RazMan
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| posted on 22/5/10 at 10:54 PM |
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Tyre Profile Change
I have been running 245/35/17 rear tyres for the last 13K miles or so but noticed the other day that both tyres had 2" splits in the sidewalls!
This is possibly a legacy of running excessive camber and soft pressures but I decided to re-fit slightly higher profile tyres and went for 245/40/17
to gain a little more comfort in the process - however the handling is completely messed up now. The car 'flops' into corners and high
speed stability has been seriously damaged. I can only put this down to the higher profile rear tyres as nothing else has been changed .... but we are
only talking about 12mm! I lowered the rear ride height to compensate but it still doesn't handle well so I am a tad confused to say the
least.
I will have to put it on the Hunter alignment setup next week to see what has changed.
How can a slightly higher profile tyre change the car so dramatically??
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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craig1410
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| posted on 22/5/10 at 11:47 PM |
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I notice that the width and rim diameter of the tyres is the same so the overall circumference will now be bigger. This will be raising the ride
height of the car (about 12.25mm) and will also be altering the scrub radius at the front. Both of these factors will affect handling.
In my opinion you should have gone for slightly narrower tyres (215/40x17 would be just 1mm different diameter to 245/35x17) to keep the ride height
and scrub the same.
Let me know if you need more info on the above.
Cheers,
Craig.
[Edited on 22/5/2010 by craig1410]
[Edited on 22/5/2010 by craig1410]
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RazMan
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| posted on 22/5/10 at 11:57 PM |
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Hi Craig, Surely the increase in ride height is half of the diameter increase - roughly 12mm. As I lowered the rear shocks by 12mm to compensate for
the extra height of the tyre, wouldn't this cancel out any geometry changes ... or am I missing something?
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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craig1410
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| posted on 23/5/10 at 12:05 AM |
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quote:
Originally posted by RazMan
Hi Craig, Surely the increase in ride height is half of the diameter increase - roughly 12mm. As I lowered the rear shocks by 12mm to compensate for
the extra height of the tyre, wouldn't this cancel out any geometry changes ... or am I missing something?
Yes, sorry mate I just spotted that and edited my post. This is still quite a lot and I'd have gone for a narrower tyre. It won't affect
grip much because the narrower tyre will have more pressure on it and will generate the same grip. As long as you don't overheat the tyre the
extra width of the 245 profile won't be a benefit.
Sorry again, I didn't read your initial post properly. You've only changed the rear tyres so the front scrub won't be changed.
Just thinking more about this - by lowering the ride height you will be (most likely) introducing more negative camber and as you have already
mentioned excessive camber, have you corrected for this excessive excessive camber?
Also, what about tyre pressures? Have you tried playing with tyre pressures?
With your lower ride height, could you be hitting the bump-stops in the corners? This will certainly screw up the handling.
[Edited on 23/5/2010 by craig1410]
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RazMan
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| posted on 23/5/10 at 12:24 AM |
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I am running 8" rims on the rear so 215/40 might look a tad on the small side but I understand your point, however I have bought the tyres now
and just need to make the best of them.
The effect is like the rear toe has been moved, or something else has shifted in the process of changing the tyre profile.
If it helps I am running 1 degree of camber front and rear (was 2 deg), toe in of 0.5mm (F) and 1mm (R) Castor is roughly 2 degrees but I am trying to
increase this to 3.5 degrees. Tyre pressures are 16psi(F) 20psi(R)
Maybe reducing the rear tyre pressure a couple of psi will help, but only a trip to the alignment jig will tell me if the camber has been increased by
the ride height change.
[Edited on 23-5-10 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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craig1410
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| posted on 23/5/10 at 12:30 AM |
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As I mentioned in my last post, try checking your camber on the rear with the lower ride height, it might have changed.
I'd also try dropping tyre pressure to 18psi because a 4psi front to rear difference sounds quite a lot.
Are the tyres the same brand as the ones on the front? Just wondering if the compound is different maybe?
What exactly do you mean by "flop" into the corner? Do you mean mid corner oversteer?
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RazMan
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| posted on 23/5/10 at 12:41 AM |
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The tyres are the same make, Falken 452. The original tyres are 451 but these are now obsolete but the tread pattern is almost identical.
Mid corner oversteer hits the nail on the head - I set the car up for a bend, turn in and find myself having to grip the steering wheel tightly as the
front end tries to tighten the bend on its own accord (although I haven't got the nerve to see what happens yet 
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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craig1410
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| posted on 23/5/10 at 01:03 AM |
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452's are a good tyre in my experience, I used them on my Leon Cupra and I have them on my wife's Altea.
My best guess is that you are running out of damper travel on the rear due to the reduced ride height. I'd try winding the rear up the 12mm and
try again and see what happens.
Another thing to check is rear toe because the reduced ride height might have affected that as well depending on your geometry. That would explain the
instability issue.
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RazMan
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| posted on 23/5/10 at 10:19 PM |
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I have no complaints with the old 451's and they gave good grip with reasonable tyre wear so I'm hopeful that the 452's will be
similar.
I reduced the rear tyre pressure to 18psi and things have improved a little. Actually they were set to 22psi by the tyre shop (their gauge must be
out) so a drop of 4psi in total. Dropping the ride height a little hasn't made the rear shocks bottom out, even with two people on board, but
the camber has increased a little - easily sorted though. I am back to running 120mm (F) 140mm (R) which has always felt 'right'
There is still room for improvement, so an alignment check will be carried out during next week but at least things are a bit more predictable now.
[Edited on 23-5-10 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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craig1410
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| posted on 23/5/10 at 10:49 PM |
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Glad to hear you have made some progress. 22psi certainly seems a bit high.
I don't know what your rear axle weight is but lets say it is 200Kg or 440lbs per corner. At 22psi that would result in a contact patch of 52mm
x 245mm and at 18psi it would be 64mm x 245mm. Having a wide but short contact patch would probably make it even more sensitive to excessive camber.
This calculation isn't exact due to sidewall stiffness supporting some of the load but it shouldn't be far off.
I was out in my Meerkat RV8 tonight, dropping my son at swimming practice and I gave it a blast in the countryside afterwards. I'm starting to
get the confidence to throw it around a lot more now that I've done a few more miles. The handling is really good and it takes some serious
corner speed to get close to the limits. I'm also really impressed with the traction from the de-dion back axle setup. I can flick the tail out
with the throttle but it settles really quickly with some opposite lock and you barely need to back off the throttle. Makes corner exit explosive.
Stick at it, you'll figure it out.
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RazMan
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| posted on 26/5/10 at 06:07 PM |
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Update: I decided to start from scratch so I reset the ride heights and cambers and had another go on the Hunter alignment jig and it seems that the
new tyres had somehow increased the rear toe-in, so that is now corrected. The front toe was also tweaked in a little (it had wandered out) and
finally I can drive with confidence again.
I didn't realise that new tyres would change the geometry that much and with hindsight I should not have checked the geometry with part worn
tyres so a lesson has been learned.
Looking at the printout from the Hunter computer, the only setting that is slightly out is the caster (now at 2.04 degrees), but I think I can live
with that for now - I might have another go when I get round to replacing the front tyres.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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