Tyre situation, Need help.

Hellmun · Postby **Hellmun** » Tue Apr 01, 2008 6:46 pm

Adam, How does a stiffer sidewall make a wheel spin more easily? Maybe those semi's aren't as good in the cold as the R888's which are known to be fantastic all round tyre, just a little slower on the track when its hot, but they last longer.

If a tyre deforms under pressure it will form a larger flat surface. If the sidewall is too stiff it resists the deformation and you get a small contact patch. Watch a drag-car take off from a side profile and it'll probaly make more sense.

Remember stiff sidewalls are only for cornering grip, it's a negative for acceleration and deceleration. Compromise as everything is in cars. Nothing does everything the best.

Alex · Postby **Alex** » Tue Apr 01, 2008 6:50 pm

Patty wrote:Force(Grip)=coefficientOfFriction(COF)*NormalForce(=mass*gravity). Your COF remains constant since it's a property between the material of your tyre and of the road, and NOT related to contact patch. This means that your grip here is not related to tyre width but to the mass of your car, material of your tyre and the type of road Thats why RWD cars should be harder than FWD to wheelspin.

If wider tyres increases contact patch (which [Will] says doesn't) it would probably only work against you because it equates to lower force per area or less equivalent mass on you wheels.

not quite, tyres can't be looked at this way because of how soft they are, unless they are on a totally flat surface
tyre's will deform to a certain extent , so there is also shear force involved not just friction

wider tyres have a larger contact patch hence the area that is in shear is increased resulting in more overall force

jules · Postby **jules** » Tue Apr 01, 2008 9:54 pm

Adam_NAclubman wrote:oops

titter...

Jules

marcusus · Postby **marcusus** » Tue Apr 01, 2008 10:24 pm

Sasso wrote:I think your flatspotginity is gone.

Might have a looksee at the car if I get a chance to jack it up. Not sure whether that's exactly the case, but it wouldn't surprise me considering I first noticed it when I was at Eastern Creek ages back.

rjastra2 · Postby **rjastra2** » Tue Apr 01, 2008 10:45 pm

Ahhh, remember the old days when 5.7L Ford GTHOs were shod with 185/70x14 tyres

Patty · Postby **Patty** » Tue Apr 01, 2008 11:08 pm

SM wrote:not quite, tyres can't be looked at this way because of how soft they are, unless they are on a totally flat surface
tyre's will deform to a certain extent , so there is also shear force involved not just friction

wider tyres have a larger contact patch hence the area that is in shear is increased resulting in more overall force

Yeah thanks SM. I retract my previous statement re:tyre width. Did some more reading :oops:

Basically what I was saying before is that contact patch itself does not directly translate to friction if it was a pure friction application. The article in that link says that it's not pure friction because the tyres can become sticky so it's gripping the road by adhesion (you called it shear force) and basically giving a COF>1. Therefore more contact patch = more grip. Aah, enlightenment.

Sasso · Postby **Sasso** » Tue Apr 01, 2008 11:48 pm

Yar the tyre basically melts to glue itself to the road - thats why semi's don't last very long on the track but can on the road (they stay cold). And they heat up alot which is why you need less cold pressure. Street tyres are hard and when they heat up they just go off. But they last longer...

Great, my dad just went to melbourne for the week, he would know where to find the answer to the whole wider tyre thing, it'd be somewhere in the study. He did say though that it would give more grip but have more effect laterally than longitudinally. Can't be bothered to read up on it now. The 185's are coming and thats it for now. Skinnier means less aquaplaning - which is good, in england. Maybe thats why my experiences in the wet with semis are better than everybody elses who run wider.

I think its generally accepted that wide tyres make the car less nimble and horrible over bumpy roads (sydney), which is not what you want on an mx5. Or you could go fat at the back and thin in front, that would look sweet and still steer well.

Marcus it sounds exactly like my car, and its the flatspot. Wouldn't be surprised if you started feeling it after the track, specially the amount of times you were facing the wrong direction at wakefield last time

marcusus · Postby **marcusus** » Wed Apr 02, 2008 9:41 pm

Sasso wrote:Marcus it sounds exactly like my car, and its the flatspot. Wouldn't be surprised if you started feeling it after the track, specially the amount of times you were facing the wrong direction at wakefield last time

I'm trying to think of a way to get the car up to that speed with you in it to compare, but legally so...

Sasso · Postby **Sasso** » Wed Apr 02, 2008 10:02 pm

marcusus wrote:
Sasso wrote:Marcus it sounds exactly like my car, and its the flatspot. Wouldn't be surprised if you started feeling it after the track, specially the amount of times you were facing the wrong direction at wakefield last time

I'm trying to think of a way to get the car up to that speed with you in it to compare, but legally so...

Ship the car to germany. And make a holiday of it. The m7 will do, its nice and smooth.

mmmm got my two new R888's today, such a gorgeous tyre. Car feels much better. $225 fitted and balanced, pretty cheap for what you get.

[Will] · Postby **[Will]** » Wed Apr 02, 2008 10:18 pm

On width and contact patch size (and shape), this article sums it up quite well:

http://www.autospeed.com/A_108915/cms/article.html

However, the bit about aquaplaning and higher pressure doesn't mention that the reduced contact patch if you go really high in pressure (e.g. 50psi) would means that although you didn't aquaplane off the road, you'd just skate off because the F=uN rule isn't perfectly linear, if N gets big enough, then F doesn't go up proportionally. Read any race car/suspension book and they will draw this graph for you.

Regardless, still a good article, but just keep in mind that at the extremes, the rules don't apply as plain and simple.

marcusus · Postby **marcusus** » Wed Apr 02, 2008 10:27 pm

Sasso wrote:Ship the car to germany. And make a holiday of it. The m7 will do, its nice and smooth.

Your shout huh?

Sasso · Postby **Sasso** » Wed Apr 02, 2008 11:36 pm

Thanks will, nice article. I still think there's more to it though. Nothing is that simple.

So it says its a heat thing. Well thats all good for a road tyre that hates heat, but what about a racing tyre that likes and needs heat, couldn't you say that the heat generated give you more grip, as long as its not overheating. Racing tyres (including semis) do actually melt to give their high grip.

Not a big deal but its a bit misleading about the profile. Profiles could be completely different but have exact same tyre wall hight and stiffness, so it only applies if the wheel itself is bigger, and profile lower. You then sacrifice ride quality, be prone to rim denting and the car will track all over an uneven road. Albeit semis with stiffer walls do the same thing but also protect the rims.

Yah you're right about the FUN rule the grip drops off heavily when the weight gets above a point. With road tyres its very gradual but with racing tyres the drop is very suddon but the graph is much higher (making it tricky at the limit).

Marcus how do you plan on doing it legally? Unless we go to a circuit club track day, they allow passengers. But I think you'll scare the crap out of me with your crazy driving.

marcusus · Postby **marcusus** » Thu Apr 03, 2008 6:56 pm

Sasso wrote:Marcus how do you plan on doing it legally? Unless we go to a circuit club track day, they allow passengers. But I think you'll scare the crap out of me with your crazy driving.

I only need to go in a straight line to show this to you. I don't get the sound unless I'm straightlining.