Gear ratio poll for 37” tires on 17” wheels 3.6 gasser

[Greg_L]

I’d just love to pick your brain and learn how to do gears. It will void the warranty for the gears. I had Jeep agree to warranty my locker sensor even though I had regeared. I didn’t take them up on it though as they were going to have it too long and I fixed it myself.

Yeah I think I'll just hold off. I do not wanna void my warranty. Mostly because my wife will void my life.

Setting up gears requires some specific tooling and ability to measure in the thousandths. Micrometers, dial indicators, various torque wrenches, maybe a cutoff wheel, and most importantly...a bearing press. This is not stuff you can do with a hammer and crescent wrench in the driveway. It also takes a lot of care and patience. It takes a lot of measuring and mocking up before final assembly. And final assembly requires extreme attention to detail. With experience it becomes pretty easy even though the job itself is large. Reading the contact patterns and what to do with that info is crucial.

Although I'm not any kind of brand loyalist...I will say the Ford 9-inch is probably the best for quick and easy setups. They use minimal shims yet the same rules apply. I used to keep about eight 9-inch pumpkins with various gear ratios in constant rotation for a clients SCCA circle track car. The track length and straightaways would dictate which gear set he used. And with drag racing you'd basically math it out so your engine is running out of RPMs in final drive as you go through the lights. Too much gear will have you bouncing off the rev limiter. Too little and you're leaving power on the table. It was a dance between tire size at speed and final drive ratio. Tires grow a lot at 150-200 mph. Anyway, with the ford 9 inch you just plop the whole pumpkin on a bench and off you go. You could service it pretty easily even in the pits, compared to a GM or Dana model where you gotta be underneath the vehicle pretty much the whole time. That sucks. Honestly I'd rather just swap in an entire different rear end on those models.

So if you wanna learn and practice, I'd suggest finding an 8 or 9 inch ford pumpkin...maybe from a junkyard or something...and just rip it apart and rebuild it correctly and incorrectly so you can see how it affects the gear contact pattern.

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[WILDHOBO]

Yeah I think I'll just hold off. I do not wanna void my warranty. Mostly because my wife will void my life.

Setting up gears requires some specific tooling and ability to measure in the thousandths. Micrometers, dial indicators, various torque wrenches, maybe a cutoff wheel, and most importantly...a bearing press. This is not stuff you can do with a hammer and crescent wrench in the driveway. It also takes a lot of care and patience. It takes a lot of measuring and mocking up before final assembly. And final assembly requires extreme attention to detail. With experience it becomes pretty easy even though the job itself is large. Reading the contact patterns and what to do with that info is crucial.

Although I'm not any kind of brand loyalist...I will say the Ford 9-inch is probably the best for quick and easy setups. They use minimal shims yet the same rules apply. I used to keep about eight 9-inch pumpkins with various gear ratios in constant rotation for a clients SCCA circle track car. The track length and straightaways would dictate which gear set he used. And with drag racing you'd basically math it out so your engine is running out of RPMs in final drive as you go through the lights. Too much gear will have you bouncing off the rev limiter. Too little and you're leaving power on the table. It was a dance between tire size at speed and final drive ratio. Tires grow a lot at 150-200 mph. Anyway, with the ford 9 inch you just plop the whole pumpkin on a bench and off you go. You could service it pretty easily even in the pits, compared to a GM or Dana model where you gotta be underneath the vehicle pretty much the whole time. That sucks. Honestly I'd rather just swap in an entire different rear end on those models.

So if you wanna learn and practice, I'd suggest finding an 8 or 9 inch ford pumpkin...maybe from a junkyard or something...and just rip it apart and rebuild it correctly and incorrectly so you can see how it affects the gear contact pattern.

I love it. Thanks. I have no problem doing something slowly and methodically. I’ve rebuilt engines in my living room with perfect results. I was single, so that’ll never happen again. And I have no problem buying the right tools. I’d even pay someone to help me the first time. It’s on my list of things to learn.

 

[Greg_L]

I love it. Thanks. I have no problem doing something slowly and methodically. I’ve rebuilt engines in my living room with perfect results. I was single, so that’ll never happen again. And I have no problem buying the right tools. I’d even pay someone to help me the first time. It’s on my list of things to learn.

One thing I've learned about our Jeep/Dana setups that I don't recall seeing before is that the pinion depth shims go between the housing and the main pinion bearing as opposed to the more common way of having those shims sandwiched between the pinion itself and the bearing. Getting that pinion depth right, and the bearing preload/crush sleeve, is super crucial. It sucks to have to keep pressing the pinion bearing on and off to swap shims to get the depth right. But with the Jeep/Dana pinion setup you could use an old bearing race and sand/grind it down a few thousands so it loses it's interference fit and it's easily popped in and out just for mock-ups. I love that. That theoretically makes the pinion portion of the setup so much easier. Once you find the right pinion depth based off the gear contact pattern via mockups you can do your final pinion assembly with the proper fresh bearings and crush sleeve. That's where you need to really crank down the pinion nut to get the crush sleeve crushing...but not too much because you need to measure rotational drag (preload) with an in/lb beam or gauged torque wrench. It takes like 120 ft/lbs to get a crush sleeve crushing...but you're only wanting about 15 in/lbs of rotational drag on the pinion. It's a delicate dance.

And we haven't even gotten to carrier preload and backlash yet.

 

[WILDHOBO]

One thing I've learned about our Jeep/Dana setups that I don't recall seeing before is that the pinion depth shims go between the housing and the main pinion bearing as opposed to the more common way of having those shims sandwiched between the pinion itself and the bearing. Getting that pinion depth right, and the bearing preload/crush sleeve, is super crucial. It sucks to have to keep pressing the pinion bearing on and off to swap shims to get the depth right. But with the Jeep/Dana pinion setup you could use an old bearing race and sand/grind it down a few thousands so it loses it's interference fit and it's easily popped in and out just for mock-ups. I love that. That theoretically makes the pinion portion of the setup so much easier. Once you find the right pinion depth based off the gear contact pattern via mockups you can do your final pinion assembly with the proper fresh bearings and crush sleeve. That's where you need to really crank down the pinion nut to get the crush sleeve crushing...but not too much because you need to measure rotational drag (preload) with an in/lb beam or gauged torque wrench. It takes like 120 ft/lbs to get a crush sleeve crushing...but you're only wanting about 15 in/lbs of rotational drag on the pinion. It's a delicate dance.

And we haven't even gotten to carrier preload and backlash yet.

All of that makes perfect sense. I’m pretty analytical, and ask questions before going forward if I don’t know the answer. Sounds like it’s definitely something I can learn. If/when my 5.13’s ever fail, I think I’ll tackle the next gears myself. By then I’ll have a garage big enough to do it at my own pace.

 

[jwilson2899]

We don't talk about it much here because it's negligible in this application in my opinion. Racing applications it is huge, even carving corners it is very important. You probably aren't doing repeated hard accelerating and braking or trying to carve corners at speed. A 5k pound box on wheels I don't think you'd feel or be able to measure any kind of difference between 2 similar sized tires with a 6lb difference.

I disagree completely as does my truck. I was running 35" Wildpeak AT3Ws that weighed 71 pounds each. I swapped them for 37 KO2s that are 65 pounds. That 6 pounds per tire made a big difference in the truck. I lost almost no power despite going up in size, and I actually gained fuel economy (17.3 on the 35's, 19 on the 37's). This was all on stock 4.10 gears. Tire weight is definitely something that really should be discussed more and taken into account.

 

[WILDHOBO]

I disagree completely as does my truck. I was running 35" Wildpeak AT3Ws that weighed 71 pounds each. I swapped them for 37 KO2s that are 65 pounds. That 6 pounds per tire made a big difference in the truck. I lost almost no power despite going up in size, and I actually gained fuel economy (17.3 on the 35's, 19 on the 37's). This was all on stock 4.10 gears. Tire weight is definitely something that really should be discussed more and taken into account.

No question. Reducing grams in bike tires makes enormous gains in efficiency. It’s immediately felt.

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