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  #1 (permalink)  
Old 06-06-2010, 06:19 PM
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Question Tire sizes related to vehicle weight?

I've had my car aligned properly for the first time, and soon I'll have it corner weighted and put a sway bar on the front end. After that a change from the Cooper Cobra tires to something a lot stickier. I read that several folks prefer a smaller combination - Tom Kirkham if I recall recommended 245 fronts and 275 rears for optimal handling.

I have 265 fronts and 295 rears - switchback handling is more important to me than launch. My car is heavier than most as it's a WCC. Do the smaller tires actually usually lead to nimbler handling (I guess the rears would tend to push less), and if so is that a set of sizes that would only apply to the majority of cobras which are 300-400# lighter than mine?

Thanks!

Dirk
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Old 06-06-2010, 08:02 PM
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The Superformance "recommendations" are 275/40 and 335/35 (or 315/35) on 17" wheels. The 15" tires are 255/60 and 275/60 (or 295/50). You have so much power I would think you want as much rubber on the road as possible. Your wheel size might make a difference what you can fit. I don't have any first hand experience of any other size than the size I have.
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Old 06-06-2010, 08:17 PM
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Default Roger Krause Tires tech page

There is a great diagnostic chart on tuning "OVERSTEER vs. UNDERSTEER" at the "information" section on the ROGER KRAUSE TIRES web site.

http://rogerkrausracing.com/pages/moreinfo.html

Gives handling characteristics vs. corrective actions for a myriad of conditions...tires sizes, rubber compounds, shock settings, roll bars...lots to think about...more than I can deal with...

I stop at fiddling with tire pressures..actually, you can get a LOT of response with only tire pressures...the rest takes more expertise than I have or track time than I can afford.

Pete
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Old 06-07-2010, 06:13 AM
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Thumbs up Two basic questions

Thanks for your replies guys! The Roger Krause chart boggles the mind...... I guess having reviewed what I've learned so far it comes down to two questions for me.

All other things being as equal as possible;

1. Is there an intrinsic "autocross" turning advantage to the 245/275 combo vs 275/335?

2. Does the heavier weight of my car influence the above choice significantly (2400 vs 2800#) ?

Ride safely!

Dirk
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Old 06-07-2010, 10:21 AM
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Default From Mark Ortiz' newsletter

June 2010
Reproduction for free use permitted and encouraged.

Reproduction for sale subject to restrictions. Please inquire for details.
WELCOME

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: markortizauto@windstream.net. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.


RELATIONSHIP OF TIRE SIZE AND WEIGHT DISTRIBUTION

Question:
Many car owners say that because their car has 50/50 weight distribution, they inherently have the best handling possible. However, many owners install wider tires at the rear to improve traction. My opinion is that by doing so, while their car may still have 50/50 weight distribution – at rest – the dynamic balance will be a car that understeers. True?

On a related note, let’s say we have a mid-engine car with 40% front / 60% rear weight distribution. If the rear tires are made 50% wider than the front tires, wouldn’t this negate the static weight distribution, essentially creating a car with more balanced traction front and rear (granted, on a

constant-speed circle?) I understand there may be an issue with getting enough heat into the tires to generate traction proportional to their width. This assumption aside, is this thinking correct?

It is definitely correct that, at least up to a point, we can compensate for weight distribution with unequal size tires. We can also compensate with roll resistance distribution, camber, tire pressure, and aerodynamics.

So it is safe to say that if the car has 50% rear weight, rear drive, and bigger tires in back, it will probably understeer in steady-state cornering if camber properties and settings, overall roll resistance, tire pressure, and downforce are equal at both ends, and if speed is low enough so that the rear tires do not have to transmit a huge amount of power just to maintain steady speed. If any of these conditions are not present, all bets are off – and usually not all of these conditions are present.

We can at least say that if we take an existing car, and add tire size at the rear only, leaving all else unchanged, the tire size change will move the car toward understeer.

Answer:
In many forms of racing, we do not have free choice of tire sizes; the rules impose a maximum size. This may be the same for both ends, or not. The rules often also impose restrictions on other aspects of the car's design, which limit what sort of weight distribution we can have, and what sort of downforce distribution we can have.

Nonetheless, it is interesting to consider what we should want in terms of weight distribution and tire size in a rear-drive car, given a free hand.

It is useful to note what course car evolution took in F1 and in sportscar racing, before tire sizes were limited. The cars were decidedly tail-heavy – around 60% rear – and the rear tires were about a foot and a half wide. Front tires had about 2/3 the tread width of the rears. The cars had more aero downforce at the rear than at the front.

Why would this be better than 50% rear, and equal-size tires? There are various reasons, but probably the main one is that the car doesn't just have to corner; it also has to brake and put power down. It brakes with all four wheels, and propels itself with only the rears.

Even without downforce, in straight-line limit braking about 20% of the wheel loading transfers from the rear to the front. Even with 60% static rear, the car does 60% or more of its braking with the front wheels. A car with 50% static rear and modest downforce does about 70% of its braking with the front wheels. It is possible to get the brake bias needed, but it becomes difficult to sustain it over the length of a race. The front brakes tend to overheat and go away.

As for putting power to the ground, there is no mystery as to why more rear percentage is an advantage with rear wheel drive.

Finally, in most cases it is difficult to accommodate foot-and-a-half wide tires on the front of the car, especially without power steering. The scrub radius ends up being really large, and the car gets difficult to steer.
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Old 06-07-2010, 03:14 PM
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Default Say what....??

Haha, Tom your technical article response was very informative and essentially what I get is YES and NO. Alright, I give up on the hypotheticals and will ask it outright! -

If you owned (please humor me) a West Coast cobra which weighed in at about 2800# and wanted to optimize the porker's handling performance using tires after everything else has been set as right as can be, which sizes in a sticky 10000 mile tire would you choose?

I'll have this project done.....in approximately 100 years..give or take a few

Regards,

Dirk
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Old 06-07-2010, 03:33 PM
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Default Autox Tire sizes

Quote:
Originally Posted by DocDirk View Post
Thanks for your replies guys! The Roger Krause chart boggles the mind...... I guess having reviewed what I've learned so far it comes down to two questions for me.

All other things being as equal as possible;

1. Is there an intrinsic "autocross" turning advantage to the 245/275 combo vs 275/335?

2. Does the heavier weight of my car influence the above choice significantly (2400 vs 2800#) ?

Ride safely!

Dirk
I use 295x30x18's front and 335x30x18 rear for autox with good results, if I were doing it again I would have installed 315x30x18 fronts. As stated elsewhere there are so many other variables that you just cannot just assign a tire size without all the facts: Roll stiffness front and rear, roll axis, chassis stiffness, aero loading, suspension geometry, track width and weight distribution all can effect tire width decisions. I use the same tire sizes for road courses with a change in rear springs (from 1100 down to 1000 lb/inch) and the car is perfectly balanced with a trace of understeer under throttle.

The car is not harsh at all and uses a good portion of available suspension travel when running, car weight is 2350 with a 53% rear weight bias.

As far as street tires go I would have to agree that you don't need to go really big on tires sizes, the big ones cost more and will just get you in more trouble if something goes wrong because you will be going faster when it does.
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Old 06-07-2010, 04:35 PM
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I think a Cobra with the same size tires front and rear would look funny.

Just my personal opinion by the way.
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Old 06-07-2010, 06:29 PM
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IMHO:
For the street Avons
245/60-15 Front
275/55-15 Rear

For the track: Goodyear Slicks
7.00-15
8.00-15

And if you are willing to go to 18 in tires...Whatever CSX3170 is using.

The weight of your car is well within the limits of these tires.
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Old 06-07-2010, 07:19 PM
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I never got understeer from any of the various tires and replicas Cobras I've owned/driven with any tires until,,, I recently went to Goodyear slicks. Now I have a slight tendency to understeer, but the good news is there is no way the back end is stepping out! I have to induce throttle over steer if needed.

I've run 16 and 15" tires. Auto Cross DOT tires, BF Goodrich TA's, Goodyear GT-11's and Goodyear Bill Boards. Always the classic sizes, 245 front, 295 rear sort of thing. ALWAYS the rear end would come out first under hard cornering conditions (over steer).

The reasoning behind smaller tires is all about "matching the traction" front and rear to arrive at a more nuetral cornering condition. I find I prefer a slight understeer rather than a big over steer.
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Old 06-07-2010, 09:01 PM
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Default Thanks for the good advice!

I'm grateful - I could have taken ten years of experimentation to find so much out!

I'm looking forward to driving this car hard sometime soon - the advice I've received here will prepare me for my first hard driving school - at least the equipment won't be the most limiting factor....

Dirk
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