tire sizes

[gmcbuffalo]

I currently have 315's all around and I'm due for a new set of drivers. I was wondering if anyone have gone to 12R22.5 for the drive axle. I'm going to keep the 315/2.5 on the tag and steer The operators manual said the tires size in preference order is 315, 12.5, 12R and 11R. But again the one aluminum and one steel wheels with 315's on the drivers makes for some very close rubber. that is why I was thinking about 12R's.
GregM

[Jon Wehrenberg]

Greg,

When your coach was built it was likely it had 12.00 tires. The front axle weights on some coaches of that vintage was right at the limit for the 12.00 tires (7400 at the max inflation pressure?) so some owners went with the 315 tires thinking that makes the weight problem go away.

The normal rim for the 12.00 was 8 1/4, and for 315 it was 9.00, but a 315 on an 8 1/4 is acceptable according to the Michelin specs.

The 12.00 tires mounted on the drive used 2 aluminum 8 1/4 rims, but to remain a legal width when 315 tires were used Prevost went with 9.00 wheels, but one steel and one aluminum. The 9.00 rims are listed as acceptable for 12.00 tires.

I question why you would want to retain the 315 tires at all. Unless the coach came that way from the factory I would go back to 12.00 all around. They are cheaper, and with all the same size tires you can correct any uneven wear pattern by moving the bad tires to the drive. If there is a weight problem just putting on 315 tires will only allow you to carry the weight at a lower tire pressure, but it does not change the weight limits on the wheels, axle, bearings, etc.

[dalej]

Just my thoughts,

I weighed in and I'm in a safe range of 12Rs all the way around. I have put 315s on my steer because some times when I roll across a bridge at 65 the front tires of the bus feels like their taking on a lot of extra stress. I just feel like the 315's can take it a little better.

[truk4u]

Jon,

I thought they came with 315's and 9.00 aluminum and 8.25 steel? Mine has two 9.00 aluminum and I hate it cause with the air dumped, the fender flairs are really stressed. I assume I'm also over 102.

I'm thinking about changing out the 9.00 inside aluminum and going back to 8.25 steel. There almost new, I can make someone a deal.

When I questioned this dual 9.00 rims at Nashville, they said they do it all the time because people hate the steel wheels.

Any thoughts....

[Jon Wehrenberg]

The width limit is exceeded with a pair of 9.00 rims (aluminum) which is why Prevost went with the steel inner wheel. That is why you hit or almost hit your fenders.

It is not as easy as just changing wheels from steel to aluminum because the studs need an increase in length. I think it is OK to go the other way as long as the inner wheel "nuts" do not bottom out on the studs.

I don't know why anyone objects to the steel inner wheels. Nobody sees them. And I am certain, that except for me, nobody polishes them.

[gmcbuffalo]

Jon my drivers are on 8.25 aluminum wheels and inner steela. I think that my tires would die of old age before they ever got a wear problem. If they did I need to find out way. I should look more closely to the other rims, who knows maybe they have been rotated and the 9.00's on in a different position.

I was thinking the 315's would help with weight since these early 90's coaches were really heavy in front from all the real wood cabinets. Beaver has alway prided themselves on their cabinetery. If they don't help with weight they still would geve me a little more rubber on the ground to brake with.

Thanks

Greg

[Joe Cannarozzi]

Greg

How much is the front axle weight exactly?

The difference in the weight ratings between those two tire sizes is ? 500lbs per tire I think, if that.

What is that the threshold for deciding how much weight do you carry on 12.22/5s before moving up to the 315s. IMO these tires are very underrated so I would switch at 90% of manufacturers rated max. capacity, if I were being very safety conscious.

Going from memory here I think it is 7250lbs and 7750lbs respectively so that would make 90% cap. for the 1222/5s anything over 13000lbs on that axle.


What is the rating on the super singles, 365's are they? They need to be at least 10000 a tire, right?

[Jon Wehrenberg]

Joe....at max pressure the 12.00 tires are around 7400 capacity and the 315 are around 9000 capacity.

But it is irrelevant because the 8 1/4 wheels I think are rated around 7400, and even if their rating was more the bearings, axles, airbags, shocks, etc. are all sized for the 7400 pounds. Prevost did a seminar about then in the early 90s and just changing tires is nothing more than a placebo.

This all goes back to converters and owners piling on the weight and this type of discussion has take place as far back as I can remember on the internet. That's why Dale used to make his wife ride back in the bedroom. He needed to get weight off the front axle.

[Joe Cannarozzi]

If you already switched to the bigger bags and went with 9 inch rims with 315's, some kona shocks, that might be a pretty good upgrade all combined?

Wheel bearings? Fill the cavity with all syn. Lucas gear lube additive;)

[gmcbuffalo]

Joe my steer axle is close to 13000#.

I could probably use the bigger air bags just to help raise the bus.

You bring up shocks I only have two on the front axle I notice that there are fittings for a second set. Did the bus come with only two shocks or has someone taken a set off to soften the ride?

Greg

[Joe Cannarozzi]

I was off a little on the 12.22/5s if they are 7400 a tire than 13320lbs on that axle would be 90% of max.

I regularly put 12000lbs on 11.22/5s, the next size down, on the steers on my Peterbuilt. They have always preformed flawlessly.

I recently had a discussion with Jon about the preferable cost and availability of the 11.22/5s combined with my current axle weights. When tire time comes I might very well size down thus being able to rotate out dated tires over to the truck before dry rot sets in.

I don't mention this to convince you to do this but to give you more confidence in your 12,22/5s.

I have 2 sets of shocks up front but have not experimented there yet. Prevo Parts in Elgin carry them and have a prt#.

Many have upgraded to kona shocks. Harry really spoke highly of the switch.

[Jon Wehrenberg]

Greg, If your steer axle is at 13000# go back to 12.00 tires. Cheaper, and more than adequate.

You may check to see if you have the "Mae West" air bags. Prevost parts should be able to give you the trade part numbers from which an internet search will give you the dimensions. When the axles were close to their limits weightwise some front ends were slow to raise or would not raise so Prevost came up with different air bags with a larger diameter. That did the trick. If my memory is correct the four shocks came into being as a retrofit around the same time and it was to deal with rebound on heavy front ends. If I am correct you are OK on your front end weight and if you do anything have the larger air bags installed (if they are not already) when you need to replace air bags.

In the FWIW category, if your air bags have about an 8" lifting diameter (I am guessing here) it will take about 125 to 130 PSI to lift your front end. This is a guess because I do not know your unsprung weight. By increasing the air bags by only 1" diameter it drops the air pressure required to around 100 PSI. Don't take these numbers to the bank, they are guesses and only serve to illustrate how a small increase in the air bag diameter has a significant effect on the pressure required to support the coach.

[Joe Cannarozzi]

Very sensable advice Jon. Bigger bags new 12.22/5s and one set of Konis.

There are quite a few of the early 90's stuff that falls into the same catagory.

[gmcbuffalo]

Thanks for the advice guys.
Greg