Break-over and departure angles—I'm an odd duck

[mrblaine]

The part I don't understand is why there is relation between tire diameter and wheelbase. More than one experienced builder has given basically the same info so I don't doubt the guidelines. Any thoughts on how tire size and wheelbase work together?

It hampers overall performance. Think of trying to climb a double ledge. Front tires are just over the edge of the upper ledge and the backs are up against a 12" tall ledge. If you increase tire size without increasing wheelbase, you are effectively shortening the wheelbase because the distance between the front and rear tires is shorter. That makes it harder to climb stuff that you would normally walk right up since the rear tires won't let the fronts get up on top.

I also have the advantage of being around a lot of rigs that were built for 37's and then the owners figured out they could toss on 40's with a small bit of trimming. Across the board with one exception, they all sold the rigs shortly thereafter due to how crappy they worked.

 

[jjvw]

35's- roughly a 19-21" belly height, 100ish wheelbase.
37's- roughly a 20-22" belly height, 104" minimum wheelbase.
40's- roughly 22-23" belly height, 108" minimum wheelbase.

Axle widths-
35's- 60-61 front
37's- 63-64 front
40's- 66- 67-68 front
All with around 4" of back spaced rims to 4.5" of BS.

To put that in clear perspective, I have two rigs here right now, one is the standard TJ 4" Currie on 35's. The other is 114" stretch TJ Unlimited on 40's. The Unlimited is 3" taller than the TJ. 2.5" of that height difference is just the tires.

It hampers overall performance. Think of trying to climb a double ledge. Front tires are just over the edge of the upper ledge and the backs are up against a 12" tall ledge. If you increase tire size without increasing wheelbase, you are effectively shortening the wheelbase because the distance between the front and rear tires is shorter. That makes it harder to climb stuff that you would normally walk right up since the rear tires won't let the fronts get up on top.

...

Doing a bit of math, I am seeing that, at least for JV, there is a sweet spot of 65-68" between tire faces. But there are a lot of other things going on. The larger the tire, the less the rig is going to fall into a hole which means the belly height doesn't need to increase at the same rate as the increase in wheelbase.

Back to my local observations. A typical short arm TJ 35 build is about a 92" wheelbase with 57" between tire faces. My wheelbase is currently at 94" with 59" between tire faces after some not too difficult work. I have yet to see where my extra 2" matters. Though I have seen occasional cases where a similar build in a ~103" wb LJ can get up and though things I can't. I've also seen occasional cases where that 103" wb is just a bit too much without more belly height.

However, for me to get to Blaine's ~100" on a 35" TJ is a huge amount of work to achieve and also do it well. I know how to get another 1-1.5" up front. It isn't all easy. And getting the rest needs to occur on the rear, which really means a frame stretch and a fuel cell. A GR stretch tank doesn't do what I want without compromises, so I don't count that as an option.

All of that is to say that for my purposes, if there is a meaningful benefit to a longer wheelbase on 35s, it is a rare occurrence. And if I were to go through the trouble, I am still stuck with 35s on stock axles. It almost feels like it would be worth it to just build for 37s.

Point being, adding wheelbase for the sake of adding wheelbase is very low on my list of things to do to make the Jeep perform better. If I'm understanding the problems it addresses, they aren't very meaningful problems for me to address given the amount of work and money involved.

 

[mrblaine]

Doing a bit of math, I am seeing that, at least for JV, there is a sweet spot of 65-68" between tire faces. But there are a lot of other things going on. The larger the tire, the less the rig is going to fall into a hole which means the belly height doesn't need to increase at the same rate as the increase in wheelbase.

Back to my local observations. A typical short arm TJ 35 build is about a 92" wheelbase with 57" between tire faces. My wheelbase is currently at 94" with 59" between tire faces after some not too difficult work. I have yet to see where my extra 2" matters. Though I have seen occasional cases where a similar build in a ~103" wb LJ can get up and though things I can't. I've also seen occasional cases where that 103" wb is just a bit too much without more belly height.

However, for me to get to Blaine's ~100" on a 35" TJ is a huge amount of work to achieve and also do it well. I know how to get another 1-1.5" up front. It isn't all easy. And getting the rest needs to occur on the rear, which really means a frame stretch and a fuel cell. A GR stretch tank doesn't do what I want without compromises, so I don't count that as an option.

All of that is to say that for my purposes, if there is a meaningful benefit to a longer wheelbase on 35s, it is a rare occurrence. And if I were to go through the trouble, I am still stuck with 35s on stock axles. It almost feels like it would be worth it to just build for 37s.

Point being, adding wheelbase for the sake of adding wheelbase is very low on my list of things to do to make the Jeep perform better. If I'm understanding the problems it addresses, they aren't very meaningful problems for me to address given the amount of work and money involved.

I need to build a TJ on 35's with as much wheelbase as we can get with a low profile tank under the back and a couple of inches up front to see how that does in JV. Not sure what the wheelbase would be but I'd certainly like to find out.

 

[jjvw]

I need to build a TJ on 35's with as much wheelbase as we can get with a low profile tank under the back and a couple of inches up front to see how that does in JV. Not sure what the wheelbase would be but I'd certainly like to find out.

That would be neat to see. When I get my winter project done, I should be at a little over 95" wheelbase. That's about 3" longer than a typical 35" build. I have no idea if I'll notice the difference in wheelbase out here.

 

[Plumber1]

Blaine's list has been out there for a long time. And I've spent quite a bit of time comparing it to my experiences and observations. My conclusion is that for what we do out here, the variances in wheelbase, belly height, tire size isn't as crucial as it would be in a more challenging area. To compare, in Colorado I don't believe the differences between short, mid, long arm make the difference between making through an obstacle or not. What does matter the most is lockers followed by tire size and belly height.

I think you forgot to add common sense to your list. I have seen many rigs with 37's struggle and some of with us w/ 35's cruise through. Picking the right line and letting the jeep do it's thing seams to work out a lot better than more gas peddle.

 

[jjvw]

I think you forgot to add common sense to your list. I have seen many rigs with 37's struggle and some of with us w/ 35's cruise through. Picking the right line and letting the jeep do it's thing seams to work out a lot better than more gas peddle.

Agreed. And to add, I often wonder how many actually exceed the limits of the rig before diving into these ambitious builds. I didn't.

 

[Irun]

Agreed. And to add, I often wonder how many actually exceed the limits of the rig before diving into these ambitious builds. I didn't.

Very few!

 

[Irun]

All,

Many thanks for the informative posts. I've learned a couple important things as a result of your input. One final question here. With the exception of weight and performance impacts, is there any reason to not run a 64.5" width axle on 35" tires? If it's useful information, my current wheels are Walker Evans beadlocks. They are a 17" wheel, 5 on 5.5 bolt pattern, with 3 3/4" BS. Although I'm on 35" tires now, I will go to 37" at some point in the future.

 

[jjvw]

All,

Many thanks for the informative posts. I've learned a couple important things as a result of your input. One final question here. With the exception of weight and performance impacts, is there any reason to not run a 64.5" width axle on 35" tires? If it's useful information, my current wheels are Walker Evans beadlocks. They are a 17" wheel, 5 on 5.5 bolt pattern, with 3 3/4" BS. Although I'm on 35" tires now, I will go to 37" at some point in the future.

Ignoring cost, if it meant a better scrub radius, then I would appreciate it. I believe the Currie front 44 and 60 are a bit wider for that reason.

 

[Irun]

Ignoring cost, if it meant a better scrub radius, then I would appreciate it. I believe the Currie front 44 and 60 are a bit wider for that reason.

I'd be looking at the currie extreme 60 front and rear. If I did move up, no more 1/2 ton ball joints for me. Expensive is a given here!

 

[JMT]

I need to build a TJ on 35's with as much wheelbase as we can get with a low profile tank under the back and a couple of inches up front to see how that does in JV. Not sure what the wheelbase would be but I'd certainly like to find out.

I have always wanted a low profile gas tank, even if it means dropping the tank to 15 or less capacity. I think the gas tank, the bumper, and the rear wheelbase are the major limiting factors for departure angles, which everyone who wheels knows will occasionally go 'bang' when coming off a rock. Wheelbase is probably the most important. I just had my exhaust chopped just south of the muffler (still 90* curved down). It's completely out of the way now. I got tired of scraping that shit. Your thoughts are always appreciated.

 

[mrblaine]

I have always wanted a low profile gas tank, even if it means dropping the tank to 15 or less capacity. I think the gas tank, the bumper, and the rear wheelbase are the major limiting factors for departure angles, which everyone who wheels knows will occasionally go 'bang' when coming off a rock. Wheelbase is probably the most important. I just had my exhaust chopped just south of the muffler (still 90* curved down). It's completely out of the way now. I got tired of scraping that shit. Your thoughts are always appreciated.

I can envision a tank that would do what we want. No one would likely pay for or build it due to the complexity. It would have a large divot in the center for the diff to swing back into which moves the fuel pump off to one side or the other. That would require a secondary pump, intricate baffling, or some method to transfer fuel across the connector. I'd have to study on it some to see if it is even feasible.

 

[JMT]

I can envision a tank that would do what we want. No one would likely pay for or build it due to the complexity. It would have a large divot in the center for the diff to swing back into which moves the fuel pump off to one side or the other. That would require a secondary pump, intricate baffling, or some method to transfer fuel across the connector. I'd have to study on it some to see if it is even feasible.

What about the GenRight 19.5 gal tank? That gives you 5” with a Dana 44. Or others that give you 7” or 9”.

 

[someguysjeep]

finding the right tank is the hardest part of any stretch.

 

[mrblaine]

What about the GenRight 19.5 gal tank? That gives you 5” with a Dana 44. Or others that give you 7” or 9”.

Not sure.

 

[hosejockey61]

Blaine I remember years back that you said you moved a gas tank up, partially into the bed (Kat's TJ?) Did that allow you to stretch at all or was it mostly just to raise it up out of the way? I understand it's not necessarily a good option for most but I'm just curious how much it allowed you to push the axle back.

 

[mrblaine]

Blaine I remember years back that you said you moved a gas tank up, partially into the bed (Kat's TJ?) Did that allow you to stretch at all or was it mostly just to raise it up out of the way? I understand it's not necessarily a good option for most but I'm just curious how much it allowed you to push the axle back.

I did one that way that let me get out to 102 in a TJ but the cost and time are not really worth it. That and the disruption of the cargo area is terrible especially in something that doesn't have much to begin with. We cut up a Genright stretch tank to do that one.

I also did one where I moved the stock tank up, again, a lot of work.

On mine, I did at one point put a skid across the bottom of the frame and then built a custom tank with clipped corners that dropped down through the tub floor and shock crossmember area to sit on the skid. Again, far more work and expense than necessary.

 

[someguysjeep]

that alien tank thats supposed to give a +9" wheelbase, still hangs to low for my taste. ^^ a lot lower than that ^^.

i even had a goofy thought to use 2x5gl vessels in the wheel wells like saddle bags. feed those down to a can with the/a pump assy in it and just endure filling 2 small tanks.

 

[mrblaine]

that alien tank thats supposed to give a +9" wheelbase, still hangs to low for my taste. ^^ a lot lower than that ^^.

i even had a goofy thought to use 2x5gl vessels in the wheel wells like saddle bags. feed those down to a can with the/a pump assy in it and just endure filling 2 small tanks.

Semi trucks with saddle tanks only have to fill one side, just sayin'.

 

[Mr. Bills]

that alien tank thats supposed to give a +9" wheelbase, still hangs to low for my taste. ^^ a lot lower than that ^^.

i even had a goofy thought to use 2x5gl vessels in the wheel wells like saddle bags. feed those down to a can with the/a pump assy in it and just endure filling 2 small tanks.

Its not such a goofy thought. Back in the day one of the well known vendors offered auxiliary fuel tanks for CJ's that fit under each wheel well behind the rear tire. Five gallons per side if I remember correctly, but they may have been smaller.