Kinematic Analysis

[Mariah]

Hi,what are the things should be considered for kinematic analysis of motorcycle suspension?I'll be grateful if U could state clearly the differences between dynamic analysis.I'm very new to all these.......
Regards.

 

[ivymike]

I suppose the simplest way to illustrate the difference between kinematic and dynamic analyses is to give an example:

Say you're simulating the behavior of the front fork of a motorcycle as it drives down a road. Your simulation includes some forcing function or excititation, in this case a ramp that the tire must roll up and over. The ramp is somewhat steep on the up-side, and drops off abruptly on the down-side. In real life, the motorcycle could "jump" a little bit as it passed off of the ramp, in that the tire could continue close to its initial trajectory for a little while before gravity (and the suspension) could force it downward. In a (representative) dynamic analysis, the tire would behave the same way - the forces accelerating the tire downward would have to overcome the upward momentum of the tire before the tire's velocity would become downward. In a kinematic analysis, the momentum of the tire is not tracked, so as soon as the tire comes off of the edge of the ramp, it will start to have downward velocity (and downward accn, of course).

The big difference between the two is whether or not momentum of the objects is tracked vs time. Kinematic analysis is akin to "zero-speed" analysis, where none of the parts have momentum. Dynamic analysis is a closer representation of reality, as it keeps track of the momentum of the moving parts.

 

[GregLocock]

Trust an engine designer to reduce suspension design to cam follower technology!

You are right, but don't knock kinematic analysis, if that doesn't work then the dynamics are going to be much harder to tune, and you may have to make some horrible compromises further down the track.

Here are a couple of web sites on suspension design:

http://www.presspley.com/racecarresearc/roll.htm

http://members.fortunecity.com/suspdesign/

As an example, it is very easy to design a 3 lateral arm+ 1 steering link suspension that cannot even move freely throughout the required vertical range. A kinematic analysis will show you this, as well as showing the motion of the contact patch, wheel inclination, shock motion ratio etc etc.

In the case of a motorbike's rear suspension yo need to also consider the loads from the chain, and on both wheels you need to think about brake loads.

If you want more details why not take this over to the suspensions forum?
Cheers

Greg Locock

 

[ivymike]

good point Greg, I didn't mean to sound like I was against kinematic analyses... I use them too (or, more accurately, psuedo-dynamic analyses), for lots of different things. One really nice thing about kinematic analyses would be the simplicity with which they can be set up (for a simple system) relative to their dynamic cousins.

 

[ProEpro]

Another simple way to look at it is

Kinematic analysis will tell you where something is.
Dynamic analysis will tell you when it is there.
ProEpro

http://www.whitelightdesign.com

 

[JamesBarlow]

Mariah,

Dynamics is divided into Kinematics and Kinetics.

Kinematics is the study of motion without consideration of the forces that caused the motion. In your case it will be moving the wheel up and down and seeing how each suspension element moves. For a constrained system, such as yours, this would be a geometic analysis of angles and distance between joints.

Kinetics is the study of motion with the acting forces being considered. This is what most people refer to as a dynamic analysis. What happens to the bike, as a whole, when you hit a bump at a certain speed.

Both of these are extremely limited in the application you are describing. You will eventually get to you systems class, which will make all of this very, very easy.

 

[awol]

Kinematics will determine position. If I move this lever 10 degrees, this connecting lever moves 16 degrees. And relationship can be determined between the corresponding angles of all linkages so an accurate mechanical advantage can be determined. Like stated above , all linkages have zero mass. Kinematics is position.

-awol