What is the drag coefficient of a motorcycle wheel?

[4stroke]

Motorcycle wheels spin and cause turbulence.  You can cover most of the wheel up with a fairing but some of it will stick out the bottom of the fairing.

What is the drag coefficient of a motorcycle wheel?

[donpearsall]

That is an open-ended question for sure! You did not say tire but I assume that is what you meant so I am including Wheel and TIRE.  I know very little about calculating Cd but you can relate the exposed part of the wheel/tire to a cylinder shape. For those the Cd is about .4 -.5. Very draggy. 

Not only that, but the rotation of the tire and wheel with spokes also creates drag all of it's own, kind of like a fan does. So not only do you have to factor in the Cd of the tire, but the work required to pump the air in a circle.

There are aerodynamic experts here who can do a much better job of explaining.
Don

[4stroke]

Thank you for your reply.  Yes a tyre spinning at speed.

I can calculate the body drag coefficient but I had not found useful information about the drag coefficient of a spinning wheel.

My understanding of calculating CdA is as follows.

Drag coefficient x frontal area = Coefficient of drag Area

So if the tire is 0.6m tall and 0.1m wide and has a drag coefficient of 0.5.

0.6m x 0.1m = 0.06m squared (frontal area of tyre)

0.06m * 0.5m (Cd) = 0.03m squared CdA

--

Alright so if I can find out the drag coefficient of the wheel I can compared the CdA of the exposed wheel with the CdA of the body to give better estimation of the total vehicles CdA.

[panic]

Dealing entirely with a fully exposed wheel & tire, the smallest OD, widest section width, and lowest aspect ratio will have the lowest drag, but IDK whether the change is significant.

[Sumner]

Alright so if I can find out the drag coefficient of the wheel I can compared the CdA of the exposed wheel with the CdA of the body to give better estimation of the total vehicles CdA.

It sure is hard to find good data on wheels/tires but I've found Cd's for tires that have the width/diameter ratio similar to Goodyear Land Speed tires with Cd's of between .18 to .58. So take your pick. I'd use numbers between .45 and .60 myself as that way you will be erring on the conservative side.

You might want to look at the "Horse Power Needed For A Lakester or a Car to Run a Certain MPH" spreadsheet down a ways on the page here...

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html

It allows you to input different Cd's for the body and the tire to get possible answers related to the total Cd.  If you can't input data to it, save it and then used the saved version. 

Also if you only want to have a body (motorcycle) and front tire then don't input the rear tire data and when inputting the front tire data only input 1/2 the front tire width if you want to incorporate the rear tire into the body.  That will reduce the effects of the tires from 4 (for a lakester) to only one for the front tire of a bike,

Sumner

[stay`tee]

You can fiddle the small stuff all you want on paper, your "never" going to get it accurate, it will always be only an assumption ("every" vehicle differs),,
Use these assumptions as a base(starting point) to help zero in with more accurate data after a number of passes have been made down the salt,  

[Rex Schimmer]

Good data on wheel/tire combinations is hard to find, there is a book, "The Leading Edge" in which they do a considerable amount of testing with what are basically bicycle wheels and tires and do come up with some interesting numbers. One of the challenges with exposed wheels is that the velocity differential between the tire surface and the oncoming air flow on the top half of the wheel goes from 2 times the speed of the vehicle at the very top of the wheel to the velocity of the vehicle at the center line to zero at the very bottom of the tire plus at the bottom there is air that the tire is attempting to draw under the tire which can cause some lift. Such things as non rotating wheel covers and the fit/shape of the wheel cover to the tire also can have an affect. One of the things that the GP type motorcycle tires have, which I think is a plus, is a very aero profile. The best ones are rated by the factories at over 185 mph and I am sure that if you reduce the tread thickness they could run faster. It does take special premission from SCTA to run these tires but I have seen a C/gas Lakester that ran them on the rear (the car was front wheel drive) and went over 275 mph with out problems.

Cal Poly in So. Cal is starting up a small moving floor wind tunnel and I have offered to them to provide a set up that would allow them test some land racing wheel/tire combinations in their tunnel. I have not heard from them but I think it would be a great project for one of their engineers, especially if they could do CFD at the same time.

Rex