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Old 05-02-2012, 06:53 PM   #21
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Re: Getting my SMB....couple questions?

Quote:
Originally Posted by TraceyAZ
When would you need 80psi?
TraceyAZ
Maybe never, maybe always. Depends on the weight on the wheel and the ratings of the tires.

First, you need to know how much your van weighs, preferably, individual weights for each tire. Then:

This is copied from old thread about tire pressure:

The best method to calculate tire pressure requires weight measurements for the front and rear of the van, even better is each individual tire weight measurement. Then calculate how much pressure you should be running. This from an earlier thread:

Old school answer:

The pressure rise from cold tire to hot tire should be 4-6psi. Less than 4psi rise means the cold pressure is too high, more pressure rise means the cold pressure is too low.

Best answer:

Get the actual weights on the tires and ask the manufacturer for the proper pressure for that weight.

My method:

Again, get the actual weight. Compare it to the max weight on the tire. Do some math.

Actual weight/Max weight X Max Pressure = Desired pressure

If you have 2500# on a tire rated at 3000#@80psi you get:

2500/3000 = .8333 .8333 x 80psi 66.66psi, which I round up to 70psi


Then I always round the pressure up. Much better to err on the side of too much pressure than on the side of too little pressure.

As a general rule, D and E rated tires carry the same load at the same pressure. The big difference is E rated tires can handle more pressure, hence more load.

My van weighs in at 9800#, fairly evenly split front/rear side/side. The tires are LT275/70-18E rated at 3640# @ 80psi. For 4 tires, that would add up to 14,560# at 80 psi. My math says I have 9800/14560 of full load. That is about 67% of the max tire ratings. 67% of 80psi is about 55psi. I run 65 psi. A good safety margin. I will never run a tire at its full capacity. Just too much at stake for me.

4 tires rated at 3000# would be too close for me. Going down the road with some lean to the road, the outside tires could be overloaded for many miles. I wouldn't do it. I always want at least a 10% safety factor, 20% is better. Hence, my 65psi in my tires.

I always round up with tire pressures since higher pressures (within the max tire ratings) will reduce tire temps and might tend to wear the center of the tread. Lower tire pressures will cause higher tire temps and can easily cause a tire to fail.

Your RB probably has less weight on the rear than our EB does, hence lower tire pressure on your rear tires.

There should be enough here to confuse the issue. Good luck.

Mike
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Old 05-02-2012, 10:32 PM   #22
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Re: Getting my SMB....couple questions?

Hey Mike,
I do appreciate all the info. I have an EB van so maybe a bit more potential weight could be stashed in there.
I guess I will just keep a good close eye on all of it.
Thanks
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Old 05-02-2012, 10:53 PM   #23
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Re: Getting my SMB....couple questions?

My experience, tell them the spacers are factory installed and not to futz with them. If they won't sell you tires go somewhere else.
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Old 05-02-2012, 11:11 PM   #24
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Re: Getting my SMB....couple questions?

Quote:
Originally Posted by jage
My experience, tell them the spacers are factory installed and not to futz with them. If they won't sell you tires go somewhere else.

Hey jage,
I stood my ground with them, and kept the tone friendly. I don't like being put on the spot to buy anything. Im sure they got the idea that I wasn't going to be pushed into something.
Im just wanting to find out what everyone in the real world of SMB's thinks about the stock set up and is it ok to run the E rated tires even if you should not run the psi above what the rim says you should.
Also I thought I read some where here on this forum that changing out those spacers could effect the rear axle life? Not sure, Ive been reading for about 8 months now lol. My brain might be a bit scrambled.
Seriously loving my SMB!! I just don't want to be making any serious mistakes....or not so serious. lol.
TraceyAZ
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Old 05-02-2012, 11:18 PM   #25
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Re: Getting my SMB....couple questions?

The spacers are there to supply a bit more side to side stability and allow the vehicle to track the front tire foot prints like when running through snow. The full float axles are cut to equal what most semi floaters measures with the spacers. There are some safety issues with spacers and is why the tire guy made that point but if installed correctly they shouldn't pose a problem. You can always call SMB for more info. If not installed right, some owners have lost a rim while driving. Also as stated, the rims must be rated for the weight of the van when fully loaded just like the tires.
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Old 05-03-2012, 10:04 AM   #26
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Re: Getting my SMB....couple questions?

Even properly installed, spacers increase the bending load on a semi-floating axle. And a couple of forum members have had those axles break!

I run rear spacers to match the front (F350) bolt pattern. Because of the above failures I swapped in a full floating rear axle from a cutaway van.

Mike
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Old 05-03-2012, 10:21 AM   #27
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Re: Getting my SMB....couple questions?

How do you tell if you have a full or semi full float axle?
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Old 05-03-2012, 02:10 PM   #28
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Re: Getting my SMB....couple questions?

Don't let it worry you too much...there are several SMB's out there with semi floating rears. Like most stuff, equipment is usually designed to exceed what we put to it. But once you start to change the stock items (like adding spacers) you begin to go outside what it was designed to do. If you tow, travel tough terrain on a regular basis, or constantly overload your vehicle, a light semi floating axle is subject to failure. Is it guaranteed to fail...no, but it's like a 300 lb guy sitting in a 250 lb rated chair. You never know when it will collapse.

More than likely, if SMB did the conversion and it has spacers, you have a semi floater. Here is a thread to help you:
viewtopic.php?f=39&t=8543
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Old 05-03-2012, 08:59 PM   #29
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Re: Getting my SMB....couple questions?

The “Full Floater” has the bolt heads showing - The “Semi Floater” has NO bolt heads. (Could be either Drum brakes or Disc brakes - The axle ends look the same)



Attached Thumbnails
Full Float.jpg   Semi float.jpg  
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Old 05-03-2012, 10:34 PM   #30
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Re: Getting my SMB....couple questions?

Yeah, just be aware you might have to take the hub cap off to see that. I finally pulled my caps off for good. Looks better IMO anyway.
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