Tire Pressure - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 2 (permalink) Old 01-19-2006, 10:17 PM Thread Starter
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Tire Pressure

I believe the general consensus is that most of us run close to the max. pressure as stated on the tire (within a few lbs. anyway). That gives an acceptable ride with maximum tread life. The max pressure on the tire is the "cold" pressure - they expect it to be more when the tires are warm, so as long as you're not exceeding the recommended max when your tires are "cold" you are OK, IMHO. I run my Metzeler ME880s at 38-40 psi both front and rear.

I read somewhere that tires normally wear more on the left because roads slope to the right to let water run off. I don't know if that explains what you are experiencing, though. I do know that most of us run much higher psi than what you stated. I know the manual says 28 lbs. or something like that, but the general consensus here is to run 38-40 instead. Gives much better tread life with little (if any) difference in handling.

I run 34 front, and 36 rear, and use pressure caps to keep them at the right inflation.
I would recomend within 10% of the TIRE manufacturers' rec. pressure. The Kaw manual says something like 28/30 IIRC. My Dunlops and Metzlers have always been at 36/39 (plus/minus one). The stock Flintstones recomend a good bit lower, I think, but not nearly as low as Kaw.

I recommend going to the tire brand web sight and looking up the specs for tire pressure for your particular bike; i.e., on the Metzeler web page, since I run ME880's, there is a lookup for the particular size ME880 we run and or a lookup of our bike by name with a pressure for "normal loads" and "greater than 200# load" which is people plus gear plus cargo. I run 36 front and 40 rear per the chart. The OEM pressures shown in the owner's manual or service/repair manuals are ONLY for OEM tires like my original Bridgestones. rgds Mike

Stock Tire Pressure-REAR

Ihave finished completing tests on using different pressures in the stock tires of my bike. My findings may contradict earlier reccomendations I have made conserning rider weight and shock settings.It should be noted that I am refering to the stock Bridgestone tires and not those that have tires by other manufactures. I belive there was some confusion there anyway looking at the tire pressure poll. The test bike was my 2002 Vulcan 750 with aprrox. 3,500 mile on it. the tires are nowhere near being worn and have excellent tread left. The first series of tests was on the rear tire.

After trying out pressures from 26 to 36 psi. I determined that the rear tire should for solo riding/and passengers under 150 lbs.. have 30 psi. Cornering tests indicate that more pressure than that for a rider under 180 pounds makes the rear a bit jumpy. However..riding with a heavier passenger/luggage weight in most cases will only require 2 more pounds of air. If you and your passenger total more than 300 pounds, I can see going as high as 34 psi...but that should be the maximum you ever use.( Passenger/luggage/rider total weight should not exceed 397 pounds according to the manual) So here..the pressure reccomended in the owners manual seems fairly spot on.Tests at lower pressures (26-28) , showed adverse handling charateristics. although the book does say that 28psi is fine for riders under 215 pounds...I got faster times through turns with the extra 2 pounds of air I added , and I weigh 148lbs.

I believe that some riders have found that the bike "rides" better with lower or higher pressures in the rear...but I am certain that this is just a bad fix...when the real culprit is the rear shocks.( See my mext post on "SHOCKS!!") Although 28-29 psi does not effect the handling that greatly and may be a good pressure to use for some riders, I myself have taken into account the fact that every time I have a passenger I don't want to keep adding air and then taking it out for just myself...I have found that the 30psi is a very good comprimise.Given the fact that sustained highway speeds also require a firmer tire...that is why I picked 30psi as a good all around pressure..even for those that never have passengers. I detected no adverse handling problems with it and hence that is my new recommendation. Knifemaker

Stock Tire Pressure - Front

The past few weeks I have been experimenting with diffrent tire pressures to determine what works best and seems the safest. After coming up with a workable pressure for the rear tire...( see post on Rear tires)..I began testing the front. the problem here is that it is very hard to detect odd handling attributes from the front end of a bike. Especialy ours , as the rear and front tires are diffrent sizes (widths being the important reason)..and the front forks playing such an important role also. The reason the widths make a diffrence is that the contact patches of the tires form more of a triangle than a rectangle. When the bike leans over..the front being thinner than the rear takes a moment longer to "set-up" for a turn...compared to tires that are closer to the same width. Most of you never notice this..but it happens and makes spotting problems a bit harder. Tire makers know this..and that's why they usualy sell front and rear tires as matched sets...they have a computer to figure out lean angles and design the tires to work as a pair...The way I solved this dilema was to devise a test where the front forks played a much smaller role and the transition time for the bike to set up for a turn was not a factor.

What I did was to test for front end slide in very slow but sharp turns. To do this, you enter a sharp turn, like a right turn from a stop at an intersection. Halfway through the turn..you abruptly make the turn tighter and roll on the throttle a bit. The bike will try to "push" the front tire in the dirrection it was orriginaly going..if all goes well , your new lean angle and countersteer will take you on the new path. If the front tire seems to slip a small amount first..this is dirrctly the fault of the tire loosing a good grip on the road. This was the basis for my test.

Everything else being equal..your tires shouldn't slip. Starting with Higher pressures first..34psi...I found that I could get the front end to slip quite easly at fairly low speeds making a right turn at a certain intersection. The road was not real smooth and was slightly crowned right near the curb.Waiting for the front tire to reach the top edge of the bank..and then executing the slight change..I found that I could repeat this without much trouble everytime I circled around and went through again. Now it was just a matter of slowly letting off some pressure for each run. ( Even though I knew what was sgoing to happen , it was still hard not feeling my heart skip a beat as the front end chattered sideways a few inches...so kids..don't try this at home....lol) After letting off one pound and repeating the test I slowly noticed the bikes reluctance to skip and found the transition going much smoother. When I got to 29 psi in the front tire I couldn't get it to slide at all. Based on this , I would have to conclude that this is the highest pressure I should run in the front end.

Tests at lower pressures than 26 psi exibited diffrent problems, unwilingness to take the turn smoothly and harder steering inputs requried to get through the turn at all. These being common indications of insuficiant tire pressure. The maual lists 28psi as the pressure to use in all load requirements...this seems to be quite logical from my tests...although I did find the bike did seem to go through fast turns..sweeping lefts and rights at 60 to 70 mph... just a bit more steadily at 29 psi than 28. Iv'e also noted posts about running much diffrent front tire pressures here...some seeming to say that the "higher" pressures gave a better ride or seemed to make the front feel "tighter"...and again I must point out that first...adding more than reccomended pressure does not absorb bumps better...higher pressures should tansmit shocks faster...not slower. And that any changes from the 28-29 psi are simply masking problems with the front suspension...Which in my opinion is in need of slightly stiffer springs and better dampening.( see my next post on shocks) Knifemaker
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post #2 of 2 (permalink) Old 01-19-2006, 10:18 PM Thread Starter
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For Indy cars - NASCAR - Formula 1 and AMA Superbike tire pressures are one of the most important aspects of race day performance. Why? Because all the technology, design, talent and money in the world won`t add up to race wins if the tires are not performing properly. Tire pressure will affect the temperature the tires run at, and excessive tire heat is the number one killer of tire performance. Riding a motorcycle on the street is of course not the same type of riding, but if a tire is going to deliver acceptable tire mileage, perform well in corners and accept braking loads properly it therefore must be run at the proper inflation pressure.

Now that I have your attention focused on how important inflation is, I will move on to finding the right pressure for your motorcycle. The information found in your owners manual will only be valid for the tires that came as original equipment on your motorcycle. If you have up graded from stock equipment and have installed a pair of high mileage tires for example, then the tire pressure indicated on an application guide is the best starting point.

There is a way to determine the perfect tire pressure for any given load configuration. First check your tire pressure when the tire is cold, load the motorcycle to the same weight regime that you are solving fo and ride the motorcycle approximately one hour at the typical speed encountered when loaded in this manner. Check the tire pressure again. Since the tire surface has now heated, so has the air pressure within, and air pressure will naturally increase with temperature. There should be an increase of 4 to 6 pounds or 10%. If the increase is less than that you are over inflated. If the increase is more than that you are under inflated, and the tire is flexing to much causing the to tire overheat.

Overheating will destroy mileage expectations and most importantly (due to overload) can cause a blow out situation. The maximum load a tire can carry lowers as the tire pressure lowers. The maximum load found on tire side walls is valid only when the tire is inflated to it`s maximum tire pressure. The tire pressure you choose should always be somewhere between what is shown in your owners manual and what is marked on the tire side wall. Under no circumstance should the tire pressure exceed the maximum pressure (cold) marked on the side wall. Over inflation will distort the profile of a tire and diminish tire performance.

Hydroplaning is not a factor of contact patch length..but tread pattern , weight and firmness of the tire compond itself. All this talk of running at or close to Max pressure worries me. Most of us do not ride the Vulcan like a bat out of hell... Waiting to the last possible second to brake and braking VERY hard. We usualy do not toss the bike into turns at the fine point of sliding the rear tire through it and don't stop only when the checkered flag comes out. Those that "race" their bike should perhaps be runnning near MAX pressure..the fact that heat build up is taken into acccount when the tire company posts the numbers. But most of us are more sedate in our riding.( most of the time...lol) so you have to figure that you are not going to heat up the tire as much as one would the "bat out of hell" way. Most of us are fair weather riders...and alot of our riding is done in the summer. Sence the heat build up is figured into the tire companys recomended pressures..one should understand that this is a good guide to use. Remember tires work better when they are "warmed up" Companys take tire tempeture readings and based on the type of rubber used and application..figure a general pressure bracket to go from. Putting near max pressures in the tire are not neccesary unless you are planning to ride in such a way to take the tire temps to the limit. These lower pressures insure that the tire reaches it's ideal tempature range..only by using less than reccomended pressures do you risk over heating the tire...running the tires at the figures given in the manual for your application does cause the tires to "heat up"..but only as much as they are supposed to. Knifemaker

Hydroplaning occours from worn tires or tires with poor tread design..but can still happen to the best of tires under the right conditions. Over braking on wet roads or going over oil/water mixes, painted lines and small puddles at high speeds can impart the same results. Our (stock) rear tires do have a center tread groove...but do have alot of un-treaded tire surrounding it. if the tire is too low..the results would be close to the same as running slicks..nowhere for water to get channeled to..slip and slide. But the length is not as important as the width here. I will give you the benifit of the doubt by saying that running lower pressures MAY increase the chances of hydroplaning..but by lower I mean pressures below reccomemded settings. Like 20-25 psi. I run about 30 front and 31 rear in mine...which is a comprimise for alot of solo riding ( to and from work) and a passenger (on weekends)..

You are correct..the Vulcan is not a "racer" and should not be treated as such..too many riders use tire pressure to adjust the "ride" when the adjustments should be made at the shocks where they belong. My overall view on this disscussion is that I am more worried about those that do not have enough air in their tires or do not check them frequently..running near max pressures will not likely cause you harm...but running too low pressures may. Overheated tires on cars blow out violently...imagine having your rear tire explode at 80 mph... And speaking of Dunlop..see my next post.. Knifemaker
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