Re: What factors affect the wheel speed of a skateboard?

mroepcke,

    It's great that you have taken the time to help this student pursue 
his interests.  I guess I never realized how far sports/hobbies such as 
skate boarding and BMX can take young boys and girls into the engineering 
realm - this is pretty exciting! (I wonder if the boys and girls recognize 
that their interests lie in engineering? - Math is power!)

    What factors affect the wheel speed of a skateboard, and how do 
bearings figure into this problem?  As far as wheel design goes, we need 
to determine what are the important characteristics of a wheel:

Roundness - to what extent is the wheel round? Assuming uniform material 
density, out of round wheels would be "out of balance" and could cause 
vibrations which would be bad.  Generally, most manufacturing tolerances 
are good enough that this should not be a problem.

Hardness  - resistance to deformation when the wheel impacts an object.  A 
soft tire deforms and would have a greater contact patch which would 
increase the friction between the wheel and the road (and slow us down).  
Harder would be better.  Additionally, the continuous deformation of the 
tire as it rolls could lead to heat buildup which is usually not good for 
materials.  (An example of this is how underinflated car tires can 
actually overheat and lose their tread because of the internal friction 
caused by increased sidewall flexing when the internal pressure is low).

Wear  - Again, harder is probably better in this case, however if the 
material doesn't wear it generally won't "grab" when lateral forces are 
applied.

Mass  - the mass of the wheel (and therefore its weight) could have some 
effect, but because the size is already pretty much standard mass wouldn't 
be much of an issue.  As a general rule, however, lighter wheels 
would "spin up" faster than heavier ones, and heavier ones would continue 
spinning longer.

However, as your student has assumed, the bearings are probably the single 
most important factor because friction is probably the single most 
important force for us to overcome.  Try a simple test with a wheel that 
has a bearing - clean the bearing and remove all the lubricant.  Now spin 
the wheel and note how long it spins for.  Next, lubricate the bearing and 
perform the same test again.  Assuming that you used the same approximate 
amount of force to spin the wheels, it should spin considerably longer in 
the lubricated state.  The lubricated state corresponds to low friction 
and the unlubricated state corresponds high friction (without the 
unlubricated bearing the frictional resistance would be even greater!).  
Generally, the higher quality your bearings are, the better they can 
overcome friction.  The quality is generally determined in much the same 
way as we looked at the wheels themselves (above).  The roundness of the 
balls in the bearing play a part (rounder is better), as does the number 
of balls (or needles in needle bearings)(more is better), and the material 
properties of the balls (harder is better).  I think the final 
consideration would be the lubricant used on the bearings - thicker 
lubricants would create greater viscous resistance than thinner ones.  
Generally, it is best to use the lubricant recommended by the bearing 
manufacturer, but if they offer a range of lubricants use the thinnest oil 
recommended.  You'll probably have to clean and reoil the bearings more 
often however.  Good bearings cost good money so take care of them as 
recommended.  To see what bearings consist of see Britannica.com @
 http://www.britannica.com/bcom/eb/article/4/0,5716,14124+1+13946,00.html?
query=bearings

Good luck - I hope this helped.  Please email me if clarification is 
necessary.

Sincerely,
Steven Miller
smiller@kahuna.sdsu.edu
Undergrad - Mechanical Engineering
San Diego State University