Kawasaki Vulcan Forum banner

1 - 20 of 30 Posts

·
Premium Member
Joined
·
499 Posts
Discussion Starter #1
My rear tire pressure when the tire is cold is 39.1 (before riding). TPMS and digital gauge are almost identical. After I'm on the road for 30 minutes or so, the tire pressure rises to 45.1. Should I lower the tire pressure 2 or 3 pounds? The ambient temp has been running from 98 to 100.
 

·
Premium Member
Joined
·
1,127 Posts
I have done a lot of research on this and contacted a number of tire mfg. Long story short: If the pressure is too low, the temperature will increase too much while riding due to excess flexing. If the pressure is too high, it will not increase enough because the tire is too rigid. From ambient to riding temp the rise should be about 4 - 5 psi. You are getting slightly more than that. I would suggest a 40 psi cold and leave it at that, since most mfg say 40 psi is the max cold pressure (the pressure limit listed on the side of the tire is for cold pressure).

If there is an extreme ambient temp change (cooler to warmer) during the day, I would let the psi rise a little more, but not much, because with every 10 degree temp rise, there should be about a 1 psi rise.

Using these guidelines, I have gotten as many miles as anyone (and more than most) on a set of tires on a Honda Sabre. Don't know yet how it will go on the Voyager, but having 33 psi in the front and 39 in the rear cold has kept the psi rise to less than 5. 39 to 40 psi is what most folks say is the best rear pressure for the most miles on a set of tires, especially MC IIs.
 

·
Registered
Joined
·
388 Posts
I have found that my ride handles best when the pressure is between 39-40 psi with the Avon tires that was mounted when I bought her. And as Sabre stated, the tire companies do extensive research, so if the reading is within the recommended range when cold, keep it there for best results.
 

·
Premium Member
Joined
·
1,127 Posts
I have found that my ride handles best when the pressure is between 39-40 psi with the Avon tires that was mounted when I bought her. And as Sabre stated, the tire companies do extensive research, so if the reading is within the recommended range when cold, keep it there for best results.
Actually, it is not a recommended range, but a max cold pressure. When it comes to what they recommend, most tire mfg cover their butts saying to use the bike mfg recommended pressures.

Well, as most of us know, Kawi's recommended pressures are WAY too low and for tires that most of us hate. While all of the tire companies are pretty adamant about not exceeding the max cold pressures they list, they are really pretty clueless on what "proper" pressures should be for any given bikes or how to determine what they should be. The party line for all but one company (Metzler) was to use the bike mfg guidelines.

Metzler actually gave me different recommendations from what Honda recommends for the Sabre. They were actually pretty close to what I determined using the method I described earlier.

The company that I got the most disappointing and useless response from was, cough cough, Bridgestone. Dunlop was next. Metzler had the best information and Michelin was a close second.
 

·
Premium Member
Joined
·
499 Posts
Discussion Starter #7
Sabre, thanks for the detailed explanation and for doing the research. I will bump up the cold pressure a tad and see what the riding pressure does.
 

·
Registered
Joined
·
2,221 Posts
The max cold pressure rating does take into account the increase in pressure from a tire heating up.

Bike manufacturers sometimes use tire pressure "recommendations" to assist with softening the suspension.
Of course, (except for Ford/Firestone and the Explorer fiasco) they've done their research and should be well within safety margins.
Tire life and handling is a different story though.

If your PSI is at the mfgs recommendation, the max cold rating OR somewhere in between; while the tires are at ambient temps (cold).
There is no reason to check pressure when hot.

If there is a big swing in ambient temps; it's a good idea to verify cold pressure though.
 

·
Premium Member
Joined
·
1,127 Posts
The max cold pressure rating does take into account the increase in pressure from a tire heating up.

Bike manufacturers sometimes use tire pressure "recommendations" to assist with softening the suspension.
Of course, (except for Ford/Firestone and the Explorer fiasco) they've done their research and should be well within safety margins.
Tire life and handling is a different story though.

If your PSI is at the mfgs recommendation, the max cold rating OR somewhere in between; while the tires are at ambient temps (cold).
There is no reason to check pressure when hot.

If there is a big swing in ambient temps; it's a good idea to verify cold pressure though.
We have TPMS (tire pressure management systems). We aren't taking IR guns out to check the tire temps, but it is good information to know. Once you have found the sweet spot for pressure for your particular bike, load and riding style, it shouldn't change much except for, like you say, adjustments for ambient temp swings.

Maybe it is a bit of overkill for folks without TPMSs to check the tires multiple times every day, but having a TPMS can save you from being stranded in the middle of nowhere with a flat tire. I would much rather be able to find a shady or dry spot to fix a tire than on the open highway in the summer sun or in a rain storm.

Yes, the max recommended tire pressures do take temperatures into account. At that pressure, the tire should not flex much at all so there should not be much as much temperature rise, so they don't expect any failures. More cold pressure and they can blame you for the tire failure. Less than bike mfg recommended pressure, they can blame you.

In between the two, statistically, they are safe, so it is extremely hard to prove mfg defects unless a Ford/Firestone situation exists. And, like you say, they don't warranty tread life or performance on motorcycle tires. Most motorcycle tire mfgs apparently don't put much research into what pressures their tires run best for particular applications. They probably take a few of the most common bikes that the tire fits and do all of their development from there. Just like for auto tires, they expect the vehicle mfg to do the fine tuning for their specific vehicles. However motorcycle tires are far more variable than auto tires, so tuning for one tire does not mean much if you get away from that brand. Unfortunately, Kawi when with the crappy Bridgestones, so we have to do our own fine tuning.
 

·
Registered
Joined
·
580 Posts
Recommended tire pressures are for cold tires.
Exactly, it says that right on most tires.....don't over think it
 

·
Registered
Joined
·
2,221 Posts
We have TPMS (tire pressure management systems). We aren't taking IR guns out to check the tire temps, but it is good information to know. Once you have found the sweet spot for pressure for your particular bike, load and riding style, it shouldn't change much except for, like you say, adjustments for ambient temp swings.

Maybe it is a bit of overkill for folks without TPMSs to check the tires multiple times every day, but having a TPMS can save you from being stranded in the middle of nowhere with a flat tire. I would much rather be able to find a shady or dry spot to fix a tire than on the open highway in the summer sun or in a rain storm.

Yes, the max recommended tire pressures do take temperatures into account. At that pressure, the tire should not flex much at all so there should not be much as much temperature rise, so they don't expect any failures. More cold pressure and they can blame you for the tire failure. Less than bike mfg recommended pressure, they can blame you.

In between the two, statistically, they are safe, so it is extremely hard to prove mfg defects unless a Ford/Firestone situation exists. And, like you say, they don't warranty tread life or performance on motorcycle tires. Most motorcycle tire mfgs apparently don't put much research into what pressures their tires run best for particular applications. They probably take a few of the most common bikes that the tire fits and do all of their development from there. Just like for auto tires, they expect the vehicle mfg to do the fine tuning for their specific vehicles. However motorcycle tires are far more variable than auto tires, so tuning for one tire does not mean much if you get away from that brand. Unfortunately, Kawi when with the crappy Bridgestones, so we have to do our own fine tuning.
Monitoring for a loss of pressure is one thing.
Using warm tire pressure to set your cold PSI is a different story though. And it's ill advised.
 

·
Premium Member
Joined
·
1,127 Posts
I didn't come up with this system from out of the blue. I read comments on a number of forums for automotive and motorcycle design engineers, professional motorcycle racers talking about street bikes, etc. This was all done 6 - 7 years ago, so I don't have any links to post, but you can possibly find this information if you want to search for it. I combined the info I found out, did my own testing and came up with the system I described. The numbers I am suggesting for heat increase range indicate tire pressures are in line with the "seat of the pants" numbers that a lot of folks on this forum with 1700s have found works for them; low to mid 30s psi for the front and upper 30s to 40 psi for the rear. So, I'm inclined to think that the system does work.

Besides, how is what I am suggesting "ill advised" (your words). I'm not saying to vary outside of the cold pressures recommended by the bike mfg and tire mfg. I'm saying fine tune your pressures by using the temp increase due to flexing to determine where in that range you should be. Too much flexing causes excessive outer edge wear, a reduced contact patch, and excessive heat. Too little flexing causes a reduced contact patch and excessive center wear. You are saying it is ill advised to use temperatures to set pressures that are in the range of official guidelines, but using any pressures within this range without considering any other criteria is OK?

If you think running your front tire at 28 psi is a good thing, go ahead. That's what Kawi recommends and according to your earlier post, if you are between the bike mfg cold recommendation and the tire mfg cold limit, that's OK regardless of tire temperature. I tried 28 psi and found it to be very sloppy handling, and the tire heated up excessively (almost to 160° on a 70° day). The pressure rose to almost 40 psi. I think that is way too much heat buildup, especially for a front tire. It only took about 10 min. on the highway at 65 - 70 mph to reach that pressure and temp.

Tire heat is important for other reasons. Every time your tire heats up, it looses plasticizers and some of the polymer cross-linking breaks down. The higher the temps at the top of the cycle, the faster this occurs. The tire starts to harden causing it to have less traction. This is a well known fact. We even discussed this in my polymer chemistry classes some 40 years ago (yes, I am a chemist)! While additives have changed some over the years, the base polymer, 2,4-polybutadiene, a.k.a. "synthetic rubber", has not. Many studies have been performed over the years on thermal degradation of polybutadiene.

Knowing the heat profile of your tire can help you determine if you have a leak. If you expect the tire to be at 45 psi when the tire temperature is 110° and it is only 42 psi at that temperature and it was 40 psi cold, you very likely have a leak. You might overlook that if you only considered pressure and never bothered to understand the relationship to temperature.

The front tire Kawi cold recommendation is 28 psi while even the Bridgestones have a 40 psi max cold pressure listed. That's a huge range. Are you saying that anywhere in that range should be just fine? If not, just how do you suggest that someone determine where in that range they should be? I haven't seen any recommendations in your posts.
 

·
Registered
Joined
·
2,221 Posts
38 front 40 rear.
But I like a firmer feel on the front.

The manual recommended settings have been tested by real experts and will be fine.
The tires will probably cup and steering may shake if you take your hands off the bars, but you can bet there's millions of miles ridden by people using the factory settings.

Usually, I would agree with Glen that it's better to over think something than to get it wrong.
But, you're reinventing the wheel with all your effort. Nobody is talking about racing these 1700s or benchmark testing a new design tire.

If you want to spend your time verifying that your tires are operating as designed go right ahead.
But if you're going to "advise" people to set their tire pressures based on hot temps you probably should post all those links so they can reinvent the wheel for themselves too.
 

·
Premium Member
Joined
·
1,127 Posts
If you go back and read my first post, I talked about getting more miles out of a set of tires. I never said anything about racing. And like I said in my last post, I am not reinventing anything. I pulled together a lot of info (and it was about street use, not racing) that already existed or was gathered from tire mfgs, combined it into one system and am suggesting a way to figure out what tire pressures should be. Tire pressure is one of the most discussed topics on most motorcycle forums. The manual and tire recommendations obviously DO NOT provide enough information and guidance. If they did, why is there so much confusion and discussion.

You say "The manual recommended settings have been tested by real experts and will be fine." You call my system "ill advised" yet, you say that the Kawi recommended settings would probably cause the tires to cup and make the bike unstable. At least I'm not advising anyone that it is OK to make their bikes unstable like you are. And, yes, that is exactly what you said. You said using the 28 psi Kawi recommended front pressure would make the bike unsteady. Would you give that advice to a new rider who is already dealing with learning how to control a bike?

edit: Also, all those "real experts" probably only tested Bridgestone tires extensively since those are the ONLY tires Kawi recommends for the 1700s. They may have done some minimal testing on other brands, but, just like any other company, Kawi has a limited research budget. Michelin, Dunlop, Avon, and other mfgs tires are all different from Bridgestones. Yet, I suspect that the majority of riders do not run Bridgestones because of their poor characteristics. Do you? If not, why? Probably because whatever you use has much different properties from the Bridgestones. The manual states: "Warning! Some replacement tires may adversely affect handling and cause an accident resulting in serious injury or death. To ensure proper handling and stability, use only the recommended standard tires for replacement, inflated to the standard pressure." So everyone should run only Bridgestones and only at the specific pressures listed! THAT's what your experts are saying.

If I had the links, I would post them, but I no longer have them after the hard disk they were on died several years ago. I am not going to do all that research, contact all those tire mfgs again. I spent a lot of time and effort on that research and I'm trying to give other the benefits of my efforts. They can take it or leave it as they choose, but at least I have offered some advise that I think will help them.

I think we should agree to disagree. I expect that you will want to respond to my comments above, then let's be done with it.
 

·
Registered
Joined
·
2,221 Posts
The OP didn't ask about mileage. He asked about a rise in psi from cold to hot.
Specifically he asked if he should let some air out; presumably to offset the increase.

You should have advised him that a rise is expected and perfectly normal. You didn't ask about about speeds he was riding, you didn't ask about braking practices, road surface or how hard he gets on the throttle. All of which will effect hot psi more than .9 pounds of air.

39.1 cold psi does not need to be adjusted because of a marginal increase in warm psi.
 

·
Registered
Joined
·
580 Posts
Always better to overthink and get it right than not think and get it wrong.
.....not rocket science, run what the motorcycle manufacturer states
 

·
Premium Member
Joined
·
1,127 Posts
.....not rocket science, run what the motorcycle manufacturer states
Got it. Only Bridgestone EXEDRA G721 E at 28 psi on the front and the G722 at 36 OR 40 psi on the rear, depending on load. No ranges, just specific psi values ("standard values"). Kawi doesn't just recommend this, it warns against using any other brand (or even any other Bridgestone) tire or pressures.
 

Attachments

·
Registered
Joined
·
580 Posts
Got it. Only Bridgestone EXEDRA G721 E at 28 psi on the front and the G722 at 36 OR 40 psi on the rear, depending on load. No ranges, just specific psi values ("standard values"). Kawi doesn't just recommend this, it warns against using any other brand (or even any other Bridgestone) tire or pressures.
Apparently I should have been more clear. Use Kawasaki recommended air pressure (within a couple lbs wither way) in whatever tires you decide to run, that's what I have done for the last 30 yrs riding different bikes and no issues......plus a little common sense goes a long way also. I guess if it isn't specific enough for some people they'll spend all their time researching minute details instead of just riding the damn thing ....just my opinion :)
 
1 - 20 of 30 Posts
Top