Vibration at unloading due in aluminum/steel

[AaronSilidker]

Hello-

As the only engineer for a small motorcycle race team, I am trying to investigate something that I never learned in school....internal damping in aluminum/steel.

How can I find internal damping values? It is a material property or dependent on the geometry of the part? Does aluminum have more/less internal damping that steel? Would a lack of internal damping in aluminum cause motorcycle riders to experience harsh feedback that they think is a frame that is too stiff, ie could large amplitude vibrations upon unloading of the frame from off road obstacles be perceived as a very stiff frame that is not forgiving?

We have a situation where all of our test riders say the bike is "too stiff, too harsh, the frame needs to be 'softer'". The engineers who designed the bike say the frame is softer than the previous one in all directions, so my gut says that this is not a stiffness issue, but a vibrational issue that I need to investigate with strain gauges possibly?

Thanks for any and all help!

 

[rob768]

the internal damping of materials is related to their hysteresis behaviour. There is very litlle damoing in steel, and not much in aluminium. I am not a driver of a motorcycle, but i cannot imagine a situation where a frame can be "too stiff". I think that you can never have too much stiffness, as stiffness means control. The damping you eek will have to come from dampeners, designed and placed for that purpose, and maybe the tyres.

 

[hydtools]

Like rob768 said, there is very little damping in the frame materials. That's why they ring when you tap them, instead of having a dull thud.
I think the suspension is stiff. The shocks are too stiff and take away from the spring travel. Or the springs are too stiff. I suggest you look at the suspension. Swap parts from a bike that rides 'softer'.

Ted

 

[GregLocock]

Some bicycle frames certainly have reputations for being too stiff. Aluminium frames tend to be too stiff because hey are made with bigger diameter tubes.

Typically a welded steel structure will have around 0.2% damping. However once you start bolting things to it the damping increases a lot.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Tmoose]

Is this in a cornering situation?

Kevin Cameron has written a few times about the relevancy of (Hesketh's ?) concern that lateral fork, etc compliance becomes the real "suspension" for a severely leaned over bike.