Question about wheel spacers

[erickdiaz019]

When should you get wheel spacers from a safety/physics point of view? Obviously when you lift the vehicle, the center of mass is higher and is more prone to rollover. At what point does it become necessary & a good idea to increase the width of your stance to offset the increase in height? Is there a lift size where this becomes important?

I have a Sport JL 4 door. My hopes are to put 33” tires and a 2” lift

thanks!!

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[azwjowner]

When should you get wheel spacers from a safety/physics point of view? Obviously when you lift the vehicle, the center of mass is higher and is more prone to rollover. At what point does it become necessary & a good idea to increase the width of your stance to offset the increase in height? Is there a lift size where this becomes important?

I have a Sport JL 4 door

thanks!!

The technically precise answer would be to figure out the height of the center of gravity of the Jeep, then increase the track width to maintain the same stability factor calculation as you increase the height of that center of gravity by lifting the Jeep: https://securitydriver.com/11/vehicle-static-stability-factor/

 

[CarbonSteel]

There is an argument that spacers in and of themselves would not increase safety as they themselves can have issues.

The way the vehicle is driven should be tailored to the lift versus attempting to offset potential rollover with wheel spacers.

 

[erickdiaz019]

The technically precise answer would be to figure out the height of the center of gravity of the Jeep, then increase the track width to maintain the same stability factor calculation as you increase the height of that center of gravity by lifting the Jeep: https://securitydriver.com/11/vehicle-static-stability-factor/

Thank you for the link, that is very interesting. My plans are to put 33” tires and 2” lift on my Sport Wrangler.

I’ll probably use that formula to gauge how much my stability changes

 

[blnewt]

Thank you for the link, that is very interesting. My plans are to put 33” tires and 2” lift on my Sport Wrangler.

I’ll probably use that formula to gauge how much my stability changes

You might consider a low center of gravity build, try to get clearance from higher fenders and a small spring lift. I did this w/ my JL Sport and got 17x8.5 +10 offset wheels w/ 295/70/17 tires. It's been a good combo for the trails I've ran. Here's a pic in it's current form and a thread on Rubicon suspension swap.
https://www.jlwranglerforums.com/forum/threads/rubicon-suspension-on-2-door-sport.40700/

 

[azwjowner]

Thank you for the link, that is very interesting. My plans are to put 33” tires and 2” lift on my Sport Wrangler.

I’ll probably use that formula to gauge how much my stability changes

This PDF gives some static stability factors and centers of gravity for 2020 models. It has a 2 door JL. https://lindseyresearch.com/wp-cont...lover-Stability-Measurements-Final-Report.pdf

So the 2 door JL Sport has a SSF of 1.09 and CG of 28.89 inches with a driver only.

If you lift a Jeep, we don't quite raise the center of gravity the exact same distance because the unsprung weight of the axles doesn't raise any (unless you add larger tires, in which case it raises by half the increased tire diameter). However, the axle weight is very low compared to the entire frame and body of the Jeep, so we can ignore it and assume that the entire Jeep weight is elevated by the amount of the lift -- which will actually make our calculations more conservative and safer.

T = SSF * 2H. So if we lift 3 inches (I'm assuming we aren't adding bigger tires, otherwise add that in too), T' = 1.09 x 2 x (28.89+3). That gives us a new track width of 69.5 inches to keep the same static rollover resistance with 3 inches extra height.

To achieve a track width of 69.5 inches, on a Sport with 66.5 inch wide axles, we need 3 inches total extra width, or 1.5 inches per wheel, which is 38.1mm. -38mm is a pretty crazy offset, but we can use wider tires to offset (no pun intended) some of that extra width. A 0mm offset wheel with 315mm tires (70mm wider than stock) might be a good choice here that gets us at least close.

I don't think I've seen anyone else do these calculations. This is actually an important calculation for people who want to safely lift their Jeeps.

 

[azwjowner]

I found JLUR data. 2019 JLUR was 29.43" CG, SSF 1.10. https://lindseyresearch.com/wp-cont...-0451-2019_NHTSA_NCAP_Final_Report_Public.pdf

Haven't found a 4 door Sport though. Nevertheless, the 2 door data would be fairly close because the width and height are the same on 2 doors vs. 4 doors.

 

[azwjowner]

Finally, consider that the above calculations ignore tire width. Although wider tires aren't the same as wider axles for resisting rollover, because they are squishy and give way, a wider tire will still offer at least a bit of resistance to pressure on the tread further out than its centerline. So if you put on tires that are 2 inches wider than stock, is it as good as increasing the track width by 2 inches total? No, but I'm sure it helps somewhat.

 

[blnewt]

Finally, consider that the above calculations ignore tire width. Although wider tires aren't the same as wider axles for resisting rollover, because they are squishy and give way, a wider tire will still offer at least a bit of resistance to pressure on the tread further out than its centerline. So if you put on tires that are 2 inches wider than stock, is it as good as increasing the track width by 2 inches total? No, but I'm sure it helps somewhat.

Nice info you provided in your posts on this thread, quite a few considerations when lifting your vehicle. I'd also add the effect of altering scrub radius when going w/ less positive wheel offset in the front on your steering components. It's a bit of a "rob Peter to pay Paul" trying to keep offset moderate but also keeping your tires from contacting suspension & fenders when lifting your Jeep.
My +10 offset on the wider 8.5" wheel seems to walk the line pretty well, I had to add a 1" bump stop in front to keep off the metal fender liners but tires aren't poking much. Keeps the scrub radius close to stock so my steering is still pretty much on point, and hopefully won't cause much of an increase in bearing and ball joint wear.

Hopefully I can get some time to review those links you posted, seems to be well worth the effort there, thanks for that!

Here's a pretty good link on scrub radius FWIW
https://low-offset.com/workshop/scrub-radius-explained/

 

[azwjowner]

Nice info you provided in your posts on this thread, quite a few considerations when lifting your vehicle. I'd also add the effect of altering scrub radius when going w/ less positive wheel offset in the front on your steering components. It's a bit of a "rob Peter to pay Paul" trying to keep offset moderate but also keeping your tires from contacting suspension & fenders when lifting your Jeep.
My +10 offset on the wider 8.5" wheel seems to walk the line pretty well, I had to add a 1" bump stop in front to keep off the metal fender liners but tires aren't poking much. Keeps the scrub radius close to stock so my steering is still pretty much on point, and hopefully won't cause much of an increase in bearing and ball joint wear.

Hopefully I can get some time to review those links you posted, seems to be well worth the effort there, thanks for that!

Here's a pretty good link on scrub radius FWIW
https://low-offset.com/workshop/scrub-radius-explained/

Yes. Rubicon axles have a huge advantage here because you gain 1.5" of track width without any change to scrub radius.

 

[azwjowner]

I've corrected a math error in my original post and also written up this analysis in more depth in a new thread: https://www.jlwranglerforums.com/fo...reduce-rollover-risk-for-lifted-jeeps.101905/

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