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Thursday, August 8, 2013

Why did my engine "blowout"?

Why is it we almost never hear the above question on various RV forums?
Could it be that we don't hear this question because people know it is not wise to exceed the RPM limit or "Red-Line" specification of their engine so they simply don't run at and above the engine speed limit?

But I do see questions of what has to be done to run 75 mph with a standard ST type tire when the RV manufacture specifies the pressure on the tire which is one of the factors limiting the tire speed capability.

The interaction of tire types, load capacity, inflation and speed "rating" are complex, as are almost all topics about the various performance trade-offs with tire selection

I did a post just on the topic of "How fast is safe to drive on your tires?"   I have also pointed out that if you are going to deviate from US industry guidelines, such as published by Tire & Rim Association, you must follow the published guidelines for your specific size, type and brand. You cannot use Goodyear document to learn the appropriate load & inflation for your Maxis tires. It even means you cannot apply specifications, including speed, load & inflation from a Goodyear Marathon with a Goodyear G614.

Now as we all know, an ST type tire has a higher load capacity rating than a similar sized LT type tire.
ST235/75R15 LR-C is rated 2340# @ 50 psi (min) and 65 mph (max).
LT235/75R15 101/104Q LR-C is rated 1985# @ 50 psi (min) and 75 mph (max).

We have also established that load capacity is nominally a function of tire size (air volume) and its inflation pressure. If however we decrease the tread depth and the lower the maximum operating speed we can increase load capacity slightly. In my post of Oct 12, 2011 I identified a Michelin trailer tire with a higher load capacity but many want to ignore its max speed rating of 62 mph which is even lower than ST type tire rating of 65 mph.

Now to the specific question of how to operate a specific tire at 66 to 75 mph. This is accomplished by increasing the inflation pressure by 10psi according to a document published by the tire manufacturer. This is not as simple as many assume.

If you select the 65 mph max speed for an ST245/75R16 LR-C you will find a load capacity of 2,600# with a minimum of 50psi if you stick to 65 mph max. But if you want to drive at speeds up to 75 you need to increase the inflation pressure in the industry standards as published by Tire & Rim Association. So now to carry 2,600# you need an inflation of 60psi but this exceeds the max inflation rating for this size @ LR-C so you either need to get new LR-D so you can run 60psi or to limit your load to 2,270# and 60psi which is the published load capacity for 50 psi inflation of the subject size tire. This higher inflation may also exceed the rating of the wheel, but again many choose to simply ignore that rating too.

One obvious item many simply ignore is that the max speed for a tire is much like the red-line rev limit for their engine. It clearly is possible to run an engine with a 6,000 red-line at 6,300 RPM and possibly higher but what about long term engine life?

There are hundreds if not thousands of posts on RV forums from people complaining about tire failure but I have to wonder why they seem to understand the effects on engine life of high RPM operation but fail to understand that running a tire faster than its design specification will also contribute to shorter tire life.

1 comment:

  1. I read your informative article on tires. I've done my research & weighed my M/H at each corner & used the tire pressure recommended for that tire by the manufacture. I was very curious about the wheel specifications as to air pressures, weight etc. where is that information located on the wheels.
    Thanks Dave


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