Date:         Tue, 3 Nov 1998 02:03:02 -0800
Reply-To:     Per Lindgren <lindgre@ONLINE.NO>
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Per Lindgren <lindgre@ONLINE.NO>
Subject:      Re: Wheels, Tires and the Laws of Physics
Comments: To: vanagon@vanagon.com
Content-Type: text/plain; charset=iso-8859-1

YauMan Chan wrote:

> 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

 All of this is good if you drive a sports car, but it seems that these guys are going off-road (like me), and ground clearance is the key factor here. You don't see any terrain vehicle of any choice with such small tires as the Vanagon. That is why VW made the infamous 16" version, so it will be more off road capable. A low profile tire will not make the same contact on uneven terrain, as a high profile would, the low profile is too stiff. The unsuspended weight is not that important for an off-roader, it is during speed that the handling of a low profile tire is needed. A side effect of the low profile tire off road is that you will also ruin the rims quite quickly.

PerL
88 Syncro