Re: Tyres
alexGS said:
So, new vs. budget, I think we all agree. I have been able to tell a difference in the past when I've put on branded tyres (I like Barum tyres myself.)
Problem here is there are budget tyres, and then there are cheap tyres (if you get my drift).
For normal day to day driving, most reputable budget brands are quite sufficient. It's when you go to the really cheap tyres (often made in China) that it starts getting a bit iffy. Then again, the Chinese are moving upwards in leaps and bounds, so even some of the 'Cheng Shin' stuff is actually ok! Not like many years ago when I fitted a Cheng Shin tyre onto my motorbike and it didn't seem to grip at all...
Even remoulds are better than they used to be. It's very much a question of finding a budget tyre that is good.
For more performance orientated cars, reputable brands are preferred because the car will push the tyre much harder than a non performance version.
alexGS said:
I'm getting nervous now thinking about my 195/45s going onto my Turbo. Chas and Morten accounts are both consistent with what I've read before.
195's should be ok - they aren't a great increase over the original 175's. However, the very low profile might cause a problem.
alexGS said:
Maybe I should lower my tyre pressures to ensure a reasonable length contact patch?
Not recommended. Always stick to the pressures recommended for the tyre and wheel combination. Going outside of these will cause excessive wear over the tyre depending on whether it is over or under inflated.
alexGS said:
Yet, why do manufacturers tend to specify *higher* pressures for the wider tyres? (check your Uno handbook) and everyone pumps tyres up to the max (e.g. 40psi) when they go racing, for maximum grip. I have to admit that I'm still a bit confused as to what works best.
Wider tyres tend to be lower profile. That means the side walls are reduced, and there is less compliancy compared with a higher profile. It's kind of the same philosphy with suspension spring lowering. If you reduce height, then you have to make the spring harder to compensate for the lack of travel. The same kind of applies to tyre walls. The lower the profile, the stronger the side walls have to be so they tend to need higher pressures. They don't cushion as well as higher profiles.
With racing, higher pressures are used to stop the tyre flexing under hard cornering, not for increased grip! Higher pressures make the tyre more rigid, so less likely to distort and therefore be more stable when cornering.
Drag racing on the other hand will often lower the pressures for more grip. You don't see a drag car with low profiles! Then again, they ARE only going in a straight line
alexGS said:
Also, I think that there is a 'feel' issue involved as well. Narrow 135 tyres on the Uno may work well (no dangerous aquaplaning) but it feels like driving on ice skates compared with the stability of my Alfa's 205/60s. And as Chas knows, there is a limit to the cornering grip as the rather high profile deforms.
I think the grip problem on my Uno is down to crap tyres rather than the profiles to be honest!
And don't forget that your Alfa is a bigger and heavier car, so can cope better than a Uno with wider tyres.
alexGS said:
Yet, someone told me recently that my 15" tyres wouldn't grip as well as 13"s would on my Uno Turbo. And, to back this up, at a recent race meeting I did the timing for, I just happened to note that the fastest Unos were running on 13"s. Plus of course, Formula 1 cars don't use low-profile tyres (do you think they could afford them?) It's all very worrying. Maybe I should have started another thread
There are many variables that need to be taken into account. One is of course the weight of the vehicle. The lighter the car, the less benefit low profiles will have. As the Uno is a light car, fitting ultra low profiles is probably making the tyres too stiff to work effectively with the suspension and the overall weight of the car. This might explain why Uno's running on 13's ended up being quicker than those on bigger wheels/ lower profiles.
As to F1 cars, they are VERY light plus have hardly any suspension travel. The tyres are used as shock absorbers as much as they are for gripping. Ultra low profile tyres would probably bounce an F1 car all over the place, rather than higher profiles that would soak up the bumps and still allow the tyre to stay in contact with the track.
There is a LOT involved in deciding what tyre/ width/ profile combination is best for a particular car. I only know the basics, but there are scientic formulas for working it out properly that I have to confess are way beyond me.
But the problem these days is that people want to fit big wheels more for looks than anything else. 13" wheels are still the best size for some cars that originally came with them, though going up to 14" shouldn't be too much of a problem. Going 2" or more oversize with much lower profile tyres is likely to cause problems unless the suspension is also modified to suit.
The one good thing for me sticking with 13" wheels (both steels and alloys) is that as everyone wants to upsize, second hand 13" wheels and also new 13" tyres are dirt cheap! And they work really well on a Uno.
The secret is out
Chas