39" vs 37" tires and handling issues

[len919]

Got a question. My Sport S Max Tow (4:10) came w/ 39" tires on it. They're BFG AT 39/13.50/17. I have a 2.5" AEV DualSport RT Suspension lift.

I think the 39's are actually a 38.5" circumference and weigh in at a whopping 88.8lbs.

The truck does not handle well. The best example I have is driving down a nearby country road. It's a long road with lots of twists and turns and of course it is poorly maintained as it is out in the country, so there are dips and bumps in it. It is a 55mph road. In my wife's Durango I can sail down the road at 65mph very comfortably. In the Gladiator I can't even do the speed limit. Anything over 45mph and it feels too close to losing control - when you take a corner with a dip in it, it bounces and drifts and there is a lot of steering involved to keep it in the lane - you wouldn't want to take your eyes off the road for a second. I do have a Fox Racing ATS Steering Stabilizer on it.

I ran into someone else this weekend w/ 37s and they said they do not have these issues. Another issue I have is if I accelerate quickly and let up on the gas in 2nd gear it will stay at a high RPM for a good 5 - 7 seconds before finally deciding it wants to go into 3rd gear.

I am thinking of going down to 37's (37 13.50 17) (same BFG AT tire).

Questions:

Do you think going from 39 to 37 will remedy some of these issues?
If you have 37s, how does your Jeep drive? Are you happy with it?
I'm leaning towards the 13.50.. Any reason not to get them (and go with 12.50)? I drive in sand quite a bit and like the larger footprint.

Thanks!

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[FREEZE451]

Can hit 80 mph no problem, rides life a dream. I'm on a Mojave though with 2 inch lift.

37x12.5x17

 

[Zero_Accel]

Man I was nervous about putting 37's on the stock Sport axles, 39 is too far for my comfort zone. I would take a look at the bushings, make sure there's no play in them. Larger, heavy tires can easily kill them faster, so check the knuckles, drag-link, pit-man arm, and the track bar. That may be why your road manners are so terrible, and the 37's might not fix it if they're damaged.

I do have one question, do you have a sector shaft/steering brace installed? When I went from the 35's to my 37's I noticed much more play in the steering, the steering box really starts to flex a lot more once you're in that territory, throwing the steer-smarts brace fixed that right away. Jeep drives great now, very happy with it.

 

[len919]

I do have one question, do you have a sector shaft/steering brace installed?

I do not. I'll grab a steering brace in a few. I'm shopping right now! lol.. I just hit 5000mi & all else is well..

 

[Jaxmax]

I drive on the beach a lot, and find that the wide tires take more effort then a reasonable sized tire, as you are pushing that little wall of sand in front of the tire, I’m happy with factory tires for now but will go to a little bit taller and narrower tire when replacing them. Not from experience but from reading way too much here once you go past 35” tires things need more attention and gear changes become a need instead of a want. Like others have said check everything out make sure you are not blaming the tires for the problem that is something else…..Jack

 

[Barnaby’sdad]

Have you tried running different air pressure in them? I think I’d look at the tire pressure, check that the axles are centered (only 2.5” of lift, but you never know), and check out the steering, as mentioned above.

I didn’t see it mentioned above, but what load rating are your tires? If they’re load rating E and you’re running incorrect (I.e. Low) tire pressure, that could explain why handling is sloppy.

If nothing else…dropping a tire size should address the transmission fishing for a gear or delayed shifting thing a bit. Just going from 285/70 to 315/70 (4.10’s here, as well)…It’s noticeable that the transmission has more work to do.

Personally…my rig (stock height JTR) feels more firmly planted and the steering more consistent with the BFG A/T’s than the factory Falken A/T’s, so I don’t think that I’d necessarily expect just changing out the tires to address your handling concerns (if they’re balanced, your tires should be good).

 

[len919]

what load rating are your tires?

Ohh, this is a good point. It appears the KO2 AT's in this size are Load Range D. I have been running them at 35PSI but I'll air up to 50 and see what happens. I just emailed BFG to make sure that's right for this size. I completely forgot about that - I had KO2s on a RAM before and had to fiddle w/ PSI to get the ride right... doh!

I did go ahead and order a Steer Smarts steering brace too.

 

[Griff21]

I've got the 37" load D, BFG KO2s and run them at 35-37 PSI with no issues. Still running the stock stabilizer and I have about 11K miles on the Jeep.

 

[Barnaby’sdad]

I've got the 37" load D, BFG KO2s and run them at 35-37 PSI with no issues. Still running the stock stabilizer and I have about 11K miles on the Jeep.

Granted mine are smaller (315/70), but I can notice a ride difference between 37 psi and 35 psi. Might have something to do with my setup though (Alu-Cab and full-size spare).

37-39 psi feels firm/planted. Getting down around 35 psi is at the point where mine start to feel “squishy” (how I’d put it).

That’s how I knew it was time to air up a bit this week. I hit my usual backroad and the ride felt off. YMMV.

 

[len919]

So many variables.. I just ran out and double checked and these have indeed fallen down to around 32. I'll play w/ some various PSIs and see what happens..

 

[Griff21]

Granted mine are smaller (315/70), but I can notice a ride difference between 37 psi and 35 psi. Might have something to do with my setup though (Alu-Cab and full-size spare).

37-39 psi feels firm/planted. Getting down around 35 psi is at the point where mine start to feel “squishy” (how I’d put it).

That’s how I knew it was time to air up a bit this week. I hit my usual backroad and the ride felt off. YMMV.

I'm usually carrying nothing extra, except my powertank and recovery gear. I do feel that squishy feeling when I drop below 33 PSI

 

[Randonexplosion]

Anything over 30 psi is probably way too much for a 39" tire. I run 30 psi in my 37" KO2s in my Rubi Diesel and about 24 psi in my 40s on my YJ. You really need to do a chalk test to find the appropriate pressure for your setup.

Caster angle is usually the biggest factor in loose/wandering steering. Do you happen to have geometry correction brackets installed? They helped the day to day handling of my truck quite a bit.

 

[Lunentucker]

Stop chasing the gimmick snake oil fixes and get your steering geometry back into spec.
See post above.

 

[bleda2002]

I have KM3 39's came up from 37's. Short answer is, 37's will make all of this less of an issue, but most likely the issue isnt actually the 39's other than the rev hangs. If the 39's are low mileage, you can actually sell them for quite a bit, and may be worth it to you.

For the rev hang, I'm assuming you are on 4.10's still. The transmission rev hangs if you been on the gas and let up because it anticipates you nailing it again. For 39's you will want to regear, I ran 37's for nearly 20K miles on 4.10s and was moderately ok with it, but 39's on 4.10s the transmission just doesnt like it, the gears just arent deep enough to get the tires rolling and the transmission is basically always in 2nd/3rd when accelerating exacerbating the rev hang as it feels like you're asking for a lot of power.

With the steering a few things are potentially happening and there are a few things to check:

1) If this is your first jeep/gladiator its not IFS, it will bounce, it will wiggle, the steering wheel moves more because its a solid axle despite the jeep still just going straight, you'll have to just kind of get used to that aspect a bit. Its hard to not want to correct every little jiggle of the wheel at first. The truck will roll, sway, and just not feel as planted as a low to the ground IFS running small street tires.

2) Check your pressures, they should be around 30-32 cold maybe even a touch less, the higher you go the stiffer that ride is going to be. If you are getting bounce after hitting a dip, higher pressure is just going to make this worse.

3) Check your caster, toe, and retorque all the components especially the trackbar, draglink, and tie rod.

4) Check to make sure you have the steel steering box (you probably do), its black instead of gray. If you do have the steel box, dont bother with a sector shaft brace it wont make any appreciable difference.

 

[len919]

Do you happen to have geometry correction brackets installed?

Yep, the AEV DualSport RT Suspension comes w/ control arm geometry correction brackets. Really nice kit!

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