Can Modified Cars Achieve 1.5G on the Skidpad?

[YellowTaxi]

This depends on the tire construction (sidewall stiffness, radial versus bias ply, ...) and the rubber compound used. Wiki has a small article:

http://en.wikipedia.org/wiki/Tire_load_sensitivity

that wiki article you quote says that coef is proportional to load ^-0.2
therefore it agrees with me entirely. I don't understand your 'argument' here, you are in agreement with my previous statements.

Of course its only an approximation, and factors like tyre geometry and stickiness of the rubber itself by definition also has an effect of the coef - so what. I believe all tyres follow that rule that coef is prop to load ^-0.2. And all what you say here has nothing whatsoever to do with the bike v car argument in any case, so why you're posting it I'm not certain. Maybe to try and confuse the issue.

You keep saying that bikes can pull a lot of G, but only on very sticky tyres. You're proving my point for me. Cheers Jeff ;>

A Caterham CSR 260 has a weight bias of 49% front, 51% rear without the driver, and with the driver virtually sitting directly above the rear axle, the actual weight bias is further rearwards. A CSR 260 with stock Avon CR500 tires pulls 1.05 g's in turns. With 13 inch wheels and bias ply racing slicks, the CSR pulls 1.4 to 1.5g's, since it can share the same very soft compound racing slicks used on light (< 1500 lbs) non-downforce race cars like the Formula Ford.

So what, it's a very light car so it has more grip. Because grip is roughly prop to load^-0.2.

The center of mass on virtually any sports or race car is low enough and track (distance between left and right tires) is wide enough that the car isn't tranferring almost all of its weight to the outside tires due to tire grip, as this could end up with car rolling over on it's side.

That simpliy isn't true Jeff. Tipping is a big problem on European Touring cars which are race cars built from stock, street legal cars like the Audi A4. The driver sits at low as possible in his specially low mounted seat, and the wheels are mounted something like 5 inches outside their normal position just to try and keep the car from tipping over. In other words they're cornering at around 100% load transfer. Occasionaly they do indeed lift both inside wheels or even flip. That's 100% load transfer, no doubt about it.

A stock porsche 911 will start tp tip at around 1.5G. ie it would have 100% load transfer at 1.5G. The tyres they're obliged to use on the road have deliberately limited max grip of 1G, so its not a propblem there. If you fit racing tyres to a porsch it would be a problem, just like it is on those Touring Cars.

 

[xxChrisxx]

that wiki article you quote says that coef is proportional to load ^-0.2

Where?

Also I don't know exactly what the CoG height is for a Porsche 911, but acutally do the calculation for 100% transfer of load.

 

[rcgldr]

that wiki article you quote says that coef is proportional to load ^-0.2

It states a range:

the maximum horizontal force Fy that can be generated is proportional, roughly, to the vertical load Fz raised to the power of somewhere between 0.7 and 0.9, typically.

You keep saying that bikes can pull a lot of G, but only on very sticky tyres.

What I stated was that cars have larger contact area, lower loading per unit area, but that sport bikes generally have stickier tires than sport cars which compensates in stock sport bike versus stock sports car comparasons. Once you're running DOT tires on both, the cars have an advantage.

 

[YellowTaxi]

Where?

It says the lateral force, also known as friction, is prop to load^0.8

well, Friction = Coef x Load
.'. Coef of friction is prop to Friction/Load.
ie [load^0.8]/[load]
ie Coef is prop to load ^ -0.2
OK ?

 

[YellowTaxi]

It states a range:

What I stated was that cars have larger contact area, lower loading per unit area, but that sport bikes generally have stickier tires than sport cars which compensates in stock sport bike versus stock sports car comparasons. Once you're running DOT tires on both, the cars have an advantage.

You said said the bike has around 1/5 or 1/6 the contact patch area of a car tyre.
And I said the car has around 5 or 6 times the weight of the bike all balancing on 2 wheels just like a bike. Therefore the 'loading per unit area' is the same...

Mentioning that adding stickier tyres gives more grip doesn't prove anything - by definition it's obvious anyway.

 

[xxChrisxx]

It says the lateral force, also known as friction, is prop to load^0.8

well, Friction = Coef x Load
.'. Coef of friction is prop to Friction/Load.
ie [load^0.8]/[load]
ie Coef is prop to load ^ -0.2
OK ?

You said said the bike has around 1/5 or 1/6 the contact patch area of a car tyre.
And I said the car has around 5 or 6 times the weight of the bike all balancing on 2 wheels just like a bike. Therefore the 'loading per unit area' is the same...

Mentioning that adding stickier tyres gives more grip doesn't prove anything - by definition it's obvious anyway.

Jeff beat me to the point, it sates a clear range. If a range is stated it's not correct to simply take the middle value and declare everything is that.

Also I'm calling ******** on your Porsche claims of 100% weight transfer at 1.5G. Please note that you can tip a car without steady cornering on 2 wheels first, there is no arguing that you have to trasistion from 4 to 2 wheel during the tipping motion, but you wouldn't really call this scenario cornering. It would be having an accident.

You made the claim, state your assumptions and show your reasoning/working. If you read it somewhere, post the source.

 

[rcgldr]

Contact area isn't just a function of pressure, it's also related to other tire parameters (like sidewall stiffness). Once contact area is beyond some optimum value, then higher pressures can make a car more responsive. An example article about Hoosier DOT R6 (race) and A6 (auto-cross) tires:

http://www.racetire.com/products/tctips.htm

List of Road Racing tires at Hoosier. Note that the softer compound road racing slicks - radial - are only available with 13 and 15 inch diameter wheels, and the road racing slicks - bias ply - don't have any sizes larger than 16 inch wheels. Click on this link, then "specs" in the left menu:

https://www.hoosiertire.com/rrtire.htm

Note, the guys who race modded Porsche 911's, generally run 18 inch wheels, and these are harder compound tires, that won't pull 1.5 gs in turns. If such tires did exist, and 1.5g's did cause 100% weight transfer, then the car wouldn't be safe. Cars generally roll because they get sideways, and dynamic compression forces (perhaps a dip or curb) on the leading side of the car are enough to create enough grip to roll the car. This amount of dynamic force isn't possible with normal turning. Most of the time, cars will spin without rolling.

 

[YellowTaxi]

Note, the guys who race modded Porsche 911's, generally run 18 inch wheels, and these are harder compound tires, that won't pull 1.5 gs in turns. If such tires did exist, and 1.5g's did cause 100% weight transfer, then the car wouldn't be safe. Cars generally roll because they get sideways, and dynamic compression forces (perhaps a dip or curb) on the leading side of the car are enough to create enough grip to roll the car. This amount of dynamic force isn't possible with normal turning. Most of the time, cars will spin without rolling.

Jeff that's funny. Modified Porsches can pull more than 1.5G and they're modified for precisely that reason - they would would tip if they weren't modified. Even a stock porsche 911 will pull well over 1G on a race circuit, since there's more grip on a racing track than out on the street.

ps. I don't know why you sent me a pm claiming that only rally cars on rough surfaces can tip in a curve, and some other such nonsense. Was that a joke ? There's plenty of videos on the net showing cars like BMW M3's tipping purely due to G forces. I have videos of my own where cars tip purely due to pulling sufficient G to lift the inside wheels. This discussion is getting stranger with every post..

As to the other posters , if you're so smart and so disbelieving of what I say, then I suggest you go work it out for yourselves and show your reasoning on here. Instead of chanting childish "Oh no it isn't"s. You people should probably be in a pantomime somewhere.

http://en.wikipedia.org/wiki/Pantomime

 

[xxChrisxx]

Jeff that's funny. Modified Porsches can pull more than 1.5G and they're modified for precisely that reason - they would would tip if they weren't modified. Even a stock porsche 911 will pull well over 1G on a race circuit, since there's more grip on a racing track than out on the street.

ps. I don't know why you sent me a pm claiming that only rally cars on rough surfaces can tip in a curve, and some other such nonsense. Was that a joke ? There's plenty of videos on the net showing cars like BMW M3's tipping purely due to G forces. I have videos of my own where cars tip purely due to pulling sufficient G to lift the inside wheels. This discussion is getting stranger with every post..

As to the other posters , if you're so smart and so disbelieving of what I say, then I suggest you go work it out for yourselves and show your reasoning on here. Instead of chanting childish "Oh no it isn't"s. You people should probably be in a pantomime somewhere.

http://en.wikipedia.org/wiki/Pantomime

Burdern of proof lies with the one making the claim. I find it hard to believe that it is phyiscally possible to get even close to 50% load transfer without the car spinning out first. Even if it were possible (which I have doubts about) as Jeff said no one in their right mind would set up a car with enough grip to roll itsself. It's for this reason I have never ever seen a roll from a steady cornering. There has ALWAYS without fail been some dynamic force at work.

You may find some older cars with floppy chassis that lift the inside front, the the rear's tend to always be planted.

Could you please link some videos that show the phenomenon of a roll caused by steady cornering?

 

[rcgldr]

Modified Porsches can pull more than 1.5G and they're modified for precisely that reason

What I meant was stock 3000+ lb Porsche modified for track use with suspension tuning and DOT tires, as opposed to a FIA racing Porsche at 2500lbs. Note also that lateral grip decreases with load transfer. A race car might be able to pull 1.5g's with a 50% load transfer (cg height = 1/3 track width), but a stock car with with a relatively higher CG won't be able to achieve that because the higher load transfer reduces the effective grip.

Some actual skidpad numbers:

http://www.modified.com/news/0708_sccp_lateral_g_skidpad_testing/skidpad.html

http://www.motortrend.com/features/112_0706_americas_best_handling_car/photo_20.html

Peak lateral acceleration (banked turn) at Laguna Seca exceeds 1.5g's, none of the cars roll:

http://www.motortrend.com/features/112_0706_americas_best_handling_car/track_conclusion.html