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Guest Message by DevFuse

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Just mulling an idea over in my head here..

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8 replies to this topic

#1 Mantis_Toboggan


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Posted 11 June 2010 - 04:07 PM

I think I may have a tentative answer to tire sizing per lift.
This is a rough guide on how much lift you will need to fit relative to original clearance. There are of course exceptions, but this guide should maximise your articulation.

Radius of original tire package:

carcass width/percent sidewall/rim size
take the carcass width, multiply by sidewall percent, divide by two, convert quantity to inches, and
add to rim size divided by two.

Here's a working example:
185(.70) = 129.5mm

This is how much of the tire is sidewall, because the sidewall exists on both sides, divide by two.
(129.5)/2 = 64.75mm

This is how much your sidewall lifts the rim above the ground, also how much your tire projects above the rim, under the fenderwell.

Convert this quantity to inches:
(64.75mm)/(25.4(mm/in)) = 2.55 inches

Add this quantity to the rim diameter divided by two
13/2 = 6.5in

Total projection of tire and wheel package above axle towards fender is:

**This is the initial value**

Do the same calculations for the larger tire you would like to equip, and take the difference (new package vs. stock package).
This will tell you how much more radius the new package takes up. You will need to lift your chassis by this much to retain the stock suspension articulation.

Edited by Mantis_Toboggan, 12 June 2010 - 01:49 AM.

#2 Mantis_Toboggan


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Posted 11 June 2010 - 04:09 PM


For offroad tires with sizes like 31x10.5, you only need to divide the first number in half.
31 is the overall tire height, so it's radius is 15.5 inches.

To equip this tire, you need to lift your car by 15.5-9.05 = 6.45in

#3 2.5_IMP


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Posted 11 June 2010 - 06:18 PM

If only 31" tires were actually 31". 31" BFG A/Ts are 30.7. You'll find that most offroad tires are smaller than what it says on the sidewall.

All your math makes my head hurt. :)

#4 Mantis_Toboggan


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Posted 12 June 2010 - 01:44 AM

Haha yeah, there are definitely variations between manufacturers. But this is one of the reasons why it is a tentative calculation.
Perhaps I should go back through the mess to make scalars that are easier to work with. I think I'll draw up some diagrams to work with tomorrow. Anybody on here a programmer? We totally could make a USMG tire calculator.

Proper lifts make the tires too big for the wheel wells (without trimming). since my subie is still stock, I will take radius calculations to see how much material must be removed from the leading and trailing edges of the fenderwell to make things ride like they were when they were stock. I also have ideas on minimising the tire rub at max steering lock.

God I love math!

Edited by Mantis_Toboggan, 12 June 2010 - 02:00 AM.

#5 Uberoo


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Posted 12 June 2010 - 05:00 AM

your math works except for one minor problem.the stock tires are narrow and they tuck into the fender wells meaning that they have a smaller fender well then what you are accounting for.Normal offroad tires are much wider than the 175-185's these cars came with. then the tires interfere at the outer edge of the fender well,where the stock tires tucked into.

stock tire size is about 23" to fit a 27" tire with your calculation would need about a 4" lift.However most 4"/27" tires rubbed when turning or when the suspension moved they needed either cutting or bashing to fit.so in reality to fit 27" tires with no cutting/bashing and no rubbing needs about a 5.5-6" lift.

a better way to figure out lift height would be to measure how far from the edge of the tire to the fender is at stock height. for example lets say it is 3.5" and you want to run a 31 which is roughly 8" taller than stock.so 4" closer to the fender well.so to figure out the lift height add the half the difference in tire size plus the stock fender well clearance.in this case:
4" + 3.5" = 7.5" of lift to fit the tires with no clearancing needed.

although I am not sure what the stock distance from tire to fender well is,but if you have a stock car it should be easy to measure.

#6 Mantis_Toboggan


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Posted 21 June 2010 - 05:05 PM

Like I said before, this is a tentative guide.
Actually, I'm about to start working on the tire rub issue. I actually made note of it in one of my earlier posts. I'm thinking longer lower control arms instead of offset upper strut mountings will be the solution. This will push the front tires away from the frame rails, the problem is, I think there will need to be longer cv's and inner tie-rods.
But, see, that's a whole different issue, it addresses what type of lift you have.

#7 monstaru


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Posted 21 June 2010 - 06:43 PM

just remember, all of things have been contemplated, some of them even manufactured, and all of the same "fuse" issues become apparent.street driveability usually diminishes the more fabwork you do to these cars.

your not the first, won't be the last, and probably not the only one that will get tired of trying to go "further " with these cars....
the wheel was already invented.just roll with it.....

its all good, build something.its fun.cheers, brian

#8 TeamCF


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Posted 22 June 2010 - 04:24 PM

Can always take the easy way out and go to the manufacturers web site and look up exact dimensions of your stock tires and the ones you want to run.
Most have the specs of all their tires available. :)
Then just compare.

#9 GeneralDisorder


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Posted 23 June 2010 - 11:10 AM

Wheel offset plays a part as well - the larger the offset toward the outside of the car the larger the sweep of the outside and inside edges of the tire when turning. And while the tire/wheel combo may fit, it may rub on turns.

Larger tires are not always better. While they may look cool, they decrease your gearing and power, and when fit to a car with only barely enough lift to clear them they will limit suspension travel. It is much better to pick a "sensible" size of tire for the amount of lift you have rather than the largest size possible to fit.

For example - a 29" or 30" tire is a much more sensible size for a 6" lift. And to practically fit a 33" tire you really need 10" to 12" of lift.


Edited by GeneralDisorder, 23 June 2010 - 11:13 AM.

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