Tire Size Help Please!

[Dzimm]

3 hours ago, Pete M said:

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

As far as I know, mine is stock.  Sits at 63" tall. 

[Eagle]

The factory service manual shows the height to the top of the roof as 63.7" for 2WD and 64.7" for 4WD. The track is specified at 57" with 6" wheels and 58" with 7" wheels.

 

If you go with the 8" Wrangler wheels you should pick up another half inch (not a full inch, because they have more backspacing), or 1" with 1/4" spacers. So let's say we can get your track to 59". You need to lower the roofline by 4.7" ... call it 5" to be safe.

 

Setting aside whatever you can gain by installing 2WD 2.5L springs, what can you get from the tires alone. You can't go by the numbers calculated from the size, because when a tire is installed on a vehicle, the vehicle weight squashes the side that's in contact with the road. Most of the tire companies used to quote a number for that in their specs, called either "static radius" or "rolling radius." It's less than the theoretical radius, but there's no way to calculate it, because it's a function of weight and tire pressure. The spreadsheet I made up for NAXJA many years ago includes the static radius for representative tires in as many sizes as I could find data for, but I wasn't looking at anything smaller than a stock XJ/MJ tire. But, to give you an idea what the order of magnitude difference might be, let's look at a common XJ/MJ tire size: 225/75R15. The theoretical tire diameter is 28.3", so the theoretical radius is 14.15". But the number I have for actual static radius is 12.8" -- so that particular tire loses 1.35" due to sidewall squat.

 

As you get into the lower profile tire series, there's just not as much sidewall there, so you won't have as big a difference between the theoretical radius and the actual, static radius.

 

Just going by theoretical (calculated) dimensions, if you're starting with a 14" radius, and 7.5" of that is the wheel, that leaves you with 6.5" of sidewall. To get 5 inches out of that, you're down to a 1.5" sidewall. I don't think that's going to happen, so it's time to start looking at your springs.

[Dzimm]

32 minutes ago, Eagle said:

The factory service manual shows the height to the top of the roof as 63.7" for 2WD and 64.7" for 4WD. The track is specified at 57" with 6" wheels and 58" with 7" wheels.

 

If you go with the 8" Wrangler wheels you should pick up another half inch (not a full inch, because they have more backspacing), or 1" with 1/4" spacers. So let's say we can get your track to 59". You need to lower the roofline by 4.7" ... call it 5" to be safe.

 

Setting aside whatever you can gain by installing 2WD 2.5L springs, what can you get from the tires alone. You can't go by the numbers calculated from the size, because when a tire is installed on a vehicle, the vehicle weight squashes the side that's in contact with the road. Most of the tire companies used to quote a number for that in their specs, called either "static radius" or "rolling radius." It's less than the theoretical radius, but there's no way to calculate it, because it's a function of weight and tire pressure. The spreadsheet I made up for NAXJA many years ago includes the static radius for representative tires in as many sizes as I could find data for, but I wasn't looking at anything smaller than a stock XJ/MJ tire. But, to give you an idea what the order of magnitude difference might be, let's look at a common XJ/MJ tire size: 225/75R15. The theoretical tire diameter is 28.3", so the theoretical radius is 14.15". But the number I have for actual static radius is 12.8" -- so that particular tire loses 1.35" due to sidewall squat.

 

As you get into the lower profile tire series, there's just not as much sidewall there, so you won't have as big a difference between the theoretical radius and the actual, static radius.

 

Just going by theoretical (calculated) dimensions, if you're starting with a 14" radius, and 7.5" of that is the wheel, that leaves you with 6.5" of sidewall. To get 5 inches out of that, you're down to a 1.5" sidewall. I don't think that's going to happen, so it's time to start looking at your springs.

Yup that's what I'm thinking.  Very good summary and explanation! 

[Jeep Driver]

12 hours ago, Dzimm said:

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. 

 "Trophies", I speak to the bigger picture and I assume you should know what I mean. I understand what autocross is, I'm speaking to the competitive nature of man. Whether you compete against yourself, the clock, or another driver.........you are competing, men don't compete because they are curious and most of us are not half-hearted, we're in or we're out. 

 

As I alluded to earlier, from Solo's official site, this is where the tire meets the road, get involved, volunteer, this is where you will learn-

Quote

 

Help Out: One of the things that makes autocross less expensive than other forms of motorsports is that competitors are also organizers and helpers. That person who helped you sign the waiver? They’re probably driving today. The person who helped you register? Yep, also driving. The tech inspector? Well, you get the idea...

There will be a time period during the day where you will need to help out. Helping out might include working in the timing trailer, helping direct cars from grid to the course or re-setting cones on course when they get knocked down.

Helpful tip – Work times and drive times are most often announced in the drivers meeting. That’s when you find out what group you’re driving in, which group you’re working in and which groups you will have some down time to relax, socialize, or watch.

 

 

Even Solo states that autocross is the 'less' expensive form of racing. 

 

Here, these are the tires I'd be looking at or similar. Perfect for a 15x7 wheel and will net you a 2" drop in overall height. (4" reduction)

https://www.summitracing.com/parts/fal-34274?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-falken-tire&gclid=CjwKCAjwtvnbBRA5EiwAcRvnpkdBTbtqFe4fd4SOC7H_oHMOLuGE6zSShkfo9kAxZ-TZU6ZbkmVYqBoCtKMQAvD_BwE

 

Suspension and steering, that's what I'd be concentrating on. Other than battery hold down and helmet, apparently they are looking to see if it's track worthy, I'd tidy it up, wiring and so forth. 

[Pete M]

someday I'd like to do some racing in my dakota rt.  It'll be in the bone-stock class though.