Avon tyres less stable at speed?

[FatBoy]

Hi,

I've just replaced my Goodyear Billboards with a set of Avon CR6ZZ tyres and the initial feeling was that they are much better at normal road speeds and much less inclined to follow bumps and white lines.
However, over the weekend I took it for a 200 mile drive and a variety of different roads and had the opportunity to stretch its legs a little and I found that the new tyres are less stable than the Goodyears at higher speeds e.g. >100mph.
The Goodyears were always rock solid at higher speeds and I would like to get the same feeling with the Avons if possible.
I had the suspension thoroughly setup to the figures provided on the Kirkham website for the Goodyears, but has anyone found that different settings work better for the Avons.
There isn't any vibration, so balance isn't the problem, but maybe the radial prefer a different toe in/out setting.

Paul

[FatBoy]

Anyone??

[mdross1]

It has been a long time since our car was on the rack but do remember it was set with toe-in and 4 to 4 1/2 degrees pos. caster. Our car has BFG radials it is steady and solid well beyond the triple digits.

[Mark IV]

You are changing from a bias ply tyre to a radial. The radial will be more sensitive to toe in/out so I would look at a good alignment with thrust angle correction.

[Cobra #3170]

Originally Posted by Mark IV

You are changing from a bias ply tyre to a radial. The radial will be more sensitive to toe in/out so I would look at a good alignment with thrust angle correction.

Excellent advice, especially getting the thrust angle correct, you will need more camber too.

[FatBoy]

Originally Posted by Mark IV

You are changing from a bias ply tyre to a radial. The radial will be more sensitive to toe in/out so I would look at a good alignment with thrust angle correction.

Thank you. That, I understand, but I was l hoping for some numbers based on personal experience. The car was REALLY well setup on the Goodyears and I just want to get some of the straight line stability back again.

Paul

[FatBoy]

Originally Posted by Cobra #3170

Excellent advice, especially getting the thrust angle correct, you will need more camber too.

Do you have any suggestion as to how much more camber?

Thanks,
Paul

[Cobra #3170]

Originally Posted by FatBoy

Do you have any suggestion as to how much more camber?

Thanks,
Paul

I would probably try -2.0 front and -1.5 rear as a start, watch inside tire wear and if is excessive go to -1.5 and -1.0. I run -3.5 and -2.5 and get even wear but the tires are seeing corner loading the majority of time.

[DanEC]

Wow - that's a lot of negative camber! Is that for the track or street?

[Chilibit]

You did not mention tire pressures. What are running the Avons at cold? I don't a Kirkham so I hesitate to suggest an optimum pressure.

[Cobra #3170]

Originally Posted by DanEC

Wow - that's a lot of negative camber! Is that for the track or street?

Yes it is, but a Kirkham is very much like an original and has very little camber gain in bump, as a result you need to run more static camber to get steering response and cornering capacity. If the car is used in "spirited" driving you will get even tire wear, if it is driven on freeways or with very little cornering load you will get inside shoulder wear. That is why it is best to drive it for awhile and adjust camber for relatively even tire wear. It all depends on the owners driving habits, that set up will give optimum handling on the street but maybe not optimum tire wear. Radials need more camber than bias plys because of the construction and can still have a relatively even foot print with 1 degree of negative camber, adding cornering load increases the camber requirement especially with stock Cobra suspension. The only way to know is to try it, I can't imagine a Cobra owner who wouldn't hit a round about hard when the opportunity presented itself, otherwise why have a Cobra? Getting perfect tire wear is like getting good gas mileage who cares?

[Bobcat]

Bruce ... speak about timing on the Avon subject . Just got an answer from the Technical Manager of Avon Tyres Motorsport/UK about using the CR6ZZ tires on the track ( I do VIR ) . Common thread for both of you is that radial tires want/need a lot more negative camber than most people are running , or are comfortable with . For example , the Hoosier R6`s a friend ran in SVRA , needed a minimum of 2 1/2 to 3 degrees negative and higher pressure than most others run . Realizing that there is no silver bullet and not all cars and drivers like the same thing , Avon suggested ( as you did ) that , a minimum of 2 degrees negative camber . They were comfortable with tire pressures of 32 to 34 psi hot in front and 30 to 32 psi hot in the rear , depending on the balance of the car . Monitoring tire temps will then be your guide . The range I was given was to shoot for was a delta of 10 to 15 degrees C with the inside warmer .
I also run 1/16" toe in per side on the front and 3/32" toe in per side for the rear for track and street .
I spend more time on winding roads and the track than Interstates and if I get 4000 miles to a set of tires , I`m happy .

[FatBoy]

This is all really good useful information.

I'll spend some time over the coming weekend playing with the settings. I don't suppose anyone knows what the effect of adjusting the steering arms by one full turn has on the toe-in on a Kirkham do you? Also, the change on camber by turning in the upper heim joints by one full turn?

Thank you,
Paul

[DonC]

Don't ignore caster angle when you are aligning after changing to radials. It goes a long way in controlling how the car tracks.
DonC

[Cobra #3170]

Originally Posted by FatBoy

This is all really good useful information.

I'll spend some time over the coming weekend playing with the settings. I don't suppose anyone knows what the effect of adjusting the steering arms by one full turn has on the toe-in on a Kirkham do you? Also, the change on camber by turning in the upper heim joints by one full turn?

Thank you,
Paul

I have modified suspension so my information won't help you but you are right on the money, knowing the effect of increasing camber on toe would be very good IE one complete turn increases camber by -1 degree and requires 1/2 turn on the tie rod to correct toe. I made up a chart for both the front and rear of the car so that I can make adjustments in the field and maintain reasonable alignment settings. Changing caster will effect bump steer, ackerman and toe but it is well worth experimenting with. Changing toe at the rear will effect upright inclination which will effect bump steer also so you need to adjust the inner link fore and aft till you get correct toe with the bump steer you want. That can vary from toe out in bump (tight autox) to 0 (street/ road course) to toe in for stability (street).

[mickmate]

Good info from Bruce but remember his is a track cornering type set up. He knows more about these suspensions than anyone I've talked to.
Rear toe also is adjustable at the rear lower inner which has much less effect on the other setting like changing radius rod lengths does.
I think Paul my first adjustment for speed stability would be more caster, especially if the steering feels pretty light at low speeds with the radials.
Check the pitch on the threads, use a ratio to make it closer to a true measurement. A good method is adjusting an upper ball joint and simply measuring between pivot points. Translate it into an angle change rather than a distance using a ratio. Best way to do that initially is on an alignment machine and noting the change. Of course that's what you were asking for initially, it's a matter of whether anyone has done this and also remembered it or written it down. When I was racing I had my own code on the decamber plate graduations so no one could read my numbers.