MadSci Network: Engineering
Query:

Re: Does a well waxed automobile get measurably better mileage than unwaxed?

Area: Engineering
Posted By: Tod Flak, Post-doc/Fellow Engineering
Date: Tue Mar 11 13:02:13 1997
Message ID: 856308135.Eg


MadSci Network: Engineering
Query:

Well, first the short answer to the question:
No, waxing a typical car will have no measurable effect on it's drag or gas mileage. There are simply too many other "bad" aerodynamic features on an automobile that completely swamp any possible effect that a smoother surface would have.

However, that answer really doesn't quite satisfy me as an engineer, because I wanted to know what the effect would be, from a theoretical standpoint, even if it is not measurable from a practical standpoint. However, not being very well acquainted with fluids and aerodynamics, I was not really sure how to approach the question to give a definitive answer and at least a guess as to the actual magnitude of the effect, even if it is very, very small. So, I asked some 'experts': I posted the question on two Usenet News groups, "sci.aeronautics" and "rec.autos.tech".

First, some basics: what we are dealing with here is a question of fluid flow and aerodynamics (air is considered a "fluid"). There are broadly speaking two types of fluid flow across a surface, laminar and turbulent:

There are also two types of drag to be concerned with:

From the Usenet groups, I got several responses; here are a couple of the more useful ones:

I told Professor Locock that it was very kind of him to offer to do an experiment in the wind tunnel for you to answer this question, but not to devote their limited time there to it. But it just goes to show that this question is rather difficult to answer in a theoretcal sense, and that even these guys were unsure of the effects of surface smoothness on drag for their vehicle. (I actually suggested that if they did do the experiement, if it might tell them that effort devoted to making the surface to a mirror-fine finish is really a waste of time! But it certainly makes it look nice.)

So, in the end, I guess I still can't quite give you a good estimate of the effects of waxing a car. It is certainly unmeasurable for a typical car, or probably even a sleek sportscar. It is possible that for a very aerodynamic vehicle, like an experimental solar car in which they are trying to get every conceivable bit of efficiency, that it may make some very small difference. But even for those vehicles, much greater fuel efficiencies can be gained by improving the shape of the shell to minimize the form drag.

I still have some inquiries out to various solar car teams at universities around the world - if I hear from them, or get other useful info from the Usenet particiapants, I will let you know by email directly. Sorry it took so long to get an answer, and that it is only a partial answer at that.

By the way, in my search I came across some interesting Web sites that you might like to look at:

Regards,
Tod

------------------------------------------------------------
Tod A. Flak
Lawrence Berkeley National Lab
1 Cyclotron Road, MS 70A-3363
Berkeley, CA 94720
phone: (510) 486-5721
fax: (510) 486-5857
email: taflak@lbl.gov
Web page: http://todflak.lbl.gov/
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