Question Regarding Wheels/Tires/Off-Sets

[CircleTrackSHO]

On my circle track car (running an ATX) I am currently running 15" wheels with 215/60 series tires. Rules restrict me to min 60 series (so I can run 60/65/70) to a Max of 235.

On the 3/8 mile track, this works out fine.. as I am hitting ~6200 at the end of the straights and around 3600 coming out. As we get the handling better, we'll carry more speed out and push higher going in. If we get to Rev limiter, I'll go to 225 or 235 series tire to get a few extra rpm.

HOWEVER -- And this is the crux of my problem..

On the 1/4 mile track, I am hitting the REV limiter about 1/2 way down the straight away if I run first gear. If I use 2nd gear, I'm coming out of the corners at ~2900 rpm and I'm not getting into the power band till I'm 1/2 way down the straight. I'm getting killed off the corner.

So, I have two options.. run a much smaller wheel/tire combo and run in 2nd gear.. or go to a much larger wheel/tire combo and run in first gear.

Pros/Cons (as I see them) are as follows..

SMALLER WHEEL/TIRE PROS: Lowers the center of gravity of the car. Reduces size of sidewall, reducing tire roll. Lower weight tire. Lower weight wheel. 2nd gear has wider usable torque band/rpm band.

SMALLER WHEEL/TIRE CONS: Smaller tire contact patch. Less engine braking assist when in 2nd gear. Limited tire choices. Will smaller tire fit over brake calibers (we can use an offset wheel) ?

BIGGER WHEEL/TIRE PROS: Larger tire contact patch. Middle of power band coming off corner. More tire choices. Better engine braking at lift.

BIGGER WHEEL/TIRE CONS: Higher center of gravity. Loss of camber. Bigger side wall. Loss of torque at rev limiter.

SO -- question becomes, can I get a big enough tire to use first gear. What are people's experiences. Any idea suggestions. Are there smaller calibers which can be used. Can they be ground at all ? will offsets help. will 17" tires fit ? Are their steel wheels available for 17". Are their 14" wheels available.

 

[zak]

Doug Lewis at FPS in GA experimented with different engine to transaxle drive ratios in the automatic cars. There is a chain and sprockets somewhere between the gearbox itself and the crankshaft, I can not recall exactly as I'm an MTX guy. Still can't believe they won't allow manual trannies. Different sprocket ratios were used on automatic trans equipped Tauri, for the 3.0 OHV (highest gearing, should let you run in second), 3.2 SHO, 3.8 SLO, etc. You should be able to run 225 60 15s by adjusting the chain drive ratio. Hopefully you can do this on 7.5 inch wide rims or at least 7 inch wide rims (if OEM is required perhaps use a 93-95 T-bird 16 x 7 with a 225/60/16, watch you don't run the coil spring perch and sway bar end link!

The compound of the tire is far more important than a 225 vs 235 section width. Search for as low a treadwear rating as possible, performance summer tire with low void volume (tread grove area). Do they allow DOT approved tires like the Toyo RA1? Think they make it in a 225-60-15 IIRC.

I would talk to Doug anyway, have a feeling you are going to need one of his massaged transmissions for what you are trying to do - what are you doing to cool the transmission? zak

 

[drivinhard]

Refresh my memory on the rules....can you only use 1 gear? Ie, why can't you just shift to 2nd?

Taller tires may allow you to not ride the rev limiter down the straight, but I'd wager that a shorter tired (and thus geared) car is going to get off the corner better anyway, and the net result is going to be the same. What you are essentially doing is taking a drag car (from the center of the corner to the end of the straight) and switching from a 3.73 rear gear to a 3.15 or something.

If you are trying to compete with larger torque V8's with a smaller disp V6, you can do it, but you're going to need some gearing advantage. You've got an RPM advantage (likely) but the only want to take advantage of it is gearing (ie, multiple).

If you are allowed 1 gear only, then it's time to cheat and dump the rev limiter. You'll be good (safe) for ~8500 rpm.

 

[SHOVNST]

Too bad you can't run anything shorter than a 60 series.....





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[CircleTrackSHO]

Refresh my memory on the rules....can you only use 1 gear? Ie, why can't you just shift to 2nd?

Taller tires may allow you to not ride the rev limiter down the straight, but I'd wager that a shorter tired (and thus geared) car is going to get off the corner better anyway, and the net result is going to be the same. What you are essentially doing is taking a drag car (from the center of the corner to the end of the straight) and switching from a 3.73 rear gear to a 3.15 or something.

If you are trying to compete with larger torque V8's with a smaller disp V6, you can do it, but you're going to need some gearing advantage. You've got an RPM advantage (likely) but the only want to take advantage of it is gearing (ie, multiple).

If you are allowed 1 gear only, then it's time to cheat and dump the rev limiter. You'll be good (safe) for ~8500 rpm.

I could flip it into second, but historically, the time it takes for the atx to change gears is too long for the relative short length of our straight away.. talking about 200 yards here.

How hard is it to bypass the revlimiter.. that sounds like the way to go.

I have a diablo chip.. shouldn't that bypass the rev limiter anyhow ?

 

[SHOVNST]

I could flip it into second, but historically, the time it takes for the atx to change gears is too long for the relative short length of our straight away.. talking about 200 yards here.

How hard is it to bypass the revlimiter.. that sounds like the way to go.

I have a diablo chip.. shouldn't that bypass the rev limiter anyhow ?

What's the exact rule on what wheel you can run...??? Your car came with 16's wheels stock, not 15's (1st GEN did/'89-'91), but no SHO ever came with 17's....

-AND-

On the 1/4 mile track, how far from the corner are you hitting the rev limiter in first gear...???





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[CircleTrackSHO]

I would say 3/5th down the straight.. just past the start/finish line... I am fairly confident that if I could shave 800 rpm off my speed OR raise my rev's by 800 to 1000 more.. I'd be good.

In all reality.. I think it would behoove me greatly to go to a 16" 235/60. Its a 7% bigger tire, which should drop my RPM's by 7%... thus moving me from 7000 to 6510.. the extra 500 rpm might get me far enough into the corner to lift early (which I should do anyhow).

 

[SHOVNST]

Here's the breakdown of the tire sizes/diameters.....

215/60r15 [currently have] - 25.15 in
225/60r15 [fit?] - 25.62 in
235/60r15 [fit?] - 26.10 in

215/60r16 [stock tire size] - 26.15 in
225/60r16 [will fit!] - 26.62 in
235/60r16 [fit?] - 27.10 in


So the tire you're currently running is smaller than what's stock meaning you're running out of RPM's sooner than you should stock. Can you run the 16" Alloy Wheel that came stock on your SHO...??? Because the taller of a tire you go the more RPM's you'll have to play with, and if you stay in first gear the whole time you'll be walking away from the competition.....





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[CircleTrackSHO]

I can't run an alloy wheel, so I'll have to find a set of steel wheels which fit.

 

[SHOVNST]

In all reality.. I think it would behoove me greatly to go to a 16" 235/60. Its a 7% bigger tire, which should drop my RPM's by 7%... thus moving me from 7000 to 6510.. the extra 500 rpm might get me far enough into the corner to lift early (which I should do anyhow).

235/60r16's are going to be your best bet for sure, BUT if you get a set of 16" Taurus steel wheels with a factory offset (42mm?) the tire tread WILL be coming off like a cheese grader. I have 225/60r16's on my Gen 2 and the tires JUST rub a little, and it's right on the corner of the tire. The 235's WILL rub ******* the sturuts and cause problems...!!! Also, with the lesser offset the car will have a wider stance and you should get some added traction with the 235's.....





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[CircleTrackSHO]

The Taurus steel wheels are 5x108 isn't that a fairly standard size wheel ?

Aren't here other mfg wheels which are 5x108 which have a bigger offset, to get the tire out past the strut ?

Don't forget.. I have max camber/caster, so I have my strut leaning way in.. and in the corners, the tire will lean UP on the right side, away from the strut.. but on the left side, they will lean into the strut.

What about a racing wheel ?? this company http://www.mrt-wheels.com/ can pretty much make anything.

the can build off-sets http://rockcrawler-mrt.com/offset01.html

 

[SASHO91]

another option- (where Im going to pick up my volvo replicas )
16's-- http://cgi.ebay.com/16-x6-5-4-New-U...hZ017QQcategoryZ43956QQtcZphotoQQcmdZViewItem
15's-- http://cgi.ebay.com/15-x6-4-New-Uni...hZ017QQcategoryZ43955QQtcZphotoQQcmdZViewItem
http://cgi.ebay.com/15-x6-4-Origina...hZ017QQcategoryZ43955QQtcZphotoQQcmdZViewItem

 

[CircleTrackSHO]

nice.. BUT.. a 235 tire is 7" wide.. will a 6.5" work ?

 

[SASHO91]

yeah...
I've seen many people run a 235 on slicers and other 6'' wide wheels. (looks a bit funny, but works).

 

[CircleTrackSHO]

yeah...
I've seen many people run a 235 on slicers and other 6'' wide wheels. (looks a bit funny, but works).

Works on a street car going in a straight line.. how will it work on a oval track car roaring into a corner on a banked track with a competition (like, competition like.. I would never not use a non-dot rated radial tire on my race car) sticky tire ?

 

[SASHO91]

True...

I don't know of any place that makes a 7''+ wide wheel that looks stock.

Why not just run the 225's on the 6.5's?

 

[CircleTrackSHO]

need the extra .5" in diameter ... hitting the rev limiter.

 

[SASHO91]

do they make a 60 series tire in your application? 225-60

 

[SHOVNST]

Yes, but the 235/60r16 is a bigger diameter (vs. the 225/60r16's) giving him more 'gear' at higher RPM's. Which is what he's looking for....


I did a search on a few websites and for a tire in that size (235/60r16) it calls for a 6.5-8.5 wide wheel. A 7-7.5 would be ideal but I don't know if you're going to be able to locate a set of wheels that are steel that will fit with the offset and BP.....

Now that I think about it if you can only find a 6.5 wide wheel you should be able to work that into you suspensions plans and make it work to your advantage.....





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[SASHO91]

I relize that... but he is going to have to make some type of sacrafice.... either a couple hundred rpm or $$$$.
I just figured that the 225-60's would be a decent medium. As they will fit on the 6.5's and still give him a slightly bigger dia. over the 15's...

just my .02