Date:         Mon, 2 Nov 1998 11:14:50 -0800
Reply-To:     YauMan Chan <YauMan@CCHEM.BERKELEY.EDU>
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         YauMan Chan <YauMan@CCHEM.BERKELEY.EDU>
Subject:      Wheels, Tires and the Laws of Physics
Comments: To: vanagon@vanagon.com
Content-Type: text/plain; charset=ISO-8859-7

Pardon me for jumping in even though I don't have 16" wheels!!!

We are comparing a 205/70-14 wheel and a 215/65-16 wheel.  Radius of wheel = height of tire +radius of rim

205/70-14 --> 205mm wide 70%height, 7" radius rim
Calculate: 205mm x .7=143.5mm + 7"x25.4mm=321.3mm=12.65" wheel radius

215/65-16 -->215mm wide 65%height, 8" radius rim
Calculate: 215mm x .65=139.75 + 8"x25.4mm=342.95mm=13.5" wheel radius

The 16" wheel is therefore 7% "bigger" than the 14".. So the speedometer will read 7% slower.. ie, when the speedometer reads 100mph, the van is really going 107mph..no big deal

The question is what is the advantage of going for the 16" rim.  These are two important consideration.  (1) For safety and comfort, you want to keep the "unsuspended" weight of a vehicle to be minimum ¯ conservation of momentum and all that ¯  and (2) You want the wheels to be as stiff as possible for good handling.

1. The answer is not  size (sorry guys!) but weight.  What you should aim for is to decrease the "unsuspended" weight of the vehicle.  i.e., anything not carried by the suspension system should be kept to a minimum. This include all the wheels, brake components and drive shaft from inner CV joint on out.  Of these, the wheels are the heaviest.  That's why alloy wheels were introduced since aluminum alloy weighs a lot less than steel.  Then why bigger rims? ¯ by having less rubber and more alloy, the net weight of the wheel can be decreased.  (While rubber is lower density than alu alloy, it takes more rubber to hold up a van than alu alloy.)  In this instants of 16" vs 14" wheel (with rubber), I don't know if the 16" actually weights less than the 14" ¯ someone here can check it out.

2. Rubber is squishy and is too flexible for good road feel.  By lowering the height of the tire and increasing the diameter of the rim, you get a much stiffer wheel.  This will greatly improve the handling of the vehicle.

The side effect of all this is that the 16" wheel have a wider contact surface..  This gives you more friction with the road and will give you more traction.. But will decrease you gas mileage.

My $0.02 worth.

Yau-Man Chan
87 GL