Sorry I was a bit vague, first, I am not talking about race cars. By handling I mean cornering on dry or wet pavement and ice/snow traction.
OK, I understand bad design but lets assume an average car with acceptable handling. If one reduces the weight considerably and fits new springs, shocks and tires that match the reduced weight, the lighter car will handle better. Why is that, I mean less weight means less traction. Also, on snow or ice why is the lighter car better? Thanks.
Because the lateral grip from a tire increases as the vertical load increases, but not as quickly. So as you reduce the weight the ratio Fy/Fz increases, which looks like an increase in the coefficient of friction.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
Greg, is there a lower limit for that effect (from the perspective of the tyre)? What I mean is, can significant decrease in weight cause (by virtue of too low vertical load) this rule to cease to work? E.g. if one puts weight equivalent to load index 40 (140 kg) through a tyre designed for LI 100 (800kg), will that tyre offer more grip than, say under a load equivalent to LI 70 (335kg)?
I only see data from about 50% of nominal (not necessarily load index) load through to 200%, so I can't answer that.
But I don't think there is a lower limit, as at some point the mechanical grip (cogging) will dominate, which does not really require any (significant) vertical load to generate a lateral load.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
As Greg says, the tire data would indicate there exists no lower limit, but, in talking to dirt track racers, I get some very strange readings on their "fixes," which lead me to believe that, under some dirt track conditions, they're relying VERY heavily on the effect of the tire edge...and not the footprint...to absorb the lateral load. Does this reverse the trend? I don't know. All I know is that I tend to think that any "help" I might offer a dirt track racer is, at times, very questionable in value.
I have seen data for tires where there is a "lower limit". In particular heavy-duty truck tires and a certain "over-tired" high-end American muscle car. In the snow/dirt I believe there is a lower limit as traction is very dependent on the ability of the tread to "dig" into the surface.
Interesting point about road conditions with reference to tire width/grip. European rally cars use wide and just road legal (have to have a minimum of tread cuts) for the dry, less wide but heavily cut for wet and very narrow high profile heavily cut(studded in some cases))snow tires. Maximum grip with lowest loading (dry) but good grip with highest loading in snow/ice. By loading I mean weight vertically against rubber area on road.
From my experience, as long as you re-tune the vehicle and change to tires that are able to get up to operating temperature with the given loads place on the them, the vehicle will keep getting faster or as you put it "handle better." It is possible to get out of the operating range of the tires, which could lead to the vehicle loosing grip.
FYI: The most "over-tired" production cars in the handling market are not an American "muscle car" but the German sleds: Porsche 911S, Carrera, Cayman S, etc. Compute their tire section to mass ratios yourself. Honda S2000 and Mazda Miata are up there greater that .6 also. (Did I mention the 1994 Geo Tracker @ 0.64)?
Thanks, guys I was aiming at exactly that- e.g. I'm fiddling with the idea for a car, and it would seem that I should use tyres for far heavier car (car weight/load capacity of tyres about 1/3 or less)... I'm concerned that tyres would be far from their optimal range of operation and I would end up with less grip than if I used smaller tyres.
If this is a FSAE car you really want to take a good look at tire compounds/carcase construction and also get some pressure sensitive paper and experiment with various tires to see which give the largest foot print for the loads you will be seeing. Much is to be gained in these areas (wink, wink --> big hint).
Thanks for the tip, but it's for a (very) light road car that I guess would top 240kph (and the narrowest tyre with W rating I found is 175/55 R16)- alack, very tight budget will neccessitate choosing tyres before the design is done.
