Choosing Tire Load Capacity

[Twisties]

Most of the tires that we see used on RT's, ST's, and GT's:

 

Michelin PR2's, Dunlop Roadsmarts, Metzeler Z6's, Continental Road Attacks, Avon Storms, etc.

 

are not rated to the full load capacity of the bikes. For example, the Dunlop Roadsmarts are rated for 520 lbs front, and 805 lbs rear at 42 psi. The Z6's are rated at 520 lbs front and 804 lbs rear.

 

The RT is spec'd as follows:

 

Unladen weight, road ready, fully fuelled 1) 571 lbs (259 kg)

Dry weight 2) 505 lbs (229 kg)

Permitted total weight 1090 lbs (495 kg)

Payload (with standard equipment) 520 lbs (236 kg)

 

BMW spec's tires by size, but without a load rating.

 

Obviously, most of the time, the load is distributed between the tires, however, that distribution may not be even. Under certain riding conditions loading could change dramatically.

 

How does one determine the minimum load rating for front and rear tires? Given that the RT for example has a max total weight of 1090 lbs, how do I know that a front tire rated less than half of this (520 for instance) is ok? Especially since BMW no longer approves tires.

 

Incidental to this, I don't see the PR2 Fitment B tires on Michelin's website today, and their fitment tool recommended a straight PR2 for the RT.

[Firefight911]

Here is a good reference chart for things motorcycle tire related - LINKY

 

The tire would be quite unbearable and not very compliant if they were individually rated for the full capacity of the bike. Additionally, there is no need to rate them for the max weight of the vehicle as that falls well outside of the design parameters, i.e., they're designed to ride on one wheel, not two.

 

One key factor in load rating is heat. An overloaded tire creates greater heat can be compromised with this heat in time.

 

Although BMW may not "recommend" tires, they supply them with your bike. Just maintain the load rating of the tire that it comes with and you would be fine.

 

As for the rating of the "B" spec vs regular spec, they are the same rating as verified by me calling Michelin two or so weeks ago. The difference is in the handling characteristics not the load capacity.

 

For fun, go look at your 3 - 4000 pound vehicle in the driveway. It is not rated for the entire load of the vehicle either.

[T__]

Jan, to be totally anal about it just load the bike to full GVW then place it on two scales.. Or just buy the tires & ride the bike like most of us do.. Keep in mind most added weight is placed on the rear not the front unless you carry an extra anvil in your tank bag..

 

Also keep in mind tires are actually rated under their maximum carrying weight by a certain percentage (that can be as high as 25% on an light truck or around 10% on a car (not sure on a motorcycle)

 

Also keep in mind the extra letter prefix on some motorcycle tires sometimes equate to load capacity,, or handling traits,, or extra ply,, but it can also mean anything as sometimes that letter refers to mounting characteristics as some motorcycles have rather weak rims that can be bent mounting a very stiff walled tire (you need to check with the tire manufacturer to determine what that extra letter refers to on certain tires)..

 

Twisty

 

[smiller]

To reiterate a bit, individual tires are not required to carry the entire gross weight (bike + payload) of the bike but both tires in combination must (or you must de-rate the bike's load capacity.) As Twisty noted each tire does not have to be rated to the total gross weight of the bike because they both share the load, and the only way to know for certain that you are not overloading an individual tire would be to have both ends of the bike weighed with an appropriate load aboard. In most cases the manufacturer has done this for you though and as long as you stay with the recommended tires sizes you will in all likelihood be OK.

[outpost22]

To reiterate a bit, individual tires are not required to carry the entire gross weight (bike + payload) of the bike but both tires in combination must (or you must de-rate the bike's load capacity.)

 

So are you saying I'm supposed to stop doing wheelies's ?

 

[smiller]

A momentary overload is probably OK, just don't ride in that mode for your entire commute...

[SageRider]

1-up moderately loaded, it appears one could continuously wheelie and not exceed the rear tire's limits.

It would appear to be not recommended 2-up and heavily loaded.....

[Bullett]

1-up moderately loaded, it appears one could continuously wheelie and not exceed the rear tire's limits.

It would appear to be not recommended 2-up and heavily loaded.....

So I guess that means you've tried it?

[der Wanderer]

*** Insert all sorts liability disclaimers. Follow manufacturer's recommendations. Whatever you do with this, I am not responsible! ***

 

I used to design and test tires and I have a bit of technical understanding of how the speed and load ratings work. I'll summarize a couple things briefly (if I can). Warning: geeks only - read at your own risks!

 

There are two major modes of failure for tires (apart from wear and road hazard):

 

- Speed failure: it's generally caused by thermal deterioration of the tire due to the heat produced via hysteresis of the rubber when the tire develops high amplitude static wave patterns (initiating in the contact patch and due to the combination of tire flex under load and high speed or due to under-inflation). Interestingly, most people believe centrifugal forces are the cause for high speed failures. Centrifugal forces in tires can be huge (sollicitation can reach 2000 g at high speed) but it's not a common cause of failure for well designed tires.

The tire speed rating (V, W, etc) is meant to express the ability of the tire in that domain. Tire ratings are self certified by manufacturers; there are standard procedures, but they are not always applied consistently and virtually most of the V, W and Z tires made by second class manufacturers that I have seen tested would have been classified at least one speed rating below by the manufacturer I was working at, meaning that those manufacturers may tend to provide a lower safety margin.

Failure happens when too much heat is created to be evacuated through conduction through the rubber and convection with the air outside the tire; it typically takes a few minutes at the max speed to fail (and if that speed is say 270 km/h, there will be very few situations where it is physically possible to ride at that speed for minutes). Failure is catastrophic (in the engineering sense - but also occasionally in the practical sense, with tires potentially "exploding") and exponential, meaning that if a given tire is rated for a speed of say 240 km/h, it likely will be able to sustain speeds just below the failure speed (say 220 km/h) for almost infinite durations. Therefore it is not uncommon in aftermarket to mount say a H rated tire instead of a V or W rated tire if the user knows he will never ride at speeds close to or exceeding 210 km/h and monitor their tire pressure carefully.

In tire construction, higher speed ratings are typically obtained by reducing the amplitude of the static waves through stiffening of side walls and longitudinal belting of the tire with "zero degree" nylon or kevlar belts. In turn, this "might" increase the cornering stiffness of the tire and make it more sporty (but that's far from a given).

Load on the tire influences high speed endurance, because more load creates more flex (in a quasi linear manner if the tire pressure remains the same) and more flex means more amplitude for the static waves; so does camber because it increase flex on one side of the tire. Increasing tire pressure reduces flex and increases high speed endurance.

If a tire is loaded close to the load rating, it will fail at lower speed than if it is less loaded - but those speeds "should" always be at least the rated speed at the appropriate inflation pressure.

With respect to the original question of using a tire with load rating less than the max weight on the wheel in operating conditions, this would potentially increase the risk of high speed failure because the speed rating of the tire is determined in conjunction with the rated load, and as stated above the max speed of the tire would be less under more load. However this is likely to be lower by maybe one speed rating, not by a huge speed difference, and can be mitigated by (reasonable) over-inflation. If the user rides significantly below the rated speed and very consistently monitors tire pressure, this mode of failure is less likely to occur even in cases of (moderate) overload.

 

- Mechanical endurance failure: as is the case for any mechanical object under cyclical loading, tires can experience fatigue failure. A truck tire carcass might run one million miles; that's not far from 1 billion load cycles... Ply separation, thread loss, and a variety of other ugly issues generally result from such cyclical sollicitation. Only rarely is the failure catastrophic though.

Evidently, the more deformation in the load cycle of the tire, the more likelihood of eventual failure. Therefore the combination of load and tire pressure is a primary driver in mechanical endurance. Higher loads might reduce in failure over time. Occasional overloading is not a significant concern in this case.

This is a domain in which no standardized test exists, and the only way to go to be safe, besides monitoring tire pressures, is by brand. In my experience, there are huge differences in this domain, primarily due to shoddy production quality in lesser brands.

Fortunately, motorcycle tires wear out quickly; this mode of failure is much less likely in 10 or 20 thousand miles than on truck tires, etc. With respect to the original question, and provided some reasonable over-inflation is applied consistently, this mode of failure is quite unlikely to manifest itself on motorcycle tires.

 

 

[motoguy128]

How much over inflation is acceptable??? I think hte the RT's case for example, fully loaded, a pressure of 42psi is recommended on the rear tire. I believe the maximum pressure listed on the sidewall is also 42psi.

 

On the other hand, the higher the pressure you run, the less heat will be genrated, so the "actual" pressure at speed running say 48psi, might be the same as if you used 42psi.

 

Of course higher pressures reduces the size and shape of your contact patch, which can have other handling effects.

 

 

Honestly, there's not a lot of reads in the world where you can safely run for long periods of time at triple digit speeds without at least occasionally slowing for cities or traffic.

 

I think the realistic top speed of the RT is well below a Z rated tires speed rating anyway.... espcailly when loaded to maximum weight, which would iinclude a passenger. Acceleration drops off much past 120 indicated with a passengner and side cases.

[smiller]

Wow, Twisty has a brother . Seriously, thanks for all the info. Regarding overinflation, I had always thought that no overinflation was ever recommended, i.e. the maximum inflation pressure specified on the sidewall was the maximum recommended inflation, period. Is this not the case?

[der Wanderer]

I have done many burst tests on tires (they are done with water, not air, which would be way too dangerous). Typically where I used to work they were designed to burst above 20 bars. No way you'll ever inflate the tires to that pressure in a normal shop. That is the least of my worries. Of course, accidents do happen, and be careful.

 

The main issue with over-inflating is contact patch reduction and as a result less adherence and less even wear. Comfort will also be affected. The way tire makes tend to determine which inflation pressure to recommend is by assessing the percentage flex - typically on car tires a flex of 10% or a bit less is a good rule of thumb. Over inflation would mean less flex, under inflation more - therefore it is very load dependent.

 

You are right, very few tires will ever be driven at speeds even close to their ratings. That's why most people survive on those lesser brands tires...

[outpost22]

*** Insert all sorts liability disclaimers. Follow manufacturer's recommendations. Whatever you do with this, I am not responsible! ***

 

I used to design and test tires and I have a bit of technical understanding of how the speed and load ratings work. I'll summarize a couple things briefly (if I can). Warning: geeks only - read at your own risks!

 

There are two major modes of failure for tires (apart from wear and road hazard):

 

- Speed failure: it's generally caused by thermal deterioration of the tire due to the heat produced via hysteresis of the rubber when the tire develops high amplitude static wave patterns (initiating in the contact patch and due to the combination of tire flex under load and high speed or due to under-inflation). Interestingly, most people believe centrifugal forces are the cause for high speed failures. Centrifugal forces in tires can be huge (sollicitation can reach 2000 g at high speed) but it's not a common cause of failure for well designed tires.

The tire speed rating (V, W, etc) is meant to express the ability of the tire in that domain. Tire ratings are self certified by manufacturers; there are standard procedures, but they are not always applied consistently and virtually most of the V, W and Z tires made by second class manufacturers that I have seen tested would have been classified at least one speed rating below by the manufacturer I was working at, meaning that those manufacturers may tend to provide a lower safety margin.

Failure happens when too much heat is created to be evacuated through conduction through the rubber and convection with the air outside the tire; it typically takes a few minutes at the max speed to fail (and if that speed is say 270 km/h, there will be very few situations where it is physically possible to ride at that speed for minutes). Failure is catastrophic (in the engineering sense - but also occasionally in the practical sense, with tires potentially "exploding") and exponential, meaning that if a given tire is rated for a speed of say 240 km/h, it likely will be able to sustain speeds just below the failure speed (say 220 km/h) for almost infinite durations. Therefore it is not uncommon in aftermarket to mount say a H rated tire instead of a V or W rated tire if the user knows he will never ride at speeds close to or exceeding 210 km/h and monitor their tire pressure carefully.

In tire construction, higher speed ratings are typically obtained by reducing the amplitude of the static waves through stiffening of side walls and longitudinal belting of the tire with "zero degree" nylon or kevlar belts. In turn, this "might" increase the cornering stiffness of the tire and make it more sporty (but that's far from a given).

Load on the tire influences high speed endurance, because more load creates more flex (in a quasi linear manner if the tire pressure remains the same) and more flex means more amplitude for the static waves; so does camber because it increase flex on one side of the tire. Increasing tire pressure reduces flex and increases high speed endurance.

If a tire is loaded close to the load rating, it will fail at lower speed than if it is less loaded - but those speeds "should" always be at least the rated speed at the appropriate inflation pressure.

With respect to the original question of using a tire with load rating less than the max weight on the wheel in operating conditions, this would potentially increase the risk of high speed failure because the speed rating of the tire is determined in conjunction with the rated load, and as stated above the max speed of the tire would be less under more load. However this is likely to be lower by maybe one speed rating, not by a huge speed difference, and can be mitigated by (reasonable) over-inflation. If the user rides significantly below the rated speed and very consistently monitors tire pressure, this mode of failure is less likely to occur even in cases of (moderate) overload.

 

- Mechanical endurance failure: as is the case for any mechanical object under cyclical loading, tires can experience fatigue failure. A truck tire carcass might run one million miles; that's not far from 1 billion load cycles... Ply separation, thread loss, and a variety of other ugly issues generally result from such cyclical sollicitation. Only rarely is the failure catastrophic though.

Evidently, the more deformation in the load cycle of the tire, the more likelihood of eventual failure. Therefore the combination of load and tire pressure is a primary driver in mechanical endurance. Higher loads might reduce in failure over time. Occasional overloading is not a significant concern in this case.

This is a domain in which no standardized test exists, and the only way to go to be safe, besides monitoring tire pressures, is by brand. In my experience, there are huge differences in this domain, primarily due to shoddy production quality in lesser brands.

Fortunately, motorcycle tires wear out quickly; this mode of failure is much less likely in 10 or 20 thousand miles than on truck tires, etc. With respect to the original question, and provided some reasonable over-inflation is applied consistently, this mode of failure is quite unlikely to manifest itself on motorcycle tires.

 

 

 

 

Only on a BMW website will you find people who will actually take the time to read this AND understand this. (at least I think I understood it )