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timbo
Advanced Member
594 Posts
Uxbridge
United Kingdom
BMW
R1100S
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 Posted - 04/20/2008 : 3:52 PM
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quote:
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The bicycle pump is hopeless even if you could get it attached. Bicycle pumps are meant to deliver a little bit of air at high pressure. You need a greater volume of air at lower pressure. Technically, the bicycle pump will eventually work but there are better solutions. For about $50 or so, you can get a pressure tank, an air hose and the necessary fittings to let you pump up your tires yourself. You can fill the pressure tank at your local gas station and (if you keep the valve shut off) it will last for several months of topping off tires. Much cheaper than buying your own air compressor.
No, you are not being too paranoid. Tire pressure is important, you only have two of them and you're betting your life on them. But from our informal survey I'd say I'm about 3-sigma cautious about tire pressure so you can judge my response from that. FWIW, I follow the manufacturer's recommendations on tire pressure.
Enjoy your Valium!
Or you could just buy a foot pump. Over here they would be £5-£10 (10-20 dollars)  |
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degaboh
 Senior Member
258 Posts
Houston, TX
USA
Kawasaki
Ninja EX250R
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Posted - 04/20/2008 : 4:06 PM
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Thanks everybody for the advice.
I will look into the different valve stems. And take a valium!
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BÖCstar
Junior Member
90 Posts
Louisville, KY
USA
Yamaha
V Star 1100 Classic
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Posted - 04/28/2008 : 9:36 PM
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I have one of these gauges that clamp on and you can fill/release air pressure to perfection. Wonderful little device!
EZ-Air Tire Gauge
œ Use with air compressor for quick tire inflation
œ Clip gauge on tire and read pressure
œ No need to remove gauge while inflating
œ Saves time and money inflating tires
Check it out here: http://www.stopngo.com/products/inflation/EZ02.asp |
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gdickelman
 Moderator
1144 Posts
[Mentor]
Annandale, VA
USA
Moto Guzzi
CA & Ducati MS1100s
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Posted - 04/28/2008 : 10:09 PM
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quote:
The bicycle pump is hopeless even if you could get it attached. Bicycle pumps are meant to deliver a little bit of air at high pressure. You need a greater volume of air at lower pressure.
Hmmm...I have a high quality bicycle pump with a built in pressure guage that works like a charm. It attaches to my valve stems without a hitch, pumps easily, and the pressure that I read on its guage agrees with the auto tire pressure guage that I use to double-check afterward.
After struggling with those impossible gas station air hoses, and paying 50 - 75 cents each time, I decided that the high quality bicycle pump (retail $49.99 pump, on sale for $29) might be a simpler solution than buying a compressor and fittings. In this case, it worked out.
I'm not referring to the portable hand pumps, but rather a floor pump. Mine is a Hurricane Team Floor Pump:
http://www.performancebike.com/shop...fm?SKU=19979
Since I am also a bicycler and shop at Performance Bike, I find many things that are applicable to motorcycles (reflective vests, reflective stickers for the helmet, compact-portable tools, little clocks that you can attach to the handlebars (my Guzzi does not have a clock) and more). |
Edited by - gdickelman on 04/28/2008 10:51 PM |
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Speedmaster07
Senior Member
287 Posts
Los Angeles, CA
USA
Triumph
Speedmaster
Peer Review:
Blocked
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Posted - 04/29/2008 : 1:45 AM
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quote:
Originally posted by bkikkert
quote:
Don't use the "maximum" value that is often printed on the tire sidewalls.
Scott...I keep getting confused about this issue but I'm sure that James recommended inflating to the maximum stamped on the tire...
I don't recall seeing that recommendation, but I do know that my bike manufacturer does NOT recommend inflating to max pressure. Doing so reduces each tire's footprint, thus reducing traction, and produces a harsh ride that further reduces traction by reducing the effectiveness of the suspension. Tires at maximum pressure are also more likely to sustain punctures and damage from sharp objects on the road. The only vehicles I've ever owned where the manufacturer recommended inflating tires to their maximum rated pressure, were bicycles, where the tire plays a negligible role in absorbing shock in comparison with motorcycle or car tires.
The manufacturer's recommended pressure, based on vehicle loading, should be listed on your bike's VIN plate/decal. |
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James R. Davis
Administrator
14193 Posts
[Mentor]
Houston, TX
USA
Honda
GoldWing 1500
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Posted - 04/29/2008 : 9:06 AM
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Still we get this wive's tale here.
Tire pressure has essentially nothing to do with traction. Contact patch size has essentially nothing to do with traction.
Yes, higher air pressure leads to a harsher ride. But you can adjust preload for that. Higher air pressure also leads to longer tire life.
But so long as you inflate anywhere within the range indicated on the sidewall, the tire is properly inflated and safe. That is true at minimum and it is equally true at maximum. |
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bkikkert
Advanced Member
796 Posts
[Mentor]
Cornwall, Ontario
Canada
Harley-Davidson
Ultra Classic-FLHTCU
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Speedmaster07
Senior Member
287 Posts
Los Angeles, CA
USA
Triumph
Speedmaster
Peer Review:
Blocked
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Speedmaster07
Senior Member
287 Posts
Los Angeles, CA
USA
Triumph
Speedmaster
Peer Review:
Blocked
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Posted - 04/29/2008 : 2:04 PM
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quote:
Originally posted by James R. Davis
Still we get this wive's tale here.
Tire pressure has essentially nothing to do with traction. Contact patch size has essentially nothing to do with traction.
No wives' tales or even higher math involved. :) I agree with most of your comments on tire pressure in this and the linked thread, other than contact patch not making any difference in traction. The coefficient of friction between your tires and road surface is dependent upon several variables, including temperature and contact area. Higher pressures decrease both, thus unavoidably decreasing the coefficient of friction, and therefore decreasing the maximum friction force available to resist horizontal forces acting on the interface between tires and road surface. The great majority of the time, you might not notice any difference, but you will reach the limits of your bike's roadholding ability more quickly with the tires at their maximum pressure than if they were at some lower pressure within the allowable range.
Note that contact area not only depends upon contact patch, but relative smoothness of the two surfaces, which is why brand new tires feel noticeably slippery and provide less traction compared to worn tires, which make more complete contact with the rough road surface on a micro level.
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James R. Davis
Administrator
14193 Posts
[Mentor]
Houston, TX
USA
Honda
GoldWing 1500
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Posted - 04/29/2008 : 3:33 PM
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quote:
The coefficient of friction between your tires and road surface is dependent upon several variables, including temperature and contact area. Higher pressures decrease both, thus unavoidably decreasing the coefficient of friction, and therefore decreasing the maximum friction force available to resist horizontal forces acting on the interface between tires and road surface.
In fact, part of what you said is accurate ... the coefficient of friction between your tires and road surface is dependent upon several factors. However, when a skid sled is used to measure the coefficient of friction between tires and a road surface, the size of the contact patch is NOT part of that measurement. The coefficient of friction depends upon the materials being pressed together, AND HOW HARD THEY ARE PRESSED TOGETHER. So, for example, a contact patch that is 6 square inches in size and carrying 300 pounds of weight has THE SAME coefficient of friction as does a contact patch of the same material that is only 3 square inches in size and also carrying 300 pounds. That, because the smaller contact patch is pressed into the roadway with TWICE the pressure per inch as is the larger patch.
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Slavik
Starting Member
8 Posts
Mt Prospect, IL
USA
Suzuki
SV1000S
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Posted - 04/29/2008 : 4:30 PM
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I just want to thank all of the participants of this and the linked thread on tire pressure. I learned something new today!
James’ logic makes total sense to me, and I get home I will check and adjust tire pressure to the max pressure recommended by the tire manufacturer on all my vehicles (or close to it).
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Speedmaster07
Senior Member
287 Posts
Los Angeles, CA
USA
Triumph
Speedmaster
Peer Review:
Blocked
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Posted - 04/29/2008 : 4:48 PM
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quote:
Originally posted by James R. Davis
quote:
The coefficient of friction between your tires and road surface is dependent upon several variables, including temperature and contact area. Higher pressures decrease both, thus unavoidably decreasing the coefficient of friction, and therefore decreasing the maximum friction force available to resist horizontal forces acting on the interface between tires and road surface.
In fact, part of what you said is accurate ... the coefficient of friction between your tires and road surface is dependent upon several factors. However, when a skid sled is used to measure the coefficient of friction between tires and a road surface, the size of the contact patch is NOT part of that measurement. The coefficient of friction depends upon the materials being pressed together, AND HOW HARD THEY ARE PRESSED TOGETHER. So, for example, a contact patch that is 6 square inches in size and carrying 300 pounds of weight has THE SAME coefficient of friction as does a contact patch of the same material that is only 3 square inches in size and also carrying 300 pounds. That, because the smaller contact patch is pressed into the roadway with TWICE the pressure per inch as is the larger patch.
You make a good point. I'll allow that the coefficient of friction for two materials with half the contact patch but twice the perpendicular force vector would be the same, all else being equal. But the sled is measuring the coefficient of friction in somewhat of an ideal environment... it seems that a tire with a bit of flex in it would conform better to the uneven surfaces found on real roads and thus keep a greater portion of its maximum contact patch in actual contact, and the tread compound in a tire running a bit warmer (due to greater flex) would have better adhesion, do you not agree?
Plus, don't dismiss the effects of tire pressure on suspension performance. A hard tire acts as an undampened spring, and bounces across imperfections in the road surface (cracks, joints, Botts dots, etc.), maintaining less contact with the road even if your suspension has been adjusted to help keep the jolts from being transferred into the frame. |
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scottrnelson
Advanced Member
4748 Posts
[Mentor]
Pleasanton, CA
USA
Ducati
ST2, 888, + XR650L
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Posted - 04/29/2008 : 5:22 PM
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quote:
Originally posted by James R. Davis
The coefficient of friction depends upon the materials being pressed together, AND HOW HARD THEY ARE PRESSED TOGETHER. So, for example, a contact patch that is 6 square inches in size and carrying 300 pounds of weight has THE SAME coefficient of friction as does a contact patch of the same material that is only 3 square inches in size and also carrying 300 pounds. That, because the smaller contact patch is pressed into the roadway with TWICE the pressure per inch as is the larger patch.
Note that at some point rubber tires no longer have a linear increase in the coefficient of friction with respect to pressure.
Kevin Cameron in his book Sportbike Performance Handbook wrote:
If you graph available grip versus vertical load you'll get an upward-sloping straight line, showing that each increase in vertical load provides a corresponding increase in grip. ... However, this rising slope doesn't continue upward forever. At some high value of load, the tread becomes "stress saturated," which is a fancy way of saying that the rubber isn't strong enough to continue resisting side force. Instead of continuing to rise on its straight line, the curve loses slope and hooks over--then actually starts downward. Further increases in load actually produce decreasing available side grip.
I would really like to better understand tire traction as it relates to air pressure. These discussions have helped me a bit, but now I want to figure out some way to actually measure it myself, or read a report where someone has proved it with their own tests.
I'll continue to run 33/36 tire pressures in my tires, because I believe that it helps the rubber deal with rough pavement better than running 42 psi would, and I frequently encounter rough pavement where I ride. |
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James R. Davis
Administrator
14193 Posts
[Mentor]
Houston, TX
USA
Honda
GoldWing 1500
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Posted - 04/29/2008 : 5:42 PM
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Scott,
We are talking about a given weight and tires within the normal air pressure range imprinted on the sidewall. There is no change of weight and the change of contact patch size is trivial.
I take you back to the comment about a skid sled that is used almost universally to evaluate the coefficient of friction available on a given stretch of roadway. The tire material is almost always a piece of a normal AUTOMOBILE tire attached to a block, the total weight of which is at least 20 pounds.
That device is attached to a calibrated weight scale and dragged along the roadway. The initial measurement is IGNORED as that is the static coefficient of friction. The scale will then wander a bit as it finds the sliding coefficient of friction and that measurement is used. The test is usually run at least three times to get a good average.
That result is totally independent of the size of the contact patch, as it should be. If the scale is not held perfectly horizontal then it can cause the sled (tire) to pitch forward or backwards while being pulled. So what? If the contact patch is made smaller as a result of such pitch, the result is the same as it would be if the pull had been horizontal (within reasonable limits) because more weight per square inch is pressing that contact patch into the ground.
The only arguments you can have with such a device is with the accuracy of the scale or, in the case of a motorcycle accident, the fact that the tire rubber used on the sled would probably be a bit harder than the tire rubber of the motorcycle tire itself and would, thus, yield a slightly lower coefficient of friction. The accuracy of the scale is typically tested both before and after the test measurements and documented along with the test results. Skid test results, unlike accelerometer devices used to measure the coefficient of friction, cannot be intentionally 'adjusted' to yield false results. (An accelerometer test depends on the braking force used while skid sleds are always measuring 100% braking efficiency.)
So, while it might be academically fun to show trivial changes in traction given changes in weight, the practical, real-world, evidence shows that tire air pressure, within normal limits, affects traction so trivially that it cannot be measured. |
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theneanderthal
Junior Member
86 Posts
Ft. Walton Bch, FL
USA
Suzuki
gz250
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Posted - 04/29/2008 : 5:43 PM
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Great thread!
I spend far too much time with tires. Silverado 2500, F350, two sports cars (MX6 MR2T), two horse trailers, two campers. two bikes,
flat bed hauler, tractor and a mountain bike.
Between the slow leaks and the extended down time on some of these
I give the compressor a fairly good work out.
I own several of the various types of tire guages but I swear by the accuracy of my little $3 digital from walmart. The pop out bar type
guages are only as accurate as a wag unless you spring for a true
professional quality commercial grade guage. The average dial type
is typically more accurate and consistant even in the cheaper home
user grade. The solid state sensor used in the digital is consistant
and accurate, within the limits of your technique of course.
Also it has been my experience with hauling horses that tires will
invariably fail prematurely if operated at the maximum inflation
as printed on the side wall. For really heavy loads I run 10psi
under max to allow for the increase of pressure generated by high
speed interstate travel with Florida's baking sun driving the road
and subsequent tire temp/pressure ever upward.
YMMV |
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Speedmaster07
Senior Member
287 Posts
Los Angeles, CA
USA
Triumph
Speedmaster
Peer Review:
Blocked
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Posted - 04/29/2008 : 7:13 PM
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quote:
Originally posted by James R. Davis
That device is attached to a calibrated weight scale and dragged along the roadway. The initial measurement is IGNORED as that is the static coefficient of friction. The scale will then wander a bit as it finds the sliding coefficient of friction and that measurement is used. The test is usually run at least three times to get a good average.
James - it's the static coefficient of friction that counts! Unless you're in a skid, your tire tread is not moving in relationship to the road surface. It's the static coefficient that is keeping you on track and shiny side up in a turn. It's also the static coefficient that keeps you from skidding when the brakes are applied (again, the tread in the contact patch is static--completely stationary--relative to the road surface). The kinetic (sliding) coefficient is meaningless in normal riding situations.
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Speedmaster07
Senior Member
287 Posts
Los Angeles, CA
USA
Triumph
Speedmaster
Peer Review:
Blocked
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Posted - 04/29/2008 : 7:22 PM
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quote:
Originally posted by James R. Davis
So, while it might be academically fun to show trivial changes in traction given changes in weight, the practical, real-world, evidence shows that tire air pressure, within normal limits, affects traction so trivially that it cannot be measured.
Come four-wheeling with me sometime on rocky desert trails, and I'll show you real-world evidence why the common practice is to "air down" at trailhead to between 10-20 psi on all four tires to increase the tires' contact patch with the rocks and help prevent wheel spin (i.e. loss of traction).  |
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kml
Junior Member
75 Posts
englishtown, nova scotia
Canada
Suzuki
hayabusa
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Posted - 04/29/2008 : 9:56 PM
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quote:
Originally posted by James R. Davis
Yes, higher air pressure leads to a harsher ride. But you can adjust preload for that.
I was under the impression that preload is for adjusting ride height, not for "harshness". Would not harshness be more quality of damping?
Quote from:
http://www.ohlins.com/Motorcycle/Se...Default.aspx
"The spring pre-load affects the ride height, it does not affect the spring stiffness. Therefore, on models with a linkage to the shock absorber, the suspension may actually feel harder when you reduce the pre-load and the shock absorber gets into the harder range of the link system."
and,
"If the motorcycle is hard and bumpy, especially over a series of bumps, then the rebound damping should be reduced."
Is the above information incorrect?
cheers
ken
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James R. Davis
Administrator
14193 Posts
[Mentor]
Houston, TX
USA
Honda
GoldWing 1500
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Posted - 04/30/2008 : 4:53 AM
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| I believe that you are more correct about how to compensate for tire ride 'harshness' than I was. Thanks. |
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scottrnelson
Advanced Member
4748 Posts
[Mentor]
Pleasanton, CA
USA
Ducati
ST2, 888, + XR650L
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Posted - 04/30/2008 : 7:35 AM
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quote:
Originally posted by kml
I was under the impression that preload is for adjusting ride height, not for "harshness". Would not harshness be more quality of damping?
Preload does more than just adjust the ride height. It adjusts where, within the full range of the shock absorber or fork travel, you'll be when riding. On the Ducati motorcycles that I've owned, there are separate adjustments front and rear for preload and for ride height. I raised the rear ride height of my ST2 about an inch without touching the preload, for example. I had a suspension expert help me set the "sag", to get the correct preload for my weight. He also adjusted the compression and rebound damping.
I guess we're sufficiently off of the topic of tire pressure now. Do we need a separate suspension adjustment thread? |
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