Heavy 35's vs lighter 37's

[J0E]

I'm just saying people say you should regear for 37's.. but what if the 37 you want to run is lighter than the 35's you currently run?.. 35x12.50 vs 37x12.50.. would you still be at a higher wear and tear.. and need for regearing even though the 37 is lighter just due to more rotational mass?

I'm on 37's with stock gearing. No need to regear unless you want to optimize for the interstate. My JLR is a dedicated wheeler so I didn't even notice going from 33's to 37's on the road but noticed a HUGE difference on the trail.

Your stock ball joints will go much faster. If you get on the skinny petal, you're more likely to break something in the driveline. T = R cross F, so the bigger lever makes it easy to break something.

On the tech side, it is an interesting equation with multiple variables.

Weight-
Location of that weight-
Actual Tire Diameter-
Width of tire-

Rotation weight is a huge performance killer and could trump just about all the other variables.

It's rotational inertial and it's only a minor perf killer on dragsters that have an enormous 1st derivative of position in time. That's why dragsters went to mag wheels. The rotational inertial is trivial compared to the mass of the vehicle. So if you're building a dragster, it's important, a crawler, it's nothing.

The tread is the heaviest part of the tire, by going to a larger diameter tire, you're moving the heaviest part of the tire (the tread) further from the axle centerline. So even if the tires weighed the same, the 37 has the weight further from the axle centerline (a larger torque arm).


And it is exponential as you move that weight further out.

Not exponential but increases with square of the radius
I = ∑miri2
Which as the sum goes to infinity becomes I = ∫r2dm
On an axial symmetric wheel, it's not a complicated computation. But @Tech Tim is right, it's the square of the mass at distance r. So a 37" tire will have significantly higher rotational inertial than a 35, but trivial compared to the mass of the vehicle. In a 0 to 60, the taller gearing of bigger tires will be more detrimental than the increased rotational inertia.

The width matters, because now you're looking at increased friction or rolling resistance.

Except for turning on pavement, increased width adds very little friction for the same PSI. If you want lower friction, run at 35 PSI. That's what I do on my 37's until I get to the trail head.

Now wider tires will cause more turbulence going 70+ MPH. If that's the friction you're talking about, @Tech Tim is right. But in general, wider tires are better for crawling in dirt, rock, sand, but worse in snow and ice.

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[2nd 392]

Ive been going through myself. I have a Rubicon. I don't want 37's since I have 4.10's. Nor do I want to deal with all the added stuff you should do to run them properly. Most spring lifts are well over 2.5" which makes 35's look funny on the Rubicon. So I only want a 1.5" lift and some 35's.

Looking at 35's I have seen weights of them range from 52lbs to 80lbs in 35x12x7 or 315/70/17 a tire.

While I don't want 37's I did check their weights and the KO2 seemed to be one of the lowest weight options at 71lbs a tire.

Some Nittos are 80lbs a 35" tire! Where as the lightest 35 I found was 52 for a GY terrority. Next in line was a KO2 35" at 62lbs.

Next in line for 35’s is Toyo AT3 35/12.50/C 59 lbs, good ride and handling, a bit louder than my KO2’s due to more aggressive tread but still quiet, sides aren’t impressive (one side slightly better) but at 59 lbs is acceptable. Lighter than factory Falken MT option so no gate reinforcement required ( used featherweight aluminum Teraflex relocation brackets) much higher than expected performance gain with new 4.56’s over stock 3.73’s and 285/70/C KO2’s. Only had ~ 3 weeks, no further review. BTW— 62 lb (110S) KO2’s “out of stock and discontinued by Michelin“—DT, 64lb (110T) Ford tire replaced it ( with tread width also decreased from 10.6” to 10” and of course -$15 more expensive). Edit— further review, when I said I was considering them my son said they are the best tires he has ever had on his F-250, but are F rated if it could make a difference.

 

[roaniecowpony]

But wait…. Isn’t 38s the new 35s in our Jeeping world ?

My 37s look smaller every time I look at them lol

Brought to you by the popularity of the big fat 5000 lb "Unlimited" 4 door (which I own).

 

[Old Jeeper]

Next in line for 35’s is Toyo AT3 35/12.50/C 59 lbs, good ride and handling, a bit louder than my KO2’s due to more aggressive tread but still quiet, sides aren’t impressive (one side slightly better) but at 59 lbs is acceptable. Lighter than factory Falken MT option so no gate reinforcement required ( used featherweight aluminum Teraflex relocation brackets) much higher than expected performance gain with new 4.56’s over stock 3.73’s and 285/70/C KO2’s. Only had ~ 3 weeks, no further review. BTW— 62 lb (110S) KO2’s “out of stock and discontinued by Michelin“—DT, 64lb (110T) Ford tire replaced it ( with tread width also decreased from 10.6” to 10” and of course -$15 more expensive). Edit— further review, when I said I was considering them my son said they are the best tires he has ever had on his F-250, but are F rated if it could make a difference.

You mentioned tailgate reinforcement. I ran 37/12:50 BFG KM2s/GY MTRs on my 2003 TJ Rubicon tailgate for almost 10 years without any issue at all, no tearing, twisting, sagging, nothing for almost 100K miles and trips down into Baja Mexico, Moab, Rubicon, all over the SW US. I did see a lot of TJs with torn sheet metal??

 

[mgroeger]

@J0E...
"I'm on 37's with stock gearing. No need to regear unless you want to optimize for the interstate. My JLR is a dedicated wheeler so I didn't even notice going from 33's to 37's on the road but noticed a HUGE difference on the trail."

FYI, while you can run 37s w the stock 4.10 gears it is far from optimal. I know everyone does it but the reality is there is a mechanical advantage being lost and the transmission is the component that has to make up for it. Running 4.88 or even 5.13 puts the ratios back to normal and makes a huge difference in performance and efficiency both on and off road.

 

[2nd 392]

You mentioned tailgate reinforcement. I ran 37/12:50 BFG KM2s/GY MTRs on my 2003 TJ Rubicon tailgate for almost 10 years without any issue at all, no tearing, twisting, sagging, nothing for almost 100K miles and trips down into Baja Mexico, Moab, Rubicon, all over the SW US. I did see a lot of TJs with torn sheet metal??

Other reports concur with yours, but not wanting to push my luck the originally purchased (without considering it’s weight) 13 lb RC relocation plate is in the garage along with the chintzy hood locks I also wasted money on, likely won’t be the only two.

 

[J0E]

FYI, while you can run 37s w the stock 4.10 gears it is far from optimal. I know everyone does it

How so, on this forum, most report getting lower gears.

Quantify far from optimal. For a DD, it's not as good for drag racing if that's what your after.

Use my GearWheelSpeed calculator and you'll see a JLR with stock gears and 37's is almost identical to a non rubicon on 31.5" tires. Using 36" for actual tire (my KM3 37's are 35 16/16), the non JLR is actually geared higher than my stock JLR with 37's. Given over 90% of JL's are not Rubicons, the factory optimizes for the masses, not the Rubicons.

the reality is there is a mechanical advantage being lost and the transmission is the component that has to make up for it. Running 4.88 or even 5.13 puts the ratios back to normal and makes a huge difference in performance and efficiency both on and off road.

There is nothing being lost, it's trivial 7th grade math higher ratios. What efficiency is lost?

makes a huge difference in performance and efficiency both on and off road.

Not in low range with the Ruby's 4.1 xfer case. With 37's, you're pushing the limits now you have a weaker ring gear and pinion. I routinely climb 36 degrees in M1 at 1,200 RPM.

If you're optimizing your DD for highway driving with lots of hills it's probably worth doing.

 

[Old Jeeper]

How so, on this forum, most report getting lower gears.

Quantify far from optimal. For a DD, it's not as good for drag racing if that's what your after.

Use my GearWheelSpeed calculator and you'll see a JLR with stock gears and 37's is almost identical to a non rubicon on 31.5" tires. Using 36" for actual tire (my KM3 37's are 35 16/16), the non JLR is actually geared higher than my stock JLR with 37's. Given over 90% of JL's are not Rubicons, the factory optimizes for the masses, not the Rubicons.



There is nothing being lost, it's trivial 7th grade math higher ratios. What efficiency is lost?



Not in low range with the Ruby's 4.1 xfer case. With 37's, you're pushing the limits now you have a weaker ring gear and pinion. I routinely climb 36 degrees in M1 at 1,200 RPM.

If you're optimizing your DD for highway driving with lots of hills it's probably worth doing.

Your pinion start getting smaller at 5:13, even then its generally not significant, in fact in my racing days we ran 5:36 and even 6:14s for the really high winders and smaller tires. That was my world on the 1320.

Craw Ratio:

Transfer Case Low Range

Transmission low gear

Axle ratio


OEM JLR, 6 spd trans:

TC - Trans - Axel

4 x 5.13 x 4.1 = 84.13 Crawl Ratio

The crawl Ratio IMO not a good index because it fails to take into consideration the height of the tire. So I calculate the craw ratio and then divide it by the height of the tire

Reality: 84.13/33 = 2.55 (OEM). Rubicon Rock Crawling Index. (where 33 is the OEM tire height)

So when it comes to Rocks the Rubicon is the Gold Standard. You can use any CR you want but using the Rubicon tells the performance level. If you have a JLR then I know how it performs simply based on the RRCI this becomes your metric by which to compare against.

RRCI with 37 tires: 84.13/37 = 2.27 RRCI. Impact: 2.55 vs 2.27, RRCI -10%. You lose 10% which is about an order of 1 magnitude which is more than compensated for in a good set of KM2s.

As you go taller in tires the penalty starts to get well beyond 1 magnitude and starts to impact your performance especially as you climb into 39s, 40s, 43s and 44s. Unlike the world of 1320 when I dumped the clutch at 7000 rpm I had wheel slippage which saved the R&P from exploding. Rocks do their damage when those big tires get in a bind due to a wedge or bind and you unload and are trying to move a 200 lb tire almost and things break. The big wheel boys trailer their rigs because the trip home has a broken rig on the trailer.

While we are on the subject of driveline, I will add: If you are SERIOUS about rocks what is important is the driveline, axles! First thing I did when I got my Rubicon was to put a set of Dynatrac ProRocks/ARBs F/R, a set of Tom Woods Drive shafts with the rear shaft having a 1310 u joint on the rear DS pinion end the rest were 1350s. WHY? After almost 100k miles and no problems that is why. The next thing I did was a 3 in lift, which came out about a year later and I went with a custom-built parts bin lift.

GO FOR IT... and remember this: YOUR Jeep, YOUR wallet, YOUR pride of ride, do it YOUR way!

 

[mgroeger]

How so, on this forum, most report getting lower gears.

Quantify far from optimal. For a DD, it's not as good for drag racing if that's what your after.

Use my GearWheelSpeed calculator and you'll see a JLR with stock gears and 37's is almost identical to a non rubicon on 31.5" tires. Using 36" for actual tire (my KM3 37's are 35 16/16), the non JLR is actually geared higher than my stock JLR with 37's. Given over 90% of JL's are not Rubicons, the factory optimizes for the masses, not the Rubicons.



There is nothing being lost, it's trivial 7th grade math higher ratios. What efficiency is lost?



Not in low range with the Ruby's 4.1 xfer case. With 37's, you're pushing the limits now you have a weaker ring gear and pinion. I routinely climb 36 degrees in M1 at 1,200 RPM.

If you're optimizing your DD for highway driving with lots of hills it's probably worth doing.

All Jeeps, YJ, TJ, JK, JL have their gears optimized for the tires that come on them from the factory which is why you see the Rubis with 4.10 gears for the 33" tires. So that 4.10 gear with that transmission and that tire size is setup for optimal performance whether it be on road or off road. It's simple 8th grade math... if you change your tire size you should be changing your gear size. As a daily driver you will see 8th gear on a regular basis and off roading you will have more bite climbing over obstacles. Myself and countless others have experienced this for years and years and years, but according to you all those people have needlessly regeared. If you jump to 37" tires 4.88 or 5.13 gears are optimal for both daily and off road. 5.38 gears are highly NOT recommended in the factory axles for various reasons including pinion strength.
Educate yourself as there are countless threads on this forum, along with charts, showing this. @chevymitchell has explained it at length many times on here.
Understand I am not saying that keeping the stock 4.10 gears with 37" tires is not doable, as plenty of people do it. I am saying it is not optimal and the transmission is the one that needs to make up the slack.

 

[J0E]

All Jeeps, YJ, TJ, JK, JL have their gears optimized for the tires that come on them from the factory which is why you see the Rubis with 4.10 gears for the 33" tires. So that 4.10 gear with that transmission and that tire size is setup for optimal performance whether it be on road or off road.

Then explain the non Rubicon gearing, which is 90% of JL's sold. Look at my bt39.com/GearWheelSpeed and explain Ruby vs non Ruby gearing being optimized. The reality is the non-ruby JL is optimized for highway speeds and the JLR's make highway speed concessions for off road performance.

Just because something is repeated enough doesn't make it true. The ultimate experts in football rarely went for it on 4th down for 97% of the history of football even though statiticans published papers showing they're better off going for it, 40 years ago. It's only the last few years the experts are doing what's right. Now you get a bunch of armchair quarterbacks asserting you must regear, therefore it must be right.

 

[J0E]

Your pinion start getting smaller at 5:13, even then its generally not significant, in fact in my racing days we ran 5:36 and even 6:14s for the really high winders and smaller tires. That was my world on the 1320.

Craw Ratio:

Transfer Case Low Range

Transmission low gear

Axle ratio


OEM JLR, 6 spd trans:

TC - Trans - Axel

4 x 5.13 x 4.1 = 84.13 Crawl Ratio

The crawl Ratio IMO not a good index because it fails to take into consideration the height of the tire. So I calculate the craw ratio and then divide it by the height of the tire

Reality: 84.13/33 = 2.55 (OEM). Rubicon Rock Crawling Index. (where 33 is the OEM tire height)

So when it comes to Rocks the Rubicon is the Gold Standard. You can use any CR you want but using the Rubicon tells the performance level. If you have a JLR then I know how it performs simply based on the RRCI this becomes your metric by which to compare against.

RRCI with 37 tires: 84.13/37 = 2.27 RRCI. Impact: 2.55 vs 2.27, RRCI -10%. You lose 10% which is about an order of 1 magnitude which is more than compensated for in a good set of KM2s.

As you go taller in tires the penalty starts to get well beyond 1 magnitude and starts to impact your performance especially as you climb into 39s, 40s, 43s and 44s. Unlike the world of 1320 when I dumped the clutch at 7000 rpm I had wheel slippage which saved the R&P from exploding. Rocks do their damage when those big tires get in a bind due to a wedge or bind and you unload and are trying to move a 200 lb tire almost and things break. The big wheel boys trailer their rigs because the trip home has a broken rig on the trailer.

While we are on the subject of driveline, I will add: If you are SERIOUS about rocks what is important is the driveline, axles! First thing I did when I got my Rubicon was to put a set of Dynatrac ProRocks/ARBs F/R, a set of Tom Woods Drive shafts with the rear shaft having a 1310 u joint on the rear DS pinion end the rest were 1350s. WHY? After almost 100k miles and no problems that is why. The next thing I did was a 3 in lift, which came out about a year later and I went with a custom-built parts bin lift.

GO FOR IT... and remember this: YOUR Jeep, YOUR wallet, YOUR pride of ride, do it YOUR way!

Agree with most. You could make your RRCI better by dividing by the tire radius, half the height.

The big wheel boys trailer their rigs because the trip home has a broken rig on the trailer.

That's what I'm thinking on my next build. The two guys in our club in JK's on 43's won't even wheel with us (too easy, boring). They trailer their rigs with the buggy guys, who have to trailer their rigs.

While we are on the subject of driveline, I will add: If you are SERIOUS about rocks what is important is the driveline, axles! First thing I did when I got my Rubicon was to put a set of Dynatrac ProRocks/ARBs F/R, a set of Tom Woods Drive shafts with the rear shaft having a 1310 u joint on the rear DS pinion end the rest were 1350s. WHY? After almost 100k miles and no problems that is why. The next thing I did was a 3 in lift, which came out about a year later and I went with a custom-built parts bin lift.

GO FOR IT... and remember this: YOUR Jeep, YOUR wallet, YOUR pride of ride, do it YOUR way!

That sounds serious. I'm hoping not to get deep in the trail and bust my front axle at the FAD. But I'm not not putting any money into this rig as I'll sell in in a couple years and do a build right.

 

[Old Jeeper]

Agree with most. You could make your RRCI better by dividing by the tire radius, half the height.



That's what I'm thinking on my next build. The two guys in our club in JK's on 43's won't even wheel with us (too easy, boring). They trailer their rigs with the buggy guys, who have to trailer their rigs.



That sounds serious. I'm hoping not to get deep in the trail and bust my front axle at the FAD. But I'm not not putting any money into this rig as I'll sell in in a couple years and do a build right.

Why did I use tire diameter? I could have used radius but I picked diameter for its commonality. 33, 35, 37 inch diameter tire. The Rubicon was built by Jeep to tackle the Rubicon trail and that was the test bed and that was the engineering objective. Keep it simple. The Rubicon and its 33 tire are the Guideon or the metric that I refer to. It also makes the math simpler, no step for dividing the 3X in half. I also think if you use the radius vs diameter you over compensate...because in the real world you air down and the 37 tire morphs into something along the lines of a 34.x.

I was having breakfast with the writers and editors of Jeep Magazine. They had laughed as walked past my Jeep on 37s out in Moab, guess you are run the easy trails and as we walked in the restaurant one guys said: Did you not apply for a Jeep run where spend 2 weeks + hitting some of the hardest trails in the SW and if you broke we left you behind other than helping you off the trail? YES I did I wonder why you guys did not pick my Jeep. Sit down at our table. We did when we first read your write and mods. Then we were getting entries and we dropped your Jeep because of 2 things: 3 in lift, 37 tires. Every entry that we picked had a 5 in lift or higher and 39 or taller tires.. Yea I guess the last thing you want is some guy with 37s & 3 drive around the guys on 40s. Then one of the writers said you even do any tough trails here in Moab? Pritchett Canyon, forward and back, by myself, no winching, no strapping.

We finished breakfast and the writers wanted to take a better look, curious as to how got 37s under that 3 in, most guys are doing 5 in lifts. Do you rub? Sure and I do so on purpose, you can adjust your 'stuff' and your droop' and I adjust to BARELY rub, I want to be able to hear it rubs at the top. Also my inner fenders have been treated to Roundhead screws and then I keep it painted in Gloss Black to keep it slick.

Joe, here is the take away: When I decided to build I went to one of the top builders in the US, he just happened to live not far from me. He is a one-man show, been building Jeeps since he opened his shop back in the 80s. I helped him do it taxes back in 2004 and he had made $482k that year. He advised me to build the drive first. He said most of the time breakage on the trial is the driveline and to go with full set of Dynatrack ProRocks and he also told me about the 1310 pinion u joint trick. He told me don't build for looks or the Sonic Drive-in unless that is where you want wheel. That is what I did, why ran 37s, I could have gone larger, but I never took a bypass or failed a trail on 37s so I stuck with them. I also noted big tires do not guarantee success. I have gone where they did not have success.

Joe go for it and start with the driveline because Chrome don't get ya home! Lifts 3-3.5 in MAX but most folks 99.9% buy a kit and bolt it on and hit the trail. Jeeps have more than 1 lift.

In no order
Suspension Lift
Body lift
Body Frame mounts lift
DRIVE LINE LIFT the BIG SECRET
NUT & Bolt lift A 3/8s bolt/screw head can and will stop ALL forward movement, deny you getting over a ledge or waterfall and you just installed a 5 in lift and cannot figure out why ole Don in his 3 in lift just drove over the ledge/waterfall with ease
FLAT skid plates

Ok Joe, now you know I really did not have just a 3 in lift, I had a whole bunch of them...enjoy

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