Calculating the H2H Distance- A math challenge

[MN_GEO]

Math Wizards and Engineers,
So I've seen many posts on various H2H measurements, but nobody seems to be answering quite the same question. I have an M100, tub width at the base is 38 inches. I want 2 inches of clearance between the tire and the outside wall of the tub. I'll be running stock JK wheels/tires, 255/75R17 (32.1/10R17). Backspace on a stock JK rim is 6.25 inches. I've estimated the H2H, but everybody seems to order a shorter axle than I've estimated. Anybody out there care to take swing at this problem. I find it hard to believe everyone just wings it.

 

[rnArmy]

Read this post and the next post (426 and 427). It should answer your question. You'll need a straight edge and a ruler to calculate it (H2H).

http://www.jeepforum.com/forum/f27/...d-up-thread-1180456/index29.html#post14977300

 

[croix]

Here is how I approach it -
A 10" wide tire is 2.5" wider than your rims. That means ~ 1.25" of tire will stick out inside your rim. This must be added to the 6.25" backspacing to get the distance from rim to inside of tire = 7.5"
.
38" (tub width) + 2" * 2 (clearance from tire to tub) + 7.5" * 2 (inside of tire to rim)
38+4+15 = 57" hub face to hub face
.
If you get bigger tires the value obviously changes. For example a 35x12.50 tire works out to 59.5" hub face to hub face
.
-Croix

edit: keep in mind that the jeep rims do not work directly with trailer axles as the center isn't big enough. you can either have the center of the rims milled or use adapters. If you use adapters then the thickness of the adapters will reduce the length of the axle.

 

[indiedog]

I don't know if tire sizes indicate the width of the face that hits the road or the overall width including the bulge of the side walls???

Get the actual trailer wheel (normally inflated tire on the trailer rim) and lay it flat on the floor with the outside of the rim facing up. Poke a tape measure through the centre and measure from the floor to the back of the rim face that sits on the hub face. Let's say that's 5.5 inches for example. This accurately measures from the outside of the side wall bulge to the face that will contact the face of the hub.

Outside face to outside face of axle hub length = chassis/tub width + 2 x 5.5" (2 x backspace of wheels to outside of tires) + 2 x 2" (clearance both sides). So if the tub is 48 inches the face to face length is 48 + 11 + 4 = 63 inches.

When I ordered my axle that's how the guys told me to measure it. Hope that helps.