37s v 35s on OME 2.5 Suspension Analysis

[Connorrw]

Hello All,
I'd like some opinions on some options I am considering. I currently have a FZJ80 with the basic OME 2.5 Inch lift running 35x12.5 tires. This has done well and let me do some great trails, but I am often scraping and am considering moving up to 37s.

My principal concern was in regards to the loss of flex. As my rear axle currently rubs my fenders, I started in the rear. I'm less worried about the front as I believe I have a lot more room under steering and full flex than the rear. If this isn't true, let me know.

To begin, I 3D-scanned the rear end of the truck. I checked some measurements and am satisfied that it is correct. My tires measured out at 34.5 inches, the axle diameter was about 0.005 inches off.

I then drew some primitive shapes and required points. Below is that showing the truck at ride height, no flex. My goal was to model how the rear axle travels as defined by either collision with the fender well (current constraint w/ 35s), collision with bump stops, or droop limit from maxing out the OME shock. The track bar is modeled in this too and helps constrain the axle.

Here is the truck with 35s modeled at full travel as the tire collides with the fender.

Here it is with 37 modeled in the same way. Constrained by the shock droop and the fender.

Overall, this results in less than a degree of articulation loss. Basically, 1 inch less of total wheel travel (measured from the bottom of the left to the bottom of the right tire at full flex)

Here are the two images overlaid. Blue is the 37s, red is 3the 5s.

0.83 Degrees between axle angles


While this is simplified a bit, this seems to go against conventional thought on running 37s. Any ideas/comments on this? To me, this suggests running 37s is a negligible loss in flex with an increase in ground clearance.

 

[COS80]

I'm amused that you did all that work just to confirm you lose exactly the difference in tire radius.

 

[Connorrw]

That's a good point. I considered that, but thought that angles would get weird and it would result in more of an impact than simply the tire radius. I don't see many people running 37s on the 2.5 inch lift but it seems like a fine decision with little impact. I was concerned on how the track bar would sweep and push the tire into the fender.

 

[JoeCova]

That's a good point. I considered that, but thought that angles would get weird and it would result in more of an impact than simply the tire radius. I don't see many people running 37s on the 2.5 inch lift but it seems like a fine decision with little impact. I was concerned on how the track bar would sweep and push the tire into the fender.

And if you put 39s on you would lose 2”.

I love the scan and the overlap and the visuals but not sure what question you are trying to answer.

Anyone can bumpstop their way to larger tires, however the principal idea is to leverage larger tires without sacrificing shock travel.

 

[mudgudgeon]

Cool scans and drawings.
I think real world use is not gonna play out quite so cleanly cut and dried
Tires flex, tires vary in tread width/ tread shoulder.
Suspension rubbers flex, compress etc.

 

[Connorrw]

That is my concern, and my question is that in a lot of other posts, it is said that cutting is required for 37s to fit on a 2.5 inch lift. This doesn't seem to be strictly true? Flex is only limited by about a degree. Is that true, and do people have to cut? Is it not 37s can fit reasonably well with a tradeoff between clearance and flex?


I do understand this is a very idealized model, but I don't want to try it and find that 35s were better than 37s for my use. As mentioned, I want ground clearance but don't want a huge compromise on flex. For only a theoretical inch of clearance gain, I don't want a large loss in flex.


For reference, just because I thought it was cool. Not an exact linear relationship.

Total wheel to wheel distance at full flex:
31 - 14 inches (Shock limited)
33 - 14 inches (Shock limited)
35 - 13.57 inches (fender/bumpstop)
37 - 12.64 inches (fender/bumpstop)
39 - 12.07 inches (fender/bumpstop)

 

[COS80]

I love the scan and the overlap and the visuals

Cool scans and drawings.

X3

 

[Mark W]

I guess I could measure and get all sciency about it. I do in fact tend to do that sometimes. But instead, I will keep this simple and simply voice an opinion based on 40 years of using Cruisers off road. Real off road. Lots of it. Been beating 80's down the trails for about 25 years now if my memory is right.

Any time I get into a discussion of how much rig someone needs for the uses that they envision, I always tell them that "here in AK" getting down the trails is more about the tires than anything else. If trail performance is the goal, then you want the biggest, most aggressive tire that you are willing to run and you build the rig to suit the tire. Among the other considerations you have to keep in mind in order for the tires to do it's job is the fact that the tire has to be able to move... It has to be able to articulate. Ideally, you want all four tires on the ground over the rough stuff... and ONLY the tires (rolling the rig on it's side is NOT the way to get through. ) The tire choice guides the gearing, the brakes, the power and the lift. (and bumpstops).

Anyway, I'll think about getting to the point.

2.5 inches of lift is NOT enough to let a (nominal) 37 inch tire live under the fenders of an '80. On flat level pavement it will be just fine. But as soon as you bump over the curb or even make a sharp turn into a steep driveway entrance, rubber is gonna meet metal and plastic in ways that was never intended.

If you travel at higher speeds over trails with some undulations, bumps, whoops, washouts, holes, drops and kickers... that rubber to metal/plastic interaction is gonna get noisy and ugly and start costing money too. At slower speeds it takes a bit more to make that happen. So, if you stay on dirt roads you might be okay. But any degree of real articulation over rough terrain is gonna create some serious interference in the tire-fender zones. You will absolutely not like it.


Now... with all that said.....




You can absolutely prevent the s*** show I just touched on by simply installing extended bumpstops. We always use extended bumpstops with lift and larger tires. Unless you are just building on of those silly superlifted parking lot queens, no matter how much lift you build into an '80 if you push it, you will still eventually find the point where rubber meets fender.

But getting carried away with bigass bumpstops to force an underlifted rig to accept too large a tire is not only a sign of a real pouser, but it also cripples your rig's rough terrain capabilities.

Bigger tires mean more lift AND bigger bumpstops if you are actually gonna use the rig on the trails or in places where you can ignore the trails too.


My "not stock" experience in '80s includes...

Lets' see... one of my tag along rigs is running 305/75R16s with no lift. For gentle use it does just fine.

Remember though... it is a tag along rig. It only goes off road when I loan it out to friends and family to join us on family friendly outings.


Another '80 sat on "3 inch OMEs" and with 33s (305s) and a couple inches of bumpstop extension for a few years. It could happily run any trail that I would want to run in an '80 wearing 33s. Without the bumpstops, tire and fender fun was not an everyday problem by any means, but it was always a factor to consider if I wanted to crawl over gnarly surfaces.

An '80 on 35s and no bumpstop extension... 4 inch lift... high speed rallying across desert type terrain (braided rivers running through wide gravel glacial valleys actually) would put the front tires into the inner fenders sometimes. I never suffered any significant damage... just scary sounding scrubbing resulting in minor tearing of plastic and burn smelling tires with buffed spots on the tread. But I was very lucky, I have seen others rip fender flares loose when this happens.

Slower speed over terrain that demanded articulation... it was worse as the tire got really up close and personal with the bodywork.

Adding a couple of inches of bumpstop extension solved that particular problem.


Notice I was talking about the front tires. The rears never had any high speed interference problems, even with no bump stop boost. However, under slow speed high articulation conditions with no bumpstop extentions... Fender flares got ripped off. That silly plastic shroud over the rear corner that is supposed to look like a bumper... that got torn up too. Even when the lift grew to 6 inches, 35 inch tires would still shred and shed fender flares when doing high articulation crawling without properly extended bumpstops. When your tires grow, your lift and bumpstops have to grow too unless all you want to do is look cool at the "parking lot car shows".

6 inches of lift and *about* 3 inches of bumpstop boost SHOULD keep everything happy with 37s depending on just how you use the rig. Keep in mind, the bumpstops inside the coils and the ones on the frame serve different purposes and may call for different amounts of extension for ideal performance.

We have a couple of '80s here on 38s with about an honest 8 inches of lift and only 3-4 inches of bumpstop extensions (if I am remembering the tape measure precisely) They rub a little bit under heavy articulation, but so far there has been no tearing of sheet metal, plastic or rubber.

It has been several years since I really looked closely at this rig, but I helped a friend of my build an '80 that sits on about 8 inches of lift (it was originally 10 but she loads it HEAVY on the trail and the coils settled a LOT and it got shorter). She is running 40 inch boggers on it and while she has broken and damaged all sorts of stuff, the tires/fenders have not had any problems. She works it hard on the trails, but she does NOT run high speed with it and she also does not push the limits of articulation.


Okay, I have babbled on too much. The short answer.... MY short answer to your question is... 2.5 inches of lift is not enough to let "37s" work. And you are gaining... nominally an inch of ground clearance at full inflation. Less if you are aired down. The compromises you will have to make to avoid a little more lift will negate any gain from that max of one inch clearance gain.

Okay... done pontificating for tonight.


Mark...

 

[mudgudgeon]

I guess I could measure and get all sciency about it. I do in fact tend to do that sometimes. But instead, I will keep this simple and simply voice an opinion based on 40 years of using Cruisers off road. Real off road. Lots of it. Been beating 80's down the trails for about 25 years now if my memory is right.

Any time I get into a discussion of how much rig someone needs for the uses that they envision, I always tell them that "here in AK" getting down the trails is more about the tires than anything else. If trail performance is the goal, then you want the biggest, most aggressive tire that you are willing to run and you build the rig to suit the tire. Among the other considerations you have to keep in mind in order for the tires to do it's job is the fact that the tire has to be able to move... It has to be able to articulate. Ideally, you want all four tires on the ground over the rough stuff... and ONLY the tires (rolling the rig on it's side is NOT the way to get through. ) The tire choice guides the gearing, the brakes, the power and the lift. (and bumpstops).

Anyway, I'll think about getting to the point.

2.5 inches of lift is NOT enough to let a (nominal) 37 inch tire live under the fenders of an '80. On flat level pavement it will be just fine. But as soon as you bump over the curb or even make a sharp turn into a steep driveway entrance, rubber is gonna meet metal and plastic in ways that was never intended.

If you travel at higher speeds over trails with some undulations, bumps, whoops, washouts, holes, drops and kickers... that rubber to metal/plastic interaction is gonna get noisy and ugly and start costing money too. At slower speeds it takes a bit more to make that happen. So, if you stay on dirt roads you might be okay. But any degree of real articulation over rough terrain is gonna create some serious interference in the tire-fender zones. You will absolutely not like it.


Now... with all that said.....




You can absolutely prevent the s*** show I just touched on by simply installing extended bumpstops. We always use extended bumpstops with lift and larger tires. Unless you are just building on of those silly superlifted parking lot queens, no matter how much lift you build into an '80 if you push it, you will still eventually find the point where rubber meets fender.

But getting carried away with bigass bumpstops to force an underlifted rig to accept too large a tire is not only a sign of a real pouser, but it also cripples your rig's rough terrain capabilities.

Bigger tires mean more lift AND bigger bumpstops if you are actually gonna use the rig on the trails or in places where you can ignore the trails too.


My "not stock" experience in '80s includes...

Lets' see... one of my tag along rigs is running 305/75R16s with no lift. For gentle use it does just fine.

Remember though... it is a tag along rig. It only goes off road when I loan it out to friends and family to join us on family friendly outings.


Another '80 sat on "3 inch OMEs" and with 33s (305s) and a couple inches of bumpstop extension for a few years. It could happily run any trail that I would want to run in an '80 wearing 33s. Without the bumpstops, tire and fender fun was not an everyday problem by any means, but it was always a factor to consider if I wanted to crawl over gnarly surfaces.

An '80 on 35s and no bumpstop extension... 4 inch lift... high speed rallying across desert type terrain (braided rivers running through wide gravel glacial valleys actually) would put the front tires into the inner fenders sometimes. I never suffered any significant damage... just scary sounding scrubbing resulting in minor tearing of plastic and burn smelling tires with buffed spots on the tread. But I was very lucky, I have seen others rip fender flares loose when this happens.

Slower speed over terrain that demanded articulation... it was worse as the tire got really up close and personal with the bodywork.

Adding a couple of inches of bumpstop extension solved that particular problem.


Notice I was talking about the front tires. The rears never had any high speed interference problems, even with no bump stop boost. However, under slow speed high articulation conditions with no bumpstop extentions... Fender flares got ripped off. That silly plastic shroud over the rear corner that is supposed to look like a bumper... that got torn up too. Even when the lift grew to 6 inches, 35 inch tires would still shred and shed fender flares when doing high articulation crawling without properly extended bumpstops. When your tires grow, your lift and bumpstops have to grow too unless all you want to do is look cool at the "parking lot car shows".

6 inches of lift and *about* 3 inches of bumpstop boost SHOULD keep everything happy with 37s depending on just how you use the rig. Keep in mind, the bumpstops inside the coils and the ones on the frame serve different purposes and may call for different amounts of extension for ideal performance.

We have a couple of '80s here on 38s with about an honest 8 inches of lift and only 3-4 inches of bumpstop extensions (if I am remembering the tape measure precisely) They rub a little bit under heavy articulation, but so far there has been no tearing of sheet metal, plastic or rubber.

It has been several years since I really looked closely at this rig, but I helped a friend of my build an '80 that sits on about 8 inches of lift (it was originally 10 but she loads it HEAVY on the trail and the coils settled a LOT and it got shorter). She is running 40 inch boggers on it and while she has broken and damaged all sorts of stuff, the tires/fenders have not had any problems. She works it hard on the trails, but she does NOT run high speed with it and she also does not push the limits of articulation.


Okay, I have babbled on too much. The short answer.... MY short answer to your question is... 2.5 inches of lift is not enough to let "37s" work. And you are gaining... nominally an inch of ground clearance at full inflation. Less if you are aired down. The compromises you will have to make to avoid a little more lift will negate any gain from that max of one inch clearance gain.

Okay... done pontificating for tonight.


Mark...

You lost me at "6 inches of lift". At "8 inches of lift", I puked in my mouth a little bit!

 

[cts]

I think you're on the right track by starting w/ how much bumpstop extension you'll need to clear the larger tires. That along with rim offset are the primary things that will determine whether or not they rub. The next step then becomes looking at what the bumpstop extension will do to your suspension. If you add 2" of bumpstop and that leaves you with something like 2.5"" of up travel, that would be rather limiting. However, that extra bumpstop would mean that you could accomodate a shock with a compressed length that is 2" longer, which could be up to 4" longer at extension. That longer shock would give you a nice amount of travel, but it would mostly all be downtravel. That in turn would warrant a taller lift, so that your static ride height is closer to the middle of that travel and you get a decent amount both up and down.

So start by figuring out what it takes to fit the tire, and then assess the other parts of the system that need to change to optimize the other variables.

What offset do you have on your rims? Given that they stick out past the flares in the scan, I assume they're probably aftermarket with negative offset, which is likely helpful for what you're trying to do.

 

[Connorrw]

Thanks Mark for the answer. I think even 2.5 inches of lift is too low for 35s but it was I have/am stuck with for a while. I chose it initially as I knew I wouldn't have any problems with driveline angles, major caster problems beyond what bushing could correct, etc.

In light of that, my logic was that in my case, an already underlifted truck, going 37s isn't much worse (the main post above) as it only reduced flex by 1inch/1 degree but increased the tire diameter. As I'm stuck with 2.5, you still think 37s will cause problems and the benefit is not worth it?

CTS- I'm not sure what offset. Definetly pushed out from stock which is definetly helping them pack into the fender.

I question the benefit of going to longer dampers really is. A tire on the ground is only helpful if it has a normal force on it and a damper drooping to its limit past the spring will not provide any meaningful reaction.

 

[Mark W]

You lost me at "6 inches of lift". At "8 inches of lift", I puked in my mouth a little bit!

That's okay.You build for your uses and we will build for ours.

And you would not need to worry about your puke breath. We'd never even notice it from the other side of the marsh... or river.


Personally, for me, 6 inch lift and 35-37 inch tires is all I ever want to run on an '80. So far as I am concerned the strength of the '80 and it's appeal is the "general purposeness" of the rig. I can run errands with it, Road Trip with it, pick people up at the airport with it and, when I chose to, I can take it down some trails that a LOT of other streetable rigs simply can't keep up on.

I occasionally run one of the larger rigs on the trails. LOTS of fun and far more capable than most would expect. But to achieve that, a lot of the general purpose appeal has been compromised. The matching pair of rigs I mentioned were built to follow along after my '40 that sits on 45 inch Tractor tires pushed by a hot 350 running through an SM 465, an NP203 a 4:1 Orion and locked (of course) 5.29:1 diffs. That thing leaves less footprint across a bog or marsh than if we got out and walked. It will climb a brick wall (until it flips over backwards!!!). But I sure would not want to make the run up to Chena Hot Springs Resort with it in Febuary... or ANY TIME for that matter.

But... even with all the mods done to them... the '80s can only follow if I play nice and route around the tougher stuff. .

Mark...

 

[Mark W]

Thanks Mark for the answer. I think even 2.5 inches of lift is too low for 35s but it was I have/am stuck with for a while. I chose it initially as I knew I wouldn't have any problems with driveline angles, major caster problems beyond what bushing could correct, etc.

In light of that, my logic was that in my case, an already underlifted truck, going 37s isn't much worse (the main post above) as it only reduced flex by 1inch/1 degree but increased the tire diameter. As I'm stuck with 2.5, you still think 37s will cause problems and the benefit is not worth it?

CTS- I'm not sure what offset. Definetly pushed out from stock which is definetly helping them pack into the fender.

I question the benefit of going to longer dampers really is. A tire on the ground is only helpful if it has a normal force on it and a damper drooping to its limit past the spring will not provide any meaningful reaction.

I think that you'd find that the 37s are gonna be enough worse in terms of clearance that it will not be a thing that you can ignore.

Unless the rig is primarily a trail toy, I would probably stick with the 35s and embrace everything that make an '80 what it is... mainly it's ability to do lots of things well, even if it will never be the best at any of them.

2.5 inches is low for '35s on the trail. Just fine on the street, fantastic for road tripping and depending on the exact usage, probably pretty acceptable on rough(ish) backroads.

It would also be a simple matter to grow that 2.5 to 3 or even 4 inches and the 35s will be a lot happier at that altitude than the 37s would be. Assuming that we are talking about the same tire, just in different sizes, any gain in trail performance from the larger tire will not be amazing by anyone's standards.


Mark...

 

[JoeCova]

For reference I run 35s on J Springs with no bump stop extensions without issue. I've ran all over moab and coloraod and the Rubicon. There are moments when a 37 would be "better" but hasnt stopped me from getting through a trail.

 

[Mdej3]

Wish I had this type of free time.

 

[lastresort576]

I see denver, as in Denver CO? Im sure you scrape your rear hitch on stuff..every 80 does. but do you really need more than 35's? you can run a ton of trails in colorado with 35's. In fact, there isnt many that you cant run with 35's and from there you have to go buggy status in order to prevent body damage regardless of tire size. Sure 37's make things a little easier and more forgiving but the mod value vs cost really isnt worth it.

Id maybe consider getting more seat time and better line choices.

 

[Connorrw]

Thanks to all for the response. 35s it is.


Also seems like there is some misconceptions on the scanning and time/effort and I think the process is pretty cool. Scanners are pretty amazing now. Scanning the rear of the truck took about 2 minutes and then bringing it into CAD and doing the quick lines was about 30 minutes. Lot cheaper than buying 37s and realizing they dont work.

I have access to some great manufacturing tools, so I do as much of the design for my own stuff as I can. Compressor brackets, sliders, etc.

 

[ChaseTruck]

…
I question the benefit of going to longer dampers really is. A tire on the ground is only helpful if it has a normal force on it and a damper drooping to its limit past the spring will not provide any meaningful reaction.

A few tidbits:
The droop is going to soften the transition when weight transitions back on that corner. The ’72 FJ40 in our stable on Skyjacker leaf springs (meaning that truck has the suspension articulation of a shopping cart) likes to lift tires, and it’s all or nothing: when there’s weight again on the corner with the lifted tire, it’ll come down quickly. Very predictably so, but sometimes a bit harsh. On an 80 with a tire in the air, the shock will help to smooth things out as soon you’re in shock range again.

Our ‘80 is on used J’s in the front, and unknown springs in the rear; truck is level, lift measured inside the front coils with stock bumpstops is 2”; tires are 315/75r16. Limits are garage door height, and the truck is too nice to dent it. On milder local trails around Phoenix, I often get the axles crossed and tires stuffed deep in the fender. That by itself doesn’t bug me much. What does bug me is what I call the “external drum brake” effect that has killed momentum during crawling uphill on many an occasion. Maybe I get around to do bumpstops this 'wheeling season.