Tire Differences 1

[BrianHa]

Guys I'm hoping you will be able to help me. My expertise is not in automotive design. I am by profession a residential home designer so my engineering knowledge and analysis of automotive engineering is very limited. I have been reading chapter 1 of the "theory of ground vehicle". In my spare time I am also Chapter Educator (safety instructor) in the Goldwing Road Riders Assoc. The topic of running a car tire on the back of a large motorcycle (Goldwing) has come and I am wanting some hard data good or bad on how the a car tire is affected versus a motorcycle tire, if there is any. I have been searching the internet but keep get lead to forums where people are doing it but they have no hard data to back up their statements only their "opinions and personal experience". I have watched their vidoes and seen the car tires up on the shoulder of the tire and know that that can't beneficial. Do any of you know if there is any published reports by independent testers or tire manufactures and if so, where might I be able to find some information or point me in the right direction. Thanks for your help.

 

[evelrod]

Advice is often only worth it's cost. True enough.

Nevertheless:

I rode a Goldwing for many years, over a hundred thousand miles. Rear tires were good for, at best, ten thousand miles. Using Goodyear 'run flats' while they were available and several other brands, I never had a failure related to "over loaded" condition. Choosing the proper size tire for the application and using a bit of common sense is paramount to a long life with motorcycles. I doubt mounting a car tire on a Goldwing would fit that description.

Rod

 

[Compositepro]

A car tire might work okay if you never turn. I don't think a car tire would last long running on its sidewall.

 

[GregLocock]

Probably OK for trikes or sidecar combinations, positively batty idea for a conventional motorbike. I'd have thought one exciting day's testing on a damp carpark would have revealed that.

Cheers

Greg Locock


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[patprimmer]

It's pretty straight forward in theory.

A car tyre is designed to run with the tread flat on the road. Car suspension designers go to some lengths to create a suspension that will keep the tread flat in the dynamic situation. The wider the tread, the more critical the camber change.

Motor cycles lean into a curve, often to a very large degree. Motor cycle tyres are designed to cope with that. Car tyres are not.

The only situation where a motor cycle behaves differently is drag racing with very wide slicks. They need a wide footprint but do not need to turn. The wide flat tread tyres used in drag racing cause a peculiar handling trait in that the bike needs to lean the wrong way to turn as that puts more weight on the outside edge of the tyre, causing it to turn the bike away from the lean. More than one Top Fuel Harley has been into the wall because of this counter intuitive characteristic.

Hopefully Capriracer will chime in with real rather than theoretical or anecdotal data.

Regards
Pat
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[CapriRacer]

Pat, Thanks for the vote of confidence.

Brian, I have been following the "darkside" (car tires used on motorcycles) for quite some time now - and I am alarmed by these folks lack of sensitivity to the warning sugnals. Let me show you what I mean:

First, motrcycle tires are built to different specs than car tires - and one of those differences is the rim diameter. A 15" motorcycle rim is nominally 15.080" in diameter. A 15" car rim is nominally 14.968" in diameter.

That means that if a car tire is mounted on a motorcycle rim, the bead has to expand 0.112" MORE than it does when mounted on a car rim. Is there evidence to support that this is a problem? INDEED!!!

Some darkside folks have reported difficulty mounting tires - and in some cases have reported explosions! Well, duh!

Does that deter these folks? Apparently not. Instead of recognizing that tire manufacturers stipluate that 40 psi is the maximum seating pressure, these folks use 100 psi and more!

And instead of recognizing the generic nature of this problem, some have created a list of tires - by brand - that should NOT be used. The list is odd because it has several different and unrelated brands on it.

The RMA (Rubber Manufacturers Association), which is the industry trade group for tire manufacturers in the US. EVERY tire manufacturer who has manufacturing facilities in the US is a member of the RMA and they will issue TISB's (Tire Information Service Bulletin) on subjects of common interest - and they have one on this subject:

http://www.rma.org/publications/tire_service_professionals/index.cfm?PublicationID=10729

This isn't just one manufacturer - it's ALL of them!

OK, so that's the first part.

But what about hard data on handling? Unfortunately, I haven't found any. It seems pretty obvious that the issue of a roundish profile is needed for 2 wheeled vehicles - and that squarish profiles are not a good idea - and I suspect it is SOOOO very obvious that every tire manufacturer, every motorcycle manufacturer, every motorcycle magazine, etc., has decided NOT to do extensive testing to prove the point.

Looking at it as an engineer, any test of this sort ought to be "double blind" - meaning neither the operator of the vehicle nor the test result analyzer can know which group is which. I see this as an insurmountable obstacle. Not only is there a potential danger to the operator - so the operator ought to be aware he is in such a situation - but putting an operator in such a situation carries with it a HUGE legal liability - hence a good reason not to even conduct such a test.

Of course, this works in the darkside folk's favor - but I suspect that even with a published report is not going to have much affect.

 

[WolfHR]

Patprimmer- I'm a bit intrigued by the behaviour you describe. Would it imply that (to me at least) counter-intuitive way of, high performance, riding gets reversed? I've been told that the best and fastest way of initiating a left-hand turn is to steer to the right (and after a good example, I began to see the point but it still seems counter-intuitive)- does that get changed too by messed up CP geometry?

 

[NormPeterson]

I'm pretty sure that to initiate a turn when riding any two-wheel vehicle you would naturally steer ever so slightly opposite to the direction of the corner in order for gravity to help get the vehicle to start leaning. I think there are some gyroscopic effects involved as well.


Norm

 

[BrianHa]

I appreciate all the information. Y'all been very helpful. I can see why and understand why no motorcycle manufacturer, or tire manufacturer would want to do a side by side comparison. When I ride, I want to know what the bike is going to do when I ask it.

At our next chapter meeting, I am going to be talking about the use of a car tire on motorcycle and I will pass out the RMA publication and explain the size differences between the car rim and the motorcycle rim and why it requires a large amount of air to seat the bead.

I am going to ask if anyone that is running a car tire, to do some "redneck engineering" (no offense to any rednecks). Take some white shoe polish and start at the rim, go up the sidewall, across the tread, down to the other sidewall to the rim. I use to do this type of "redneck engineering" when I raced auto cross in the SCCA in the early eighties. I ran H-Stock with 1976 Dodge Dart Sport with a slant 6/225 on stock street tires. I would inflate my tires until I wasn't rubbing off any shoe polish off the side walls. It normally was around 45 psi to keep my tread flat and on the ground. Maybe then they might see how bad it is and how much they actually ride on the side wall.

Thanks,
Brian Handleson

 

[BrianHa]

Countersteering works by moving the wheels out from under the bike.

Try this experiment: Balance a broom upside down on your finger. With a few minutes' practice you can keep it upright pretty effectively. Once it's reasonably stable, try moving it to the left. You'll quickly find that for the broom to move to the left, it must be leaning to the left. You do this by moving your finger to the *right*, which moves the end of the broom handle out from under the center of mass of the broom. This is exactly the same mechanism as countersteering. Your bike has some inherent stability when it's moving, which will tend to keep it upright. When you want to turn, you must lean the bike.

Countersteering moves the wheels out from under the center of mass of the bike, causing it to lean in the opposite direction. Gyroscopic precession has little bearing on countersteering. It does have a significant effect on the feel of the bike, since it tends to keep the front wheel from being turned. However, consider this: If gyroscopic precession were the primary driving force in leaning the bike, one would expect that bikes with large front wheels would turn in very quickly. As it turns out, though, this is not the case, and in fact is just the opposite of what is seen in actual practice.

Once the bike is leaned over, the trail of the front end causes the front wheel to turn into the curve, and the round profile of the tires causes the bike to experience camber thrust steering (similar to rolling a cone, which travels in a curved path), which cause the bike to go around the curve. When it's time to straighten out, countersteering is again used, this time to move the wheels back underneath the center of mass of the bike and cause it to stand up.

Hope this helps y'all understand counter steering

 

[NormPeterson]

I wasn't suggesting that gyroscopic precession was responsible for helping to lean the bike -

http://en.wikipedia.org/wiki/Precession

actually suggests otherwise, and that you'd need to give the bike a little more of a nudge to get it leaning.


Norm

 

[BrianHa]

Yes, a little nudge or counter steering makes it respond like a scalded ape versus just leaning. Counter steering is crucial for emergancy maneuver.