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GT40 Tech - Chassis,Brakes,Tires,&Wheels Chassis and Handling.

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Old 14th October 2011, 12:28 PM   #1 (permalink)
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Front/Rear Wheel Width

I'm making good on my threat in my new member introduction to seek advice from the forum in specifying a build based around a RCR 40 chassis. The proportions most pleasing to my eye are those of the Shell Oil cars best illustrated by Racing Icon's restoration of P1036 as shown on their web site. Up 'til now I didn't know I had an affinity for big butts. With big butts often comes large thighs and 1036 has 12" wide rubber on 15" rims on the rear. I like that look.

Minimal availability of rubber for 15" rims has been thoroughly discussed here as has use of racing tires for street use and my intent is not to address those issues once again. I have a devil on one shoulder and an angel of common sense on the other in this matter. The devil says to do whatever it takes to keep to the proportions of the original which would entail the use of racing tires, specifically Dunlop CR82's to go on 12" wide rims. Fran at RCR was playing the devil's advocate last we talked. He seems enthusiastic to do something a little different than the last guy did. The angel is saying go with the +2 configuration of the rear suspension and mount 10" wide rims and accept somewhat thinner thighs. There are many other reason to justify the narrower rims on roads that may see the occational patch of water, etc. but the devil is powerful. The angel could use some help.

A long time ago there was brief mention in a post about a large differential between front and rear wheel width perhaps building in a push to the handling. As the fronts would be 8 inches wide we are talking of a differential greater than 3 and as much as 4 inches. I'm asking those of you out there with experience in suspension configuration to state your opinions on the subject and give recommendations, leaving aside the race-rubber-on-civilian-roads issue that has already been discussed.

Additionally, if in the process of discussing the issue, you would care to recommend a book or two on the subject of suspension tuning, that would also be appreciated. While suspension design I'd best leave to the experts, tuning of such is something I should get smarter about. I've already taken the advise of the forum in buying Mr. Monroe's book 'Small-Block Ford Racing Engines' to get smarter specifying the engine build; I'm a motivated student.

Thanks, Rich McD
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Old 14th October 2011, 04:14 PM   #2 (permalink)
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Re: Front/Rear Wheel Width

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do whatever it takes to keep to the proportions of the original which would entail the use of racing tires, specifically Dunlop CR82's to go on 12" wide rims.
Rich -- thanks for articulating this question so well. I'm very intersted in exactly the same issues.

Given the research you seem to have done I assume you know that one street-tire option for those rims is this Pirelli
Pirelli PZero System
and also the Hoosier tires Ron has mentioned.

So my question for you is: the sentence I quoted seems to imply CR82's are the only solution to "keeping to the proportions"? Did you mean that?
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Old 14th October 2011, 04:24 PM   #3 (permalink)
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Re: Front/Rear Wheel Width

As to books, I have Tune to Win by Carrol Smith, and it's a good introduction to tire, brake and suspension setup and dynamics. Not too technical but thorough and understandable. About $20 at Amazon.
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Old 14th October 2011, 04:30 PM   #4 (permalink)
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Re: Front/Rear Wheel Width

Sir Mix-A-Lot - Baby Got Back (I Like Big Butts) [ORIGINAL] - YouTube

I met Sir Mix one time - he used to live in Seattle. Seemed like a nice guy.
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Old 14th October 2011, 09:35 PM   #5 (permalink)
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Re: Front/Rear Wheel Width

Alan- Perhaps between the two of us we can keep this thread alive until a suspension guru or two weighs in on the subject. The Pirelli option that EgilTom used certainly has a suitable road tread if only it filled the wheel well like the original, 26-27 inch diameter. They also can be as hard to acquire as a race tire given a once annual limited production run and they are priced accordingly. I revisited the Hoosier option since you brought them up. Their H rated road tire for 12" rims are 29 inches in diameter, about 2" too tall. That leaves us with the 44316 DOT race tire suitable for the rim. It is accompanied by warnings of imminent death if used on the road so I classify it as an affordable option to the Dunlop. I admit to using the Dunlop CR82 as an example in the hope of flushing out some opinion given the negative press afforded them in the past. So our question remains, is a wide tread width differential between front and rear suitable only for dragsters or can a road car run this setup?

John- Thanks for the recommendation. I've seen this on a list previously and if it is non-technical and an easy read it will not be relegated to a bed-time story and may actually sink into my head. Consider it ordered.

Cliff- You feed my recently acquired addiction. If the Butts sisters had been outfitted in blue spandex, as opposed to gold, this video would have become my theme song for the build.

Evil sends

Last edited by EvilRich; 14th October 2011 at 09:45 PM. Reason: poor spelling
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Old 16th October 2011, 06:36 PM   #6 (permalink)
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Re: Front/Rear Wheel Width

Either I've made the topic too specific or I should have posted to the Paddock for good participation.

So go ahead and comment on race tire use on civilian roads or lack of availbility of tires for 15" rims.

Perhaps someone would comment from a historical perspective. The GT40 saw quite the inflation of tire width in it's '60's racing days. I assume this was not to the detriment of handling. Maybe I'm answering my own question(maybe I'm talking to myself). I'll quiet down now.
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Old 16th October 2011, 07:21 PM   #7 (permalink)
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Re: Front/Rear Wheel Width

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Either I've made the topic too specific or I should have posted to the Paddock for good participation..
Possibly nobody knows the answer....

We do have one "instance proof" if you will: the later Lamborghini Countachs, that used 225/50-15 front (8" tread width) and 345/35-15 rear (13" tread width). We have at least one Countach owner here (Jack Houpe). Has anyone heard of a downside to the Countach's use of wide rear tires?

Last edited by awatkins; 16th October 2011 at 07:34 PM.
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Old 16th October 2011, 11:09 PM   #8 (permalink)
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Re: Front/Rear Wheel Width

In the post titled "Front clip width with gulf flares?" dated 12-17-07, MReid stated in reply to the thread that he thought the wider rear of later generation cars(Gulf?) was to address oversteer. While I see the logic to such in a racing application, I wonder how that translates to street driving? An opinion on the handling of the Countach would be beneficial.

Alan, I had read or heard that Pirelli continues to produce the P zero you referenced specifically to continue to supply tires for the Countach.
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Old 17th October 2011, 05:02 PM   #9 (permalink)
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Re: Front/Rear Wheel Width

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Alan, I had read or heard that Pirelli continues to produce the P zero you referenced specifically to continue to supply tires for the Countach.
That's my understanding as well from reading Lambo discussions.
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Old 17th October 2011, 05:55 PM   #10 (permalink)
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Re: Front/Rear Wheel Width

We race a 8" wide front wheel with 10" wide rears. Naturally, we're using the Hoosier R6 and you can find lots about those here and on other sites. We run 225 front rubber and 275 rear, and, while not the uber wide stance you refer to we do have a F/R differential and have no ill handling effects to report. I wouldn't mind having some 295 section width rear R6s but they aren't made in that size.

We have tuned the chassis on the track and skid pad to have very slight push, figuring that is a good handling characteristic for the car that we want to be easy to drive for endurance racing and not bite anyone. With a slightly less aggressive front bar bite we'd end up with a slight tendency to oversteer, so the choice can be yours. Even if we had 295 size rears I'm pretty sure we could tune it to oversteer or understeer.

R
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Old 17th October 2011, 08:51 PM   #11 (permalink)
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Re: Front/Rear Wheel Width

I have 12" rears and 9" fronts on both race and street cars, as well as lots of experience in cars with a 17" wide rear with a 12" front. I assume nobody reading this is the type of person that would be driving down a tree lined country lane on cold tires and punch it going in to a tight corner, and then complain of a "push".
In a normal street driving application
the car never reaches the limit of front adhesion and understeer is not noted. Not a problem if there is a bit of consistant scrub, right ?
On the track understear is desired and can be changed to slight oversteer by giving more, or less, throttle.
For tires in either case, I use treaded Avons which were made from slicks. New ones for the track and when they are "old" I use them on the street. I generate several sets of these per year and street use is my personal form of recycling...
I do not drive the cars in the rain.

Last edited by Johan; 17th October 2011 at 09:00 PM. Reason: clarity !
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Old 18th October 2011, 01:42 PM   #12 (permalink)
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Re: Front/Rear Wheel Width

What I've taken from the discussion thus far:

If the car has the horsepower, wider rear tires are beneficial to apply it. The gulf cars ran 14" rims apparently to put that power to the road more effectively when exiting a turn. From what I've seen in videos, the cars could snap quite rapidly into oversteer under that circumstance and the wider tires would lessen that tendency.

Ron- I conclude your car would benefit from wider rears, if available, and they would not overpower the fronts to create excess scrub. Setup and tuning of the suspension are key.

Johan- Your setup is most akin to where I was contemplating going. Again, suspension setup is what drives handling. Might I conclude that, once the Avons have been through their useful number of heat cycles, the rubber takes a set such that they perform like an average street tire, assuming dry road conditons?

Under the circumstances your last statement should not go underappreciated, "I do not drive the cars in the rain."
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Old 18th October 2011, 02:18 PM   #13 (permalink)
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Re: Front/Rear Wheel Width

Correct, sort of. The subject of heat cycles is faily involved. When during the race does a tire come in and when does it go away ? Is it a race or a track day ? How many seconds per lap, and which lap, are we giving away with x-cycles tires? Can we afford to give away those seconds ? Can we afford new tires ?
"Just off the race car" tires are still better than street tires for track use on a test day, but no longer suitable for racing because it would not be racing, if that makes sense. As far as street driving a race car, I do it all the time and I don't go very fast. I just putt around. I could drive to the track and do a test day and I would be several seconds off my lap times with fresh race rubber. Also, I don't run race engines on the street very often. I am thinking more of a race car with a street engine. A race car with a race engine needs fresh rubber every time it moves under its own power. I have more than one car so this works for me.
The good thing is that you really don't want to pick up all that gravel and sand with a street use x-race tire anyway.
Rain makes them run too cool to be effective with the pattern I use. Just to be clear, I am talking about fairly new tires and not some old has been dry rotted things that were fresh in the 1970s three owners ago.
If you want to buy new Avons, I would be happy to race on them and condition them for street use at no charge!

Last edited by Johan; 18th October 2011 at 02:40 PM.
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Old 18th October 2011, 02:54 PM   #14 (permalink)
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Re: Front/Rear Wheel Width

Just to go a bit further, we attempt to set up a car that pushes when it reaches the limit of Gs in a corner under moderate accelleration. This push can be eliminated , even over eliminated, with application or reduction of throttle. The next setup quest is to move the oversteer phenomenon to the highest limit possible. We don't want the car to push, but it beats oversteer. Wider tires allow sooner application of more throttle coming out of corners and this is crucial to a good lap. I have 4 1/2 degrees castor, -1 degree front camber, -1/2 degree rear camber that increases with squat, and 1/8 total toe in front and rear as my treaded tire mid engine V8 setup. This setup might be a bit tiresome or darty on cold tires as used on the street, but who cares, you are just going to Starbucks !
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Old 18th October 2011, 05:54 PM   #15 (permalink)
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Re: Front/Rear Wheel Width

If you want to buy new Avons, I would be happy to race on them and condition them for street use at no charge![/QUOTE]

Oh ya, sign me up!

I understand that a specific tire compound is meant to operate within a limited heat range and when not within that range either will not provide adequate grip or will proceed to wear rapidly. What I don't yet understand is what takes place in the compound with heat cycles such that it no longer performs to spec. after a certain number. Is it a chemical transaction or a physical one associated with wear or a combination? Do underlayers of the rubber not perform the same way outer layers do?

What you say of setup makes perfect sense and is custom to the individual car and track it is to be raced on. Of course it is up to the operater to select the tire compound that he/she feels will yield the best result given the current conditions. And that may not be the fastest compound given the length of the race. Always compromises.
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Old 18th October 2011, 07:30 PM   #16 (permalink)
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Re: Front/Rear Wheel Width

The Daytona 500 race makes different demands on a tire than the 24 hours of Daytona. It is common to see the very thin 500 tires worn to the cord after a single stint if run just a lap or two too long. It takes 30 sets of tires to run the 24 including practice even though the tires are more durable. Vintage race tires another thing.
Weight of the car, length of the race, horsepower, and ground effects all play a part. The type of racing I do usually involves a sprint race of 1/2 hour and the Enduros last up to 4 hours. The tires overheat.
The more heat cycles, the less sticky the tire is to begin with and the sooner it begins to feel greasy because it is sliding and that causes it to overheat sooner than a new tire. The tires are cooking and what is cooking out of them is the petroleum emulsion (I am guessing) that makes them sticky and not slip which makes them run cooler than a tire that is not sliding. A sliding cooked tire wears down pretty quick. Sometimes chuncks of rubber come off. A Lola or GT40 does not weigh enough or, in vintage racing, race hard and long enough to eat a new tire down to the cords but the tires do get greasy after a number of laps. How many laps until they feel good and how many laps until they feel bad is why new tires are welcomed as often as possible !

Last edited by Johan; 18th October 2011 at 07:38 PM.
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Old 19th October 2011, 02:24 PM   #17 (permalink)
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Re: Front/Rear Wheel Width

Really wide rear tires are beneficial where the hp/tq of the engine over powers skinnier rubber. And, generally, a wider tire will allow for higher lateral loading as well. Wider rubber tends to deform less when laterally loaded, however, precise management of camber through the working motion of the wheel from fully loaded to unloaded becomes increasingly critical. Related, a wider tire has difficulty evacuating water, so, if you're driving in wet conditions then a really wide tire will periodically struggle to stay in contact with the road.

As you've also seen in this thread, the delta between front and rear tire width is often used to manage understeer/oversteer, and there are notable examples on both road and track. Wider rear tires, for example, was one way that porsche tamed the 911 beast and its tendency to oversteer.
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Old 21st October 2011, 12:45 PM   #18 (permalink)
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Re: Front/Rear Wheel Width

Thank you for your input gentlemen. I conclude the function of overpowering the front tires is not a result of excess rear tire differential width but rather speed and acceleration imparting excessive lateral G forces in a turn. Wider rear tires are beneficial, if the engine produces enough torque, to reduce sudden onset of oversteer. Of course, my written conclusion is simplistic, because as Cliff points out, tire design and road condition needs also to be taken into account along with suspension geometry.

So, referencing my original post, the angel has no allies here. I think the next step is to lay hands on examples of 12" tires to see what I think would be most versatile and durable. I'm most curious about Hoosier's advertised increased sidewall stiffness in their latest series of tires. From what I've read here and elsewhere, the Dunlops would be at the end of the spectrum representing very thin side walls; therefore, durability could be an issue.

I did an internet search regarding heat cycling of race tires. As usual with the internet, not everything you read should be believed, some of the information was directed to advance commercial causes. But there was enough repetition of concepts related to cycling of race tires that I've formed a general understanding. This information coincides with what Johan was putting across so many thanks for steering me down that road.

At some point in the future, when the financial burden of an ex-wife is off my back, I'll announce to you the final configuration of a build that I've chosen.
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