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Old 10-12-2014, 07:06 PM   #1
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Safe minimum dual tire clearance?

As I am beginning to think about new tires for my drive axle over the winter, and the fact that I think if you ask 5 different tire shops you'll get a combination of 12 different incorrect answers and old wives' tales ... I figured I'd ask here.

My coach was originally delivered with 255/80R22.5's all the way around. There is a notation in the manuals that back in 2001 the factory swapped the tires to 275/70R22.5's on the steer and 265/75R22.5" on the drive axle. I believe this was done because the coach, as delivered, was tire-limited as far as axle weights -- certainly, as I have it loaded (which is not stuffed) it would be slightly over the ratings for the 255's on the front.

Now, as built, the coach actually has a 19,000 pound rear axle and a 12,000 pound front end, so bumping to the bigger tires didn't impact the safety of the coach itself. And I've been through the coach, underneath, etc., and see nothing negative, so to be honest, I'm not worried.

...so getting back to the point of the thread... I would like to put 275/70R22.5's on the drive axle because there seems to be a MUCH better tire selection in that size. And the diameter should be the same, so no worries there... but of course, I will be narrowing the gap between the duals by 20mm (almost an inch). There seems to be a bunch of clearance between them now (far more than my old 16" tires on my gas motorhome) but I have no idea what the standard for "safe" is.



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...and F-Troop: Fearghus, Fiona, and Frankie (Cairn Terriers)
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Old 10-12-2014, 08:45 PM   #2
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As my memory serves me, 1.5" is the minimum between duals as established by USDOT. I cannot find that now though. This is the best I have to offer as documentation: http://www.trucktires.com/bridgeston...ualSpacing.pdf
Note: the dual spacing numbers are from center of one rim to center of the other. You may calculate the actual distance between tire sidewalls from tire dimensions from mfgrs. website. For example, divide 275 by 25.4 = 10.8" section width for that tire. Hope this helps.

2000 Winnebago Ultimate Freedom USQ40JD, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member,FMCA."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert the Constitution. "Abraham Lincoln"
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Old 10-12-2014, 08:48 PM   #3
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Dual spacing is determined by rim offset. My 22.5 inch Accuride rims have an offset of 6.44 inches, giving a spacing of 12.88 inches. (In this drawing, HDS is rim offset or Half Dual Spacing) Easy way to get your spacing is to measure from the center of one tread to the center of the other, regardless of the tire size.. Easier than finding the center of the rim....

The tire manufacturer lists diameter, tread width, revolutions per mile and required dual spacing for each tire.
Check your load rating, the 275/80 has a higher maximum load capacity, depending on whether they are load range H or J.
Changing tire size on the rear axle with affect the speedometer, the 275/80 turns 516 revs per mile the 275/75 does 545. About a 5% error.
When the tire mfgr lists the minimum dual spacing, it takes in to account the tire flexing and necessary clearance between the tires.

Hope this helps...
Hooligan, Pensacola, Fl -U.S. Coast Guard 1956-1985
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Old 10-13-2014, 09:18 AM   #4
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As Hooligan says, the tire manufacturer sets the minimum required dual spacing for each tire size & model, but the wheel (rim) style and offset limits how much you can actually achieve (without changing to a different wheel, that is). So, you need to establish how much spacing you have with your existing wheels and compare that to how much is required by the tire you want.
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