Slip and Axle Stiffness

[GEspo]

Hello Eng-tips experts. Is there a short answer to why increasing axle stiffness at one axle will create more slip at that axle relative to the other axle? This question is related to how to get more oversteer/understeer ie suspension tuning/roll bar tuning.. Thanks!

 

[CapriRacer]

A word of caution here:

I think most of the folks responding work for an OEM and their data is of OEM tires. That's important because OE tires are different than aftermarket tires in that OE tires are designed to OEM specs, which usually include rolling resistance as an important parameter - unlike aftermarket tires where rolling resistance is barely an issue and sometimes is not an issue at all!

Further, if the data is from an OEM source, it will be individually unique as the OEM specs vary widely and no 2 OE tires are the same - even if the name on the sidewall is the same.

As an example, the tire company I used to work for supplied 5 different tires of the same size and name on the sidewall to 5 different OEM's and the only way to tell the difference was 1) the symbol on the sidewall required by the OEM and 2) the DOT code - and, of course, each of these was different than the non-OE version of the tire.

 

[cibachrome]

Not necessarily true. My OEM tests plenty of performance tires, especially for the 'max' handling Level 3 versionsbeing persued. This is necessary to push the OE TIRE manufacturers to advance the ball, AND because the rules of "Forseeable Misuse" require you to know what a 3 sigma tire does on a car on which it fits. Called "Tire Intermix", the most common issue is a "high performance" tire replacing just the 2 front tires while the worn out rears remain in the rear.

Also, if a 911 GT3 or a Ferrari F430 is brought in for 'fingerprinting' you'd better believe I have tire F&M tests of both front & rear tires off them. The OEM tires, not ones a dealer might substitute on them to sell separately. so, yes I have this information. A few years old because I'm long gone, but not out of date because there are limits to a tir'e sizes capability no matter what compound you slap on it.

My OEM also challenged new engineers to create a suitable SCCA autocross vehicle, requiring plenty of race slick data, usually Hoosiers, in fitments suitable for the weight and performance class selected.

The tires I showed ARE from production competitive cars, meaning they are likely to be purchasable thru various outlets in quantities needed. In this case, it's tires likely to fit in the wheelhouses without wheel travel restrictions. But, run what you brung if that's what your budget permits.

 

[GEspo]

Im a bit limited for tires for the sti as far as R-comp, not sure a non track tire would last out there. I’ll probably end up trying all the track rated stuff before too long.. I went through a practically new set of stock front pads in 2 20 min sessions, completely gone.. needles to say im hard on the car. Upgrading to large cooling ducts and Ferodo race pads for next session. Tires with temps in the 180 deg range max(i have a IR temp monitor mounted).. what would a street rated tire do with 180deg temps?

 

[cibachrome]

Is that an outer, inner, or center temperature? A single channel is NFG (IMHO) if the working section of the tread is not identified. Here is the kind of display it takes. I've had to alter the load scaling factors because manufacturers DO NOT approve of lab test data distribution. Hopefully you get the idea. Road tire temp data is available, just nobody usually cares about it. If the tread comes off, it's too hot.

"Being hard on a car" is generally NOT considered to be an attribute. "Slow & Smooth is Fast" usually prevails. Figure-8 Racing is the exception. More fun, too. Owners generally prefer to have their entries finish, then top 10, then top 3 without the thrashing that tire, suspension, and engine abuser's can be noted for. Watch some Rally car videos to see what I mean.

 

[CapriRacer]

cibachrome,

I am glad you chimed in. I wondered if any OEM tested off the shelf tires, especially hard-core grip tires. I would like to hear from others.

But my point stands. OE tires are different than aftermarket tires and people need to be aware of that.

 

[cibachrome]

Absolutely, but my local Audi dealer will put them on an R8 without hesitation. Just a warning about wet weather. The new hires here put them on an S-10 "race truck' to autocross. Wound up with hoos your daddy's, Wide Rims matter. Hey ? WRX = Wide Rims Xtra ?? Subaru= Substantially Underpowered, But Accelerates Uphill.

This is just 1 degree data but you get the idea. "Yes or No" as Claude would say ....

 

[GregLocock]

Yes OEMs do test aftermarket tires for a bunch of reasons. One that people don't think about is trailer tires.

Cheers

Greg Locock


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[GEspo]

Google turns up a quick description of shredded street tires at track temps.. looks expensive and dangerous. Not sure if this is a good case of the exception vs the rule, but the saying “bring a knife to a gun fight” may apply..

https://www.s2ki.com/forums/wheels-...ole-position-s-04-failed-temperature-1034074/

 

[cibachrome]

The guns I take to gun fights are hollow points or wad cutters. There won't be any ties. Because some manufacturers forbid testing of their tires, you have to sneak them onto a Flat-Trak on the weekends.

BTW: Make sure any race tires you 'acquire' have the exact same build date. They are like Grandma's chicken soup. You know it's 'chicken soup' made that day, just not sure how much chicken you'll be getting next week.

 

[CapriRacer]

Greg Locock said: " ..... One that people don't think about is trailer tires. ...."

OK, I gotta ask: Why?

GEspo: That link took me to a discussion page. The tires in those photos look to me like cold tires driven hard. I've seen this before on some street performance tires driven on a track.

 

[cibachrome]

People pull trailers with their vehicles. Trailer dynamics gcan greatly affect the tow vehicle's dynamics. Most trailer tires are made at sausage factories. Then there is the braking topic. Trucks now have built-in trailer brake controllers. You need to know that the tires are capable of providing FX forces. Tires made in some countries usually disintegrate when they see stopping loads as well as cornering loads.

Then the hitch design: Bumper, load equalizing, goose-neck, and fifth wheel. Now add to this list a drunk driver whose empty wine bottle gets trapped under the brake pedal going down a hill. The resulting death(s) injuries and set up all manufacturers for liability requirements (Deepest Pockets). The drunk has no money but the OEMs do.

 

[CapriRacer]

cibachrome,

So you are saying that computer modeling of vehicles with trailers is something that is done - and for that, you need trailer tire data.

OK, makes sense.

 

[GregLocock]

Sure, ADAMS or some rule of thumb type simple calculations on the stability margin. Apparently Carsim has a trailer module, so we could run playstation simulations of trailer-tow. Double lane change would be fun.

here's some ADAMS trailer stuff,

https://www.researchgate.net/publication/264812314_Tractor-trailer_modelling_and_validation#pf5

And here's some real fun, not too sure about the health and safety, but damn it makes the point (although it is more complicated than it looks as they've changed the towball load as well).

https://www.youtube.com/watch?v=i2fkOVHAC8Q

Cheers

Greg Locock


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[GEspo]

Does a similar principal apply here: a higher moi is more resistant to change in direction but can correct more easily than a lower moi, ie mid vs rear engine loosing it.. Wouldnt the toy car on the treadmill more easily correct with the higher moi? (it needs a tiny driver)

 

[SwinnyGG]

(although it is more complicated than it looks as they've changed the towball load as well).

That's the point of that demo - to illustrate why how a trailer is loaded matters a great deal, and that a trailer can be very dangerous even if it's loaded within limits.

 

[GEspo]

(car/trailer system differs from car.. makes sense)

Why is a mid engine harder to control once traction breaks vs rear engine?

 

[gruntguru]

I thought mids and rears were both less stable than front engine with rear being the worst.

Keen to hear from the experts on this and setup changes that reduce the effect on mid-engine cars.

je suis charlie

 

[BrianPetersen]

All else being equal (i.e. same tires and inflation pressures front and rear) - The heavier end of the vehicle deflects the tires more (i.e. increases the slip angle) for a given "g" lateral loading. For a front-heavy vehicle, that's understeer. For a rear-heavy vehicle, that's oversteer.

The Chevrolet Corvair attempted to offset this via specifying very low front tire pressure. People didn't read the manual and inflated all of the tires to a "normal" same-ish-front-and-rear pressure. Oops.

Staggered tire sizes (i.e. differing tire specifications) front and rear - obviously wider ("stiffer") in the rear - can kindasorta compensate for this. Many sports cars are like this.

I've never heard of mid-engine being "worse" in any way than rear-engine, in terms of handling characteristics.

 

[GEspo]

Something makes sense that a mid engine is harder to get to break traction than a rear engine but harder to get corrected when it does, possibly because the higher moi rear is quick to break traction back the other direction? ie pendulum.. The math must be there to back this up because ive heard this from a few racing guys(one being one of the oldest porsche mechanics in the country)..

Does the 911 GT3 holding the record for the double lane change confirm this? (last i read it did, HIS excluded)

 

[GregLocock]

Having driven a Lotus Esprit at /my/ limit, once, I'd far rather be fanging around a gravel road in a Ford Falcon, noisy thing at the front, huge polar moment of inertia. Very easy to set up a stable drift. I've never tried driving a 911 fast, they are always borrowed cars. Modern 911s are hugely overtired at the rear, or if you prefer, they use pram wheels at the front, because they have a functionally silly architecture.

The fastest car through the difficult double lane change (the moose test) is some old Citreon thing, not a 911.

Cheers

Greg Locock


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