Tires - The Overlooked Component

By Clifford Meier
December 23rd, 2006

 

How often do you check the air pressure in your tires? Do you check them once a month or when you change your oil? If you are like me, the tire pressure gets checked two or three times a year at best.  What is the result of running tires at other than optimal pressures?

Do you really know the optimal inflation pressure? Does the ambient temperature have any effect on the pressure? Do you need to recheck or even change the pressure when you are two up or loaded with luggage?  

I see posts all the time asking "What is the correct pressure?" We all have an idea from our personal experience or from what we have read. But again, if you are like me you don’t have the specific answer.

Tires are probably the single most important item on our bike. The condition of our tires, how they grip the road and handle the load, is paramount to our safety.  Yet we seem to neglect them and take them for granted.

If it is possible to put safety aside for a moment and focus on the cost of our tires, they are the single most expensive cost to the operation of your bike, excluding fuel.

So, the real question is with the safety factor and the cost involved with tires, why do we guess at or ignore the pressure?  Why are they so neglected?

Several things have happened within the last month to put me on this quest for answers. First, Carol hit the 10,000 mile mark on her 1300 Retro. And like virtually every other 1300 VTX, the front end has encountered the elusive wobble.

The available information on the “Wobble Demon” is all speculation. It's the tires, it’s the head bearings, it’s this or that.  But actually, nobody is sure what is causing this problem.  Tires are replaced and the problem is solved. Head bearings are tightened or replaced and the problem is solved. The problem IS solved, temporarily.  A few hundred miles later and the “Wobble Demons” come back. Many have even contacted Honda and of course Honda says there is no problem.

Looking at Carol’s bike I found nothing loose nor did I find any obvious cause for the wobble. The stock Dunlop’s look almost new, 12/32nds or more of tread. Next, I pull the front tire and check the balance. Could I be that lucky to find it out of balance? Nope, a perfect bubble. Damn.

Before I reinstall the front tire, I run my hands across the tread from both directions, with and against the rotation.  I feel it. Not even visible to the eye yet, but it’s there, a slight cupping of the tire.

In my opinion and I have said this all along, I believe the "Wobble Demon" is being awakened by the tires. In every case of wobble there has been cupping on the front tire. The cupping is found even on tire brands other than the OEM Dunlops.

Those of us riding the VTX with the OEM Dunlop tires have determined the main cause for the cupping to be low air pressure. Currently I am running 40 psi in the front and 42 psi in the rear. That doesn’t seem low to me, but how do I know? No matter, I’m done with the Dunlop’s and I have purchased the new Metzeler 880’s front and rear.

Once a wear pattern develops on a tire, that pattern will continue throughout the life of the tire. No matter what you do, there is no way to stop or correct the wear pattern. So before I install the new tires, I would like to know the proper inflation to eliminate any cupping.

So, I head back to the VTXOA message board to read some more. Hopefully I can find something about tire pressure. I find every combination of pressures ranging from 35 to 45 and every one is subjective. The Metz 880 for the front of the 1300 is new this year so there is no real information available to draw on for this tire.

In reading to solve this problem, I find the single most important reason for my quest of answers. Considering the years our VTX has been in service and the importance of our tires, no one really knows what the correct tire pressure should be for any tire brand. We all have very different ideas.

The correct tire pressure provides the optimum tire wear and fuel economy not to mention the best handling tendencies, yet everywhere I looked for information on tires I saw the same question, “What is the correct tire pressure?”

What happens if the pressure is too low? The tire can't maintain shape, it becomes flatter than intended, it could weaken the tire's internal structure and eventually lead to tire failure. Lower inflation pressures allow more deflection as the tire rolls. This will build up internal heat, increase rolling resistance (causing a reduction in fuel economy of up to 5%) and reduce the tire's tread life by as much as 25% while increasing irregular tread wear. You will also find a loss of steering precision and cornering stability. (Under inflation information source – “Tire Rack” Online performance source.)

What happens if the pressure is a few pounds too high? The tire is stiff and the size of its footprint in contact with the road is reduced. If tires are over inflated they could be damaged more easily when encountering potholes or debris in the road, as well as experience irregular tread wear. Higher inflated tires cannot isolate road irregularities as well causing the vehicle to ride harsher. However, higher inflation pressures reduce rolling resistance slightly and typically provide a slight improvement in steering response and cornering stability. (Over inflation information source – “Tire Rack” Online performance source)

Riders talk about different tire brands, the handling characteristics of each, how they perform in adverse conditions, high speed and various loads, yet we all tend to use the OEM (Dunlop / Honda) recommended pressures.

The owner’s manual provides a pressure range but what happens to that range when you are not running the OEM tire? A motorcycle is affected much more than a car when you add or remove a passenger. How about the added weight from luggage? A passenger or loaded bags can change your load weight as much as 80% and GVW as much as 20%, so how does 150 lbs or more of added weight affect your tires?

Will a change in ambient temperature of only 10 to 15 degrees affect your tire pressure? Can sitting idle in the garage for a couple of weeks affect your tire pressure? If the pressure is only checked while doing an oil change, how many times is the bike ridden with under inflated tires? 

Just how important are our tires? These two tires play a crucial role in our safety. As the only part of the bike that physically touches the ground, tires are one of the key factors affecting a vehicle’s handling, performance and overall safety. I would say they are VERY important.

Have you ever calculated the cost per mile on your tires? These numbers are mine of course and they are determined by my riding style and tire choice. If you want to check out your cost per mile, use the equation and input your own numbers.

Operating Cost Per Mile (CPM)
Front Tires - $.015 CPM (tire cost + mount) divided by average frt tire mileage
Rear Tires - $.022 CPM (tire cost + mount) divided by average rr tire mileage

Combined Tire Cost $.037 or rounding up  $.04 cents per mile

By comparison AAA shows the average tire CPM on medium and large sedans to be $.008 cents per mile. Think about that for a second. Our cars average .03 cents LESS per mile than the motorcycle and they have four tires! If you ride 10K miles this year on your bike at a cost of .03 per mile that is an additional $300.00 over what your car tires cost for the same mileage.

Tire cost compared with other operating expenses.
Fuel - $.047 CPM
Insurance - $.024 CPM
Oil and Filter - $.006 CPM

Better care of our tires and we will ride safer and cheaper. Is there a hard and fast formula that we can use for all tires?

Sometime back I had a conversation with Nick at RMCG - Cycle Parts and Accessories. He said their tire vendor gave him a formula to use and this, according to the vendor, was the most accurate way to determine the correct tire pressure in Avon tires.

According to Nick, this is what the vendor told him.  Air your tires cold. Ride the bike at highway speed for 20 minutes or so to get them to operating temp, pull over and recheck the pressure. If the hot pressure is more than 5 pounds above the cold pressure the tire is under inflated, building heat and adding pressure. If the hot pressure is 5 pounds or less above the cold pressure the tire is properly inflated. If the hot pressure is close to or the same as the cold pressure, the tire is over inflated.

Yes, this is hard work and I understand we can't perform this before or during every ride. But if you can rely on a formula and you check it a few times in varying conditions of ambient temperature, loaded / unloaded, two up or whatever scenario you find yourself riding, you should be able to get an average cold pressure that you can use for your varying riding conditions.

To prove or disprove this formula I do a search, here is what I found at "Dan’s Motorcycle Tires and Tubes."

“With the tire cold, take the tire pressure. Now ride for 15 minutes or so at freeway speed. Stop and take the pressure again. If the pressure has gone up more than three PSI the tire pressure is too low. This allows the tire to heat up too much, raising its pressure. If it's the same, the tire pressure is too high. It's not allowing the tire to warm up right. If it's hot pressure is three PSI higher than its cold pressure, the cold pressure is correct. The tire is the correct temperature and all is right with the world!”

To again prove or disprove I went a step further and contacted Avon Tyres. They make it so easy, under contact us it says if you have a technical question contact our engineers. Well… now we’re talking.

There were 3 emails back and forth with Avon. To make it easier to follow the process here, I did a cut and paste to remove the needless chatter and allow you to see my question with their answer.

From Cliff
I have a few Motorcycle Tire Pressure Questions for you.

I have had many conversations and opinions vary on correct motorcycle tire pressure. I was curious if Avon would have a formula or calculation that keeps in mind the factors affecting a motorcycle like rider weight, occasional passenger, luggage etc.

From Avon
Hi Cliff,

There are formulas for working out pressures for certain loads. These are kept confidential due to competition between tyre companies however. It can be quite complex with each bike weighing differently, different loads but also different ambient temperature.

From Cliff
Currently I am riding a 2002 Honda 1800 VTX R, the wife rides a 2005 Honda 1300 R with cast wheels. When looking at your fitment guide I see your recommended pressures but they seem to be a bit low from what I have found.

From Avon
Pressures stated are normally the best compromise between mileage, grip and comfort.

From Cliff
OEM with Dunlop, these bikes have had major wear and cupping issues. Bumping the pressure up to 40 - 42 psi seems to slow this down but not eliminate it.

From Avon
It’s normally better to be slightly over inflated (reduces cupping and wear) on heavier bikes.

From Cliff
Currently I am on my fifth set of Avon's. I would be considered an aggressive rider with varying GVW and average 12K miles on the rear and 17K on the front running 38 - 40 psi front and 40 - 42 psi rear. Most people tell me I am over inflated.

No info on the 1300 with Avon as you don't have a tire to fit that application.

Any help on a PSI suggestion / formula would be appreciated.

From Avon
With regard to the 1300, we haven't tested one as we don't make the tyres so I can't help there, sorry.

Pressures we normally quote are for solo use (170lbs average rider). For heavier loads i.e. over 240 lb rider, 2 up or 2 up with luggage, we suggest an extra 1-2 psi in the front and 2-3 psi in the rear.

From Cliff
Peter, Thanks so much for the speedy response and the limited info, I appreciate it.

It is difficult for the end user to find a good pressure and I would expect the OEM Dunlop information that Honda offers with the bike will not transfer to an Avon after market very well.

There is such a wide range of suggested pressures and with the 1300 and 1800 both having major wear issues I was hoping for a bone.

From Avon
You are correct that OE pressures may be different to other manufacturer’s tyres. We test all our tyres at OE pressures and ensure they work at that. This is because many people will set them to that anyway. If we find it’s better at another pressure, we state it in our fitment guide.

From Cliff
Online where all info can be found, I saw a rule of thumb where you check pressure cold, ride at highway speed for 30 minutes and recheck the pressure. If no increase in pressure, you're over inflated, 8 to 10% increase in pressure you are fine, over 10% you are under inflated. I'm not asking you to give away any of Avon's secrets but hypothetically speaking, is this something a person could live with? I really am just curious and trying to solve the cupping on Carol's and my VTX.

From Avon
No direct Response this time on the formula.

From Cliff
Another quick question if I might, sorry, I have seen several people running the Avon's on the 1300. Venom R on the rear and bias on front. How much of an issue is it to run a radial with a bias ply? A few years back this was considered a BIG NO, has this changed?

From Avon
It is ok to run a bias ply with a radial, but only with the radial on the rear. It is illegal to run them the other way round.

The above emails with Avon were with Peter J McNally M/C Technical Product Manager. Thanks so much Peter for taking the time to respond.

There isn’t much information we can glean from Peter, but what stands out to me is there are formulas used to calculate accurate tire pressure. Peter isn’t fessing up as to what they are so we need to do some experimentation.

The most significant tid bit to me was this, and I quote “Pressures we normally quote are for solo use (170lbs average rider). For heavier loads i.e. over 240 lb rider, 2 up or 2 up with luggage, we suggest an extra 1-2 psi in the front and 2-3 in the rear.” I would venture to say, most of us are over the 170 range and certainly over that range with any luggage or passenger. Whatever psi is suggested on their fitment guide, we should increase 2 PSI for the front and 3 PSI for the rear.

I believe what Nick heard from his Avon tire vendor to be fairly accurate and that would be a good place for me to start. I would definitely start with the PSI on the top end of the spectrum. The reason for starting so high, I will quote Peter again to refresh your memory. “It’s normally better to be slightly over inflated (reduces cupping and wear) on heavier bikes.”

I am going to install the Metzeler ME880's on Carol’s bike and to be sure they agree with my assessments, my next step is to contact Metzeler. This was not an easy task compared to Avon. After several phone calls and getting past the condescending attitude, this is what I receive from Metzeler Technical Support.

Explaining the situation I tell him, OEM with Dunlop, these bikes have had major wear and cupping issues. I’m replacing them with Metzeler and to insure no cupping and proper wear, what is the correct tire pressure?

The OEM Dunlop is produced to Honda's specifications. They are intentionally soft and run at a low pressure to enhance the ride and handling of the VTX. The VTX is a cruiser and Honda would not want you to feel every bump in the road or handle poorly.

We live in the States and as such we don't run high speeds or hit many corners. Most of our riding is on the Super Slab, cruising in a straight line and there is no need to use the entire tire. The center of the tire is the main contact patch and the side or curved area of a motorcycle tire is seldom used on a cruiser in the States. As such, we (Metzeler and other tire manufactures) purposely suggest a lower PSI for the cruiser tire and that type of ride. Lower pressure gives you better handling and a better ride on a cruiser.

I asked him if he was familiar with the VTX, Colorado and how the bike was actually ridden. This is not Kansas or Chicago, we have twisties, run two lane back roads most of the time and seldom ride a Super Slab. I have no chicken strips on my tires!

His response, "Good for you!" I continue with, what about the serious cupping issues we experience with the VTX and if your fitment guide gives us a low PSI what do we do to find the correct pressures? Would the above mentioned formula help us to determine the correct pressure?

His response, An old wives tale and a waste of time. Just air it up to what you want and ride. To which I responded, we don't mind replacing a tire that is worn out but we’re tired of pissing money down the drain every time we pull a tire at 4,000 miles that should have much more life.

The Metzeler fitment guide suggests 36 PSI for the front and 38 PSI for the rear, your printed brochure suggests 36 to 42 PSI front and 40 to 42 PSI on the rear. I asked him if these pressures were for a solo rider of 170 pounds with no luggage?

His response, Yes and with the information you are giving me, I would suggest you start with 40 PSI front and 42 PSI rear. If you notice your front tire cupping go to 42 PSI in the front. If you are loading a passenger or luggage add 2 PSI to the rear.

A bit of clarification here, this was a phone call and even though I took notes, I don't have an exact quote from him. The bolded areas are a fair representation of his comments and I use the bolded areas to better separate his part of the conversation from mine. He did give me his phone number and asked me to follow up with the results after 3,000 miles or so.

The most significant information in my mind, from both Avon and Metzeler is that the suggested pressures are for solo use (170lbs average rider) and pressures are purposely set low for the cruiser riding style and a better ride.

You can use the above information or not. Personally I think the formula is the most accurate way to determine and maintain proper inflation and worth the extra effort to formulate it.

I’m going to start with 40 psi in the front and 42 psi in the rear. If I notice any start to the cupping I am going to bump the front tire up to 42 psi. Two up or loaded with luggage, I will bump up the rear tire to 45 psi. I will also test the cold and hot pressure and from those results make whatever adjustments seem appropriate.

After my research, I found that tires are indeed affected by sitting idle in the garage. In fact, they will loose approximately 1 psi per month on average, idle or not. This means that if air isn't added for two to three months, the tire's inflation pressures will probably be 2 to 3 psi low.  

As to changes in ambient temperatures, I found the rule of thumb is for every 10 Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

My source for this information was "The Tire Rack" and you can read more about it from this direct link. Temperature Fluctuations.

I believe tires are the most neglected item on our bike and they should be checked before every trip, especially if you have a significant change in ambient temperatures or a change in load weight.

I hope you found this information interesting and that you were able to gain some insight into your tires. If you have anything you would like to add, ideas or comments please post them on the Colorado Range Riders Message Board under the topic of "Tires - Tires - Tires." We can discuss the topic further there. To find the post "Tires - Tires - Tires" all you need to do is... click here

My findings and overall results, along with any comments you add will make a good follow up and part 2 to this information.

As if you don't have enough to think about now, I thought I would mention one more topic. Some of you may have noticed a trend that has been dubbed going over to “The Dark Side.” The topic has drawn much national interest and it's gaining momentum.

Some riders, for various reasons have installed a car tire on the rear of their bikes. In Colorado we have BigVTXMan – Saul and Dennis who are currently trying it.

Many questions have been raised, is it really safe, how will it corner, and what type of mileage are they getting? I have put in a few links for those of you that would like to check it out.

Follow this link for a view of the car tire in action - Dark Side The Video  This link will take you to the VTX OA post - Discussions on the Dark Side Video  This link is called - Life is a Road - The Dark Side  This link will take you to the VTX OA post - How many have come over to The Dark Side?  If you are looking for several hours of light reading just do a search on the VTX OA for Dark Side.

Is this something that you're interested in? If so, let me know, this would be a good topic for another day, maybe part 3.

I hope you were able to find something of interest and pick up a few ideas. I'm looking forward to hearing your comments.

I have included some links that I ran across during this quest. Some were used as reference material and some are here because I found other interesting information in them. I hope you have the opportunity to check them out.

The Tire Rack - Online Performance Source  Dan's Motorcycle Tires and Tubes  Sport Rider - Tire Pressure Not a Lot of Hot Air  Pace - How much does it cost to drive?  Common Motorcycle Tire Wear Patterns 

Until we ride again ...