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(OP)
Hi

I was wondering if there is data on the coeficient of restitution for tires or even if such a thing exists.  I am trying to get an idea of fatigue on my motorcyle frame from road impact.  I am hoping to calculate the forces caused by road impact.  To do this I have been using impact formula's that suggest a needed coefficient of restitution.  I also need an idea of the approximate time interval of an impact.  I realize this is probably dependant on the speed but I figure someone has to have some data which takes into acount these variables.  Perhaps there is a better way to go about figuring all this out.  Can anyone help or does anyone have a suggestion?

Take Care:)
Replies continue below

### RE: Road Impact On Tires

Timms,

I would think that any competent tire engineer could answer your question immediately although in view of the difference in aspect ratios, tire compounding and reinforcement, diameter etc, it will not be a one answer fits all situation.

Perhaps an easier way to ascertain the fatigue life is to place either accelerometers and/or strain gages on critical positions and from these determine what the maximum stresses are in the various parts of the frame. With most road bikes, I would think the maximum stress will allow an almost infinite fatigue life. Not so for a dirt bike though...

### RE: Road Impact On Tires

(OP)
Kiwinjuneer

Thank you very much for you response!  I've had a little success with tire engineers.  Mostly with testing company's.

Your comments regarding strain gages and accelerometers are encouraging because that has been one of my conciderations.  If I used an accelerometer would I just multiply the g's with the mass of the moving parts components? For example, If I hooked it up to the axel of the bike I would have to find the mass of all the fixed components to that axel and multiply the total mass with the g's?

Thanks also for giving me an idea of the fatigue life as well.  Are their standards that I can use as guidlines for my design testing?

Blessings
Timms

### RE: Road Impact On Tires

Timms

I would suggest you mount the accelerometers on the triple clamp and near the swing arm pivot rather than the axles as the springs and dampers reduce the load the frame sees.

The loading at these points would then be the g force x the weight of the bike plus rider less the unsprung weight i.e. wheels, brakes, fork sliders, swingarm etc.

If the peak von mises stresses are <20,000 psi and the frame is steel and free of cracks and with nicely blended welds then you probably have a unit that will last indefinitely. If made of another material or stresses exceed 20,000 psi, would suggest that you gather g data over a period of some months and using an FEA model with fatigue analysis capability, determine the fatigue life.

My experience in this area is limited in what is a very specialised field so would strongly recommend you confer with someone who does claim expertise. A couple of useful documents you might also check are the API drilling tubular specification RP7G and the structural steel related AISC specification.

### RE: Road Impact On Tires

(OP)
kiwinjuneer

Thanks again.  You have been a great help!  At the present moment I don't yet have the vibration technology to analyze with accelerometers.  I have to do everything the hard way and manually calculate the stresses at these points.  Currently I am trying to figure out the stresses on the pivot.  I am using an impact of approximately 2000 pounds vertical on one of the rear tires (trike).  I have assumed the shocks to have a counter load equalling the impact load minus the wieght of the axel, tire etc making all vertical loads cancel each other out.  With this I have calculated the reaction forces on the pivot needed to counter the twisting moment created by the rear tire impact.  I think the bending moments about the shocks will help cancel these reactions.  I hope I am using a good approach.

If I get the funding I will invest in vibration equipment and FEA.  I have a little FEA experience and am concidering ANSYS along with Soldworks.  This forum has given me the impression that they are quite compatible.

Thanks for your recomendations concerning the approximate Von mises stresses.  It will give me something to compare to.

Blessings
Timms:)

### RE: Road Impact On Tires

Timms

If you want to avert lots of testing with accelerometers, you might also consider going to www.sae.com and pulling papers relating to suspension dynamic loads. At $9.60/paper (assuming you are a member) you may save yourself a lot of time and$.

There is a forum on eng-tips relating to FEA and if you started a thread there, would likely find that someone has already done just what you propose. Most of the software houses provide their software for free trial and subscribers could likely point you in the right direction and make recommendation as to the most user friendly programs. I don't want to put down anyone's product, but there is at least one that I would NOT recommend but have heard good things about Solidworks.

### RE: Road Impact On Tires

the analog devices ADXL series of accelerometers would do you a world of good. they are available in ranges of +- 1g to +- 50g with various amounts of noise cancleing and cost about ten to fifteen \$us.

I think you will find it worth your time.

### RE: Road Impact On Tires

(OP)
kiwinjuneer and carnage1

Thank you bothe for your suggestions.

kiwinjuneer I have been looking at the sae papers available and have purchased a few.  I think I will have to look for more though.  I have started a thread in this forum and have already started getting replies concerning the software

carnage1 thanks for the referal.  I will call the company and get the details.  I have previuos vibration experience and we needed a data pac or computer to take the readings.  Perhaps these analog accelerometers have a less costly output device.

Blessings
Timms

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