Tires: reduce tire size?, tire placard, inflation pressure

Question
QUESTION: I am wanting to go in another direction from most folks. I want to put the smallest(as in footprint) size tire that I can on a 15 x 7 rim. I now have 24560R15's, but I have too much sidewall flex for my application and hydroplaning is a real problem. What is the smallest tire I can fit to this 15 inch wheel and still keep something close to overall width with the stiffest sidewall possible. I don't mind loosing a little overall width if I have to.

ANSWER: Roger,

There are a couple of things that impact this issue.

First is load carrying capacity.  This is controlled by tire size and inflation pressure.  Here's my standard statement on that subject:

On every vehicle sold in the US, there is a sticker – commonly called the tire placard - that lists the original tire size and the proper pressure for that size.  The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.

I have a book that summarizes the placard tire size and inflation pressure for recently produced vehicles.  It's organized by make / model year / model - and there is a version of this is organized by tire size, but it isn't accessible right now.  But the tire size you listed doesn't sound right for an original equipment tire size.

So I suggest you post a followup with what the placard says:  tire size and inflation pressure.  It would also be helpful if you listed the year / make / model of the vehicle in question - because of what I am about to write.

Hydroplaning is not controlled so much by footprint size as by the width of the footprint.  But hydroplaning is also controlled by such things as inflation pressure, tread depth remaining, depth of water, and vehicle speed. Notice the sidewall height is not mentioned in this list.

If you are considering changing tire size in order to improve hydroplaning resistance, then going narrower in footprint width - which also means section width - is the way to go.

But you have to also consider that the source of your problems might be something else - like the tires are nearing the end to their wear life and need to be replaced.  The hydroplaning is just a symptom of the state of wear.

So if you would post a followup with the items listed above, I can better answer your question.

---------- FOLLOW-UP ----------

QUESTION: These tires are on the rear of a motorcycle trike. The total weight of the trike is around 1200, but loaded with passengers and baggage is around 1500 pounds. I don't think there is enough "weight" to really get these wide profile tires down in a wet roadway and then there is a handling issue. The manufacturer of the trike suggested 22-24 pounds, but this seems much too low and results in too much "flex". Recent model trikes have different tires sizes and some have gone to a 16-inch rim. Harley is now using a 20570R15. Can you tell me what "handling" differences I may experience between the 24560R and the 20570R. Any loss in height will not be a big issue as the trike needs to be geared-down a bit anyway. What is the overall height between the two on a 15-inch rim? Will the 20570R15 fit the same rim as these 24560R15's? Is there any particular tire type/size you could suggest that would give me a stiffer sidewall?

Thanks!

Answer
Roger,

A P245/60R15 has a load carrying capacity of 1753# at 35 psi, and at 26 psi (the lowest pressure listed in the standard) the load carrying capacity drops to 1521#.  If I do an extrapolation, at 22 psi the load carrying capacity drops to 1403 #.  This far exceeds the weight of the trike.  It's no wonder you have hydroplaning problems.

Also, a P245/60R15 has an allowable rim width range of 7" to 8 1/2", while a 205/70R15 has an allowable rim width range of 5" to 7".  So the smaller tire would fit on the 15X7" rims.

So let's go backwards and look at the load carrying capacity of a P205/70R15 at 35 psi, 26 psi, and 22 psi = 1499#, 1301#, and 1213# (estimated).

Now let's compare diameters:  P245/60R15 = 26.6" and P205/70R15 = 26.3":  a 1.2% difference!

OK, back to the original question - hydroplaning.  Clearly a narrower tire would be better for this property.  But you should be aware that different tires could have more difference in hydroplaning properties than the change in tire size would.  So careful tire selection is called for.

But you've twice mention sidewall stiffness and I am wondering why?

First, sidewall stiffness is more a function of inflation pressure than sidewall height.  Given that both of these tires are being used at loads so far off the scale of normal usage, I would think that sidewall stiffness between different tires - even tires of different sizes - would be minimal compared to the effect inflation pressure would have.  Not to mention that trikes are quite different than cars, and the way cars work result in certain handling properties than aren't applicable for trikes.

And I have to admit, I don't know enough about the way trikes work to be definitive in predicting the handling changes that would happen in a size change of this type.