Tire Size Help Please!

[Dzimm]

OK so for my autocross truck, I want to try to bring it into legal racing requirements while being stock so I can get an idea of what the change will be after suspension changes and a turbo.  In order to do this I need to bring the truck height down 3.5" by only changing wheels and tires to remain on the stock street classes.  I have zero knowledge of tire sizes and am struggling real hard to understand it and figure out my course of action. 

 

Current setup is P225-70r15 which is roughly 27.5" tall.  Track width is 58" and truck height is 61.5".  

 

To be legal, the track width must be equal to or wider than the height of the truck.  I am also only allowed to change stock backspacing by .275" and rim size by 1".   I've attached a screenshot of the applicable rules for those of you who want a better explination.  I've talked to the rules guy in the local group and he wasn't really much help. 

 

Basically I need the truck to drop or become wider, that equates to a 3.5" change.  

 

Stock backspacing is 5.25" according to my research and stock rim size is 15".

 

So my thought is drop the rim size to 14" to get me into smaller tire sizes and find rims with 5" of backspacing.  Anyone know of a tire size that is roughly 3.5" shorter than a P225-70r15?  

[Jeep Driver]

The answer to the equation is (and), not either, or. 

 

Tracking width is center to center, correct?

 

 15X8" wheel and tire with aspect ration of 60 or 50 or even 40 will gain you width and drop, the rest in the suspension. 

 

 

The whole height must equal width thing sounds like BS to me. So, you could not race a Fiat in the stock class?

 

 

I'm sure there is a local forum for you, these are the kinds of people to ask- http://etrscca.org/etrsccaforum/viewforum.php?f=13&sid=b10a509d34376cf8f8917e98f1dd9070

 

and apparently there are multiple classes within Solo...........don't know what rule set you are trying to squeeze into. 

 

 

 

Edit: just skimmed a thread...........turbo tossed you out of stock class anyway...........

You really need to get involved locally. 

[Pete M]

yeah, you'll want to get input from other racers or rules-people.  I'd hate to see you spend money only to find out the regulations actually mean this instead of that.  

 

but also, this is my fav tire size calculator.  

https://tiresize.com/converter/

 

and then I use tirerack.com to see if the size I wanted actually exists.  by the way, 195/45/r15 are the shortest but still reasonable width tire I could find.  specs to 21.9" tall. 

 

finding 14" rims in a 5on4.5 pattern with the correct backspacing might be the hardest part.  

 

edit: and perusing the 14" tire sizes I don't see any tire that would be shorter than the 195/45/r15 anyways.   

[Eagle]

AMC cars from the 60s and 70 used 14-inch wheels with the same bolt pattern that the Cherokee and Comanche use, but I'm not sure a 14-inch AMC rim will clear the disk brakes on the front. The factory rims on the XJ and MJ are 5-1/4" backspacing. It's not hard to find wheels that widen that track -- there are (AFAIK) NO aftermarket rims that provide the factory backspacing, most are in the 40" to 4-1/2" range.

 

Be careful about going too small in combination with wide rims to increase the track. The first number in a metric tire size (the 195 or 225 part) is the cross-section width at the sidewall, in millimeters. Pete mentioned a 195/45-15. 195mm divided by 25.4 mm/inch works out to a tire width (not tread width) of 7.68 inches. Jeep mounted the 195/75-15 base model tires on 6-inch rims. The 45 series have much less sidewall to play with than 75 series. If you try to mount those little tires on very wide rims, you may have other problems. 7-inch rims will probably work, but I think 8-inch might be pushing the envelope. Most tires list the acceptable wheel width range, so be sure to check that.

[Dzimm]

There is a local forum but it has had one post since 2014 and that one had no responses.  I talked to the rules guy and he basically gave me the run down on the rollover rule, which is the one on vehicle height based on track width and said the only modification I really want to make right now is changing tires and wheels to get the truck height down.   As for talking to other racers I definitely am planning on it for modifications moving forward, however the next race I can make it to is in October. 

 

As for the classing, I know the turbo will put me out of anything street but I wanted to start small and work my way into the higher classes.  The rules guy told me that would be the best approach to learn and develop the truck and still be competitive along the way.  So plans for lowered suspension are midyear next year and turbo is end of next year at the earliest.  

 

I did find the tire rack tools as well as tiresize.com has a chart of sizes.  My options are looking to be 195-60r15, which is 24.2" tall and 7.7" wide and require a different wheel or 175-65r15, which is 24" tall and 6.9" wide and I can use the wheel I have.  I was really hoping to find a tire in the 8" width range or higher. 

[Dzimm]

8 hours ago, Jeep Driver said:

The answer to the equation is (and), not either, or. 

 

Tracking width is center to center, correct?

 

 15X8" wheel and tire with aspect ration of 60 or 50 or even 40 will gain you width and drop, the rest in the suspension. 

 

 

The whole height must equal width thing sounds like BS to me. So, you could not race a Fiat in the stock class?

 

 

I'm sure there is a local forum for you, these are the kinds of people to ask- http://etrscca.org/etrsccaforum/viewforum.php?f=13&sid=b10a509d34376cf8f8917e98f1dd9070

 

and apparently there are multiple classes within Solo...........don't know what rule set you are trying to squeeze into. 

 

 

 

Edit: just skimmed a thread...........turbo tossed you out of stock class anyway...........

You really need to get involved locally. 

Yes track width is center to center.  

 

Would changing tire size change track width?  I thought that would only change by changing wheels because your tire should sit centered on the wheel correct? 

 

I attached the rules relating to width/height.  Yeah I'm not sure tho because there were some minis racing at the event last weekend and they looked pretty dang close to too tall. 

 

The turbo is coming later.  I want to start stock and build it to an end goal of lowered and turbo, which would put me into modified at least. 

[Pete M]

tire size will not change track width as the tire will want to be centered on the rim.   

 

what was wrong with the 195/45?  

[Jeep Driver]

I'll disagree with the tracking change to this extent

If you take two stock wheels one a 6in width and 1 with 8 in width both with five and a quarter backspacing

You have effectively moved tracking with out 1 inch in each Direction

[Pete M]

yes.  changing wheels can change the track width.   

[Dzimm]

1 hour ago, Pete M said:

tire size will not change track width as the tire will want to be centered on the rim.   

 

what was wrong with the 195/45?  

Ah that size wasn't on the chart.  Definitely a good option, tire rack only shows two tires in that size, one of which is on closeout.  I'll have to go to a local tire shop and see what they have available in these sizes. 

[Dzimm]

Welp so new problem.... The interwebs says my current tire size equates to 27.5" well thats not the case, they actually sit at 26" tall.  This means I need 5" smaller tires.  That's a 21" tire with no other changes.  Basically now my only choice to run stock is to find the smallest tire possible AND find production springs that came stock on an MJ or XJ that will lower mine slightly to help this situation.   

 

I saw in another post here, there was a list of about 20 coil spring part numbers that came with these trucks.   Anyone know of a spring that would be softer or shorter than the springs that come on a 2wd MJ with 4.0?   A spring from a 2.5l perhaps? 

[Pete M]

I imagine 2wd 2.5 would have to be the shortest  

 

are you measuring the tire diameter? or the height of the top of the tire off the pavement (presuming there's a truck squishing it down).   

 

are you trying to lower your truck 5"?  cause that's a 10" reduction in tire diameter.    at that point I think you're just driving around on rims.  

[Eagle]

3 hours ago, Dzimm said:

I did find the tire rack tools as well as tiresize.com has a chart of sizes.  My options are looking to be 195-60r15, which is 24.2" tall and 7.7" wide and require a different wheel or 175-65r15, which is 24" tall and 6.9" wide and I can use the wheel I have.  I was really hoping to find a tire in the 8" width range or higher. 

 

8-inch cross-section width, or 8-inch tread width? 8-inch cross section works out to 203.2mm, so a 205 x anything is an 8-inch tire ... but the tread width is never the same as the cross section width. What are you running now for wheels? The factory 195/75-15s and 205/75-15s came on 6-inch rims, and everything else came on 7-inch rims. I can't imagine that you can't fit 195s on Jeep rims.

 

Would changing tire size change track width?  I thought that would only change by changing wheels because your tire should sit centered on the wheel correct? 

 

Correct. Tires are centered on the wheels. To increase track, either go to a wider wheel with the same backspacing, or go to a wheel with less backspacing. Changing tire size in the same wheels does not change the track dimension.

[Dzimm]

49 minutes ago, Pete M said:

I imagine 2wd 2.5 would have to be the shortest  

 

are you measuring the tire diameter? or the height of the top of the tire off the pavement (presuming there's a truck squishing it down).   

 

are you trying to lower your truck 5"?  cause that's a 10" reduction in tire diameter.    at that point I think you're just driving around on rims.  

I measured off the pavement since that's the direction I need to go.  I imagine tires of a smaller size will squish relatively close to the same. 

 

I didn't even think about the reduction in tire size only applying to half the tire.. 

 

I'm starting to think there is no way to run an MJ stock.. 

[Dzimm]

22 minutes ago, Eagle said:

 

8-inch cross-section width, or 8-inch tread width? 8-inch cross section works out to 203.2mm, so a 205 x anything is an 8-inch tire ... but the tread width is never the same as the cross section width. What are you running now for wheels? The factory 195/75-15s and 205/75-15s came on 6-inch rims, and everything else came on 7-inch rims. I can't imagine that you can't fit 195s on Jeep rims.

 

 

 

 

Correct. Tires are centered on the wheels. To increase track, either go to a wider wheel with the same backspacing, or go to a wheel with less backspacing. Changing tire size in the same wheels does not change the track dimension.

They are the super basic stock wheels.  

[Eagle]

1 hour ago, Dzimm said:

They are the super basic stock wheels.  

 

Nope. Those are not super basic wheels. Those are the 15x7 factory wagon wheels. The base wheels are 15x6 and have nine rectangular slots around the periphery of the web where it meets the rim.

 

 

As I mentioned above, there are NO aftermarket rims that provide 5-1/4" backspacing like the factory rims. That means you can buy the cheapest steel aftermarket wagon wheels and increase your track by at least two inches, maybe more -- without spacers. Do the rules require that you run factory wheels?

[Eagle]

Check out springs for TJ Wranglers. They are the same diameter and will fit an MJ, but my understanding (subject to verification) is that they are shorter. Or don't your rules allow using springs out of a different vehicle?

[Dzimm]

6 minutes ago, Eagle said:

Check out springs for TJ Wranglers. They are the same diameter and will fit an MJ, but my understanding (subject to verification) is that they are shorter. Or don't your rules allow using springs out of a different vehicle?

The rules state that backspacing cannot be changed by more than 1/4" so no go on extending track width and the springs have to be a production part for the same vehicle unfortunately. 

[Eagle]

Can you use wheels from another vehicle if the backspacing matches? Wheels for the TJ Wrangler (at least some of them) are 15x8, so that would gain an inch for you compared to what you have now.

[Dzimm]

8 minutes ago, Eagle said:

Can you use wheels from another vehicle if the backspacing matches? Wheels for the TJ Wrangler (at least some of them) are 15x8, so that would gain an inch for you compared to what you have now.

I could do that.  Backspacing is the only thing that is talked about for wheels. 

[Jeep Driver]

1 hour ago, Dzimm said:

The rules state that backspacing cannot be changed by more than 1/4" so no go on extending track width and the springs have to be a production part for the same vehicle unfortunately. 

There are plenty of aftermarket wheels with 5" BS, mine are 5", my new ones will be 5". 

You are allowed .25" so it's a moot point. 

 

To make the point further moot.....

You are allowed spacers, you can use spacers to come out at 5" or 5.25" or 5.5", you can use any wheel/spacer combination.

 

As for springs and other dilemmas..........

If you need a 2" drop leaf pack, Call GS and order a custom pack. 

 

This is racing, racing is expensive, cobbling crap together will got gain you trophies. 

 

I guarantee you that you will fail a tech inspection for the most ridiculous oversight and it won't be for a leaf missing in a pack or you are off 1" in tracking. 

 

 

 

Take your truck to the track as is, pay your $50 entrance fee, get your first tech inspect knowing you will fail..........then you'll know what they are looking for. 

[Dzimm]

2 minutes ago, Jeep Driver said:

There are plenty of aftermarket wheels with 5" BS, mine are 5", my new ones will be 5". 

You are allowed .25" so it's a moot point. 

 

To make the point further moot.....

You are allowed spacers, you can use spacers to come out at 5" or 5.25" or 5.5", you can use a wheel with 3.5" BS and a 1.5" spacer or any combination thereof. 

 

As for springs and other dilemmas..........

If you need a 2" drop leaf pack, Call GS and order a custom pack. 

 

This is racing, racing is expensive, cobbling crap together will got gain you trophies. 

 

I guarantee you that you will fail a tech inspection for the most ridiculous oversight and it won't be for a leaf missing in a pack or you are off 1" in tracking. 

 

 

 

Take your truck to the track as is, pay your $50 entrance fee, get your first tech inspect knowing you will fail..........then you'll know what they are looking for. 

I'm not in it for trophies, I'm in it for fun and to see what vehicles I can compete with in a jeep.   Having said that, I'm not in it to spend a bunch of money.  I want to build the cheapest possible truck to run with.  I know I could drop a ton of money into it and build a true racecar but that's not what it's about.  That's not what autocross is about either. 

 

I've talked to the rules guy and I know what it takes to pass tech and I know what's gonna fail me, and that's the height of the vehicle.  That's all I'm trying to fix right now and it doesn't seem that is gonna happen and be able to stay in street classes. 

[Pete M]

is a stock MJ too tall? or is it your MJ that is too tall?

[Eagle]

According to JeepForum.com, the TJ Canyon wheels are:

 

Canyon
* 15x8 aluminum
* 5 on 4.5" bolt pattern
* Backspacing = 5.5"
* Optional wheel with 30" tire package
* Offset = +25 mm

 

Be careful, because the ZJ Grand Cherokees had a wheel that looks almost identical, but it's only 15x7. Here's the 8" Wrangler version:

 

[Jeep Driver]

There are Canyons and there are Deep Canyons. 

 

From what I understand.