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Tire Diameters
One of the more amazing pieces of information to come from the Michelin seminar was the importance of matching tire sizes when the tires are duals such as on our drive axle.
If for example one tire has a rolling diameter of 42.25 inches and the other has one at 42 inches due to tread wear, tire size, or inflation pressure the smaller tire is going to be dragged down the road. That seems impossible, because both tires are going to have the same revolutions per minute, but the key is in the distance traveled for each revolution.
One revolution for the larger tire is equal to 132.73 inches in this example. But the smaller tire has only 131.94 inches per revolution. That is less than an inch difference, but the larger tire is going to rotate 477.36 revolutions to go a mile. If the smaller tire goes a mile it would have to make 480 revolutions to go the same distance, but it cannot because the two tires are locked together so for each mile traveled it is the equivalent of being dragged 350.4 inches or 29.2 feet.
Think of that as locking up the wheel 29 feet for every mile traveled.
The point is if you have to replace a tire on the drive axle the rolling diameters have to be exactly the same or the smaller tire is going to experience the equivalent of being dragged or skidded down the highway.
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10 Replies
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We talked about this once before so I wonder if Michilen said anything about our outside drive wheel being 9″ and the inside being 8.25″ if this would make any difference in tire circumfrence if you inflate each tire to the same poundage.
Alot of crap to worry about Jon since I don’t have anything else to do !!!!!!!!!!
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I neglected to raise the question, but I will see if I can get an answer. Good point, but it is an example of something you cannot do anything about if it does influence the rolling diameter of the tires.
We did discuss both heat and tire wear on the inside drive axle wheels. We did thermal imaging and saw tire temps rise and reach equilibrium. The inner tire was warmer than the outer tire. On this forum we speculated it was due to proximity to the engine or transmission. What it turns out to be is the axles flex (hard to believe but evidently proven by Michelin) and the inner dual gets slightly more weight on it as a result of the flex. Not enough to require any changes, but enough to be able to measure the increased temp. They take temps the same way we do and the hot spots on all tires is at the edge of the tread just where it merges with the sidewall. This is where the bands are flexing the most and it is the flexing of the bands inside the tire plies that increases the temp.
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Interesting. Any discussion of duals vs super singles?
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No formal discussion, but anecdotally fleets that have used them and tested along side of their trucks with duals have gone back to duals, with those saying ability to get roadside service and back to the shop or to the customer as the primary reason. Michelin did not discuss one tire versus another except to discuss things like fuel efficiency or wear and the super singles were never discussed in that respect.
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Jon,
Just want to say thank you for posting the information you learned about tires.
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Kurt, We who attended the seminar got information overload and I am going to try to continue posting information to refute some of what we believed, and to reinforce some knowledge that is very important.
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Jon, I always notice that the temps on my Pressure Pro show different readings on my drive axle after I have been driving for a while. Before running anywhere, they are exactly the same, but as I run, they do not stay the same. Usually, it’s the inner, but other times, if the sun is beating down on that side of the coach, the outer will have a higher reading. I imagine that the tire revs per mile must also change too. I see that most of the big fleets are running the jumpers between the duals, probably to keep the pressures equal, but also to make it easier to check & fill for large operations. Was anything on this discussed? It seems to me that it would make sense to do this if there is so much concern over the revolutions per mile effecting the tire.
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The regulators and pressure balancing systems were discussed but you can still have different rolling diameters even though the wheels have the same pressures.
On our coaches the best we can do is match drive axle wheels if we can. Remember a lot of the discussions related to truck fleets where tire wear and fuel mileage are important factors. If we have mismatched drive axle tires (within reason) we lose a little efficiency and one of the tires gets more wear than the other. But in our case if our rolling efficiency drops by a percent or so and one tire gets 2/32″ more wear over its life it has no impact on us. But if we are Perdue with a large fleet of trucks the combination of the poorer rolling resistance and increased tire wear and be tens of thousands of dollars annually.
They also discussed the Halo Tire Inflator but it also was in the context of minimizing service calls and tire life issues from running low pressures.
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Bill,
When I bought my coach it had one of the “cat’s eye” systems which connects the dual tires. I finally checked the inner and outer pressures individually and to my surprise the inner was significantly lower that the outer which was at the correct pressure. Both sides were the same (uneven pressure). Both units had failed.
They are now in the trash can and I won’t recommend them to anyone. I’m scheduled to get new drive tires this Saturday. Otherwise I would be doing a lot of soul searching trying to determine if I had run the inner tires under inflated by more than 20%.
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Michelin made a point out of owners checking the regulator on that system to prevent the problem you describe.
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