger rear tyres to improve rake" hypothesis, it may be something else entirely, or as you say, just wrong

 

1999 VX 8v (Q41 ACF)

[CaterBram]

I think the car has mainly run on 60's as it's done most of it's mileage either on a sprint / hill climb circuit or on the back of a trailer.

 

So with the "Race" 60's on the rake was probably correct but now it's got it's road tyre's back on it's probably got too much rake on, i'll have to have a measure when it's on the flat.

 

(The car's only done around 200 miles per year for the last 7 years, majority of that is either on the track or the return trip to the MOT testing station)

 

Newbie Owner of Q469 WET Caterham 1.6 Crossflow in Ali except for the red bits.

[CaterBram]

Well just been on the floor measuring the rake angle (finally)

 

The measurement from the underside of the sump to the floor is 85mm

The Measurement from the lowest point of the rear frame to the floor is 115mm.

 

So if i'm understanding it right i've currently got 30mm of rake.

 

The OD difference between 185/60 and 185/70 is 37mm which would make 18.5mm difference to the wheel centre height and about 15mm to the measurement at the sump.

 

So if I change all the tyres to the same profile I should come back to a rack angle of about 15mm, so it's either 185/60 or 185/70's all way round.

 

So it's either Yokohama A539's in the 185/60 or

Yokohama S306 in the 185/70.

 

I will be using the car for both pleasure and sprinting so i'll need to go for a list 1A tyre, so i'm unsure which way to go,

I don't think that there will be much handling difference however it will effect the speedo if I go to 185/60's

 

Has anyone used the Yokohama S306's ?

 

Newbie Owner of Q469 WET Caterham 1.6 Crossflow in Ali except for the red bits.

[Stationary M25 Traveller]

No !! you don't compare gap under sump to rear frame.

 

Measure from under side skin just behind front wheels, and under side skin at front of rear wings.

 

edit ... do a search on 'rake' in Tech Talk .... plenty of reading material there !

 

 

 

Edited by - Stationary M25 Traveller on 17 Sep 2009 13:41:13

[CaterBram]

Now I understand, i'll measure again, must be getting mixed up between the threads on ground clearance and the ones on rake :(

 

Newbie Owner of Q469 WET Caterham 1.6 Crossflow in Ali except for the red bits.

[Stationary M25 Traveller]

Checked mine after 2 years ownership, with our weight in concrete blocks in the car to simulate driver and passenger (that was a shock !!). Found it was 10mm low at the rear, instead of 15mm high.

 

Don't forget to load the car with the weight of the driver (if that is how you want it set up), or sit in the seat while someone else measures.