Date: Sun, 30 Oct 2011 16:56:31 -0700
Reply-To: BenT Syncro <syncro@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: BenT Syncro <syncro@GMAIL.COM>
Subject: Re: An interesting article on gas mileage vs wheel size
In-Reply-To: <004a01cc9743$0059f440$0301a8c0@korky2>
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Hi Ken,
The trouble I find with using "backspace" is people either use a calculated measurement whilst others physically measure the wheel. One or the other frequently do not take into account that wheel size often does not take the dimensions of the area past where the bead would seat.
Anyway, here's how I read your wheel. ET42 at 7.5" wide compared to a factory alloy wheel of ET30 at 6.0" wide. On the inside edge, you are likely to see the inner edge move inwards 31.05mm compared to stock. That's 12mm for the offset difference plus another 19.05mm (0.75") because of the wider wheel. You will have clearance issues unless you correct the offset with spacers as you already surmised.
You could get away with a range of spacer thicknesses depending on what you what to accomplish. Personally, i would adjust to the factory offset of ET30 w/ 12mm spacers. You will need proportionately longer bolts up front and longer studs in the rear unless you have a big brake kit with studs in front. If you do, then longer front studs are also in order. Don't forget that you will need to factor in the thickness of the wheel mounting pad unless you recontour the lugholes to a 60 degree conical seat. An alternative would be bolt on spacers which have studs.
There will be .75" more wheel on the inside and outer edge. Suspension geometry should be unchanged. You should be able to clear the steering components. The downside is you will have more 'splash' from the tire compared to stock. Tire selection can become critical if you go to a larger offset depending on sidewall bulge.
On the upside, thick spacers often make up enough distance between the tall 2wd dust cap so machining the centerbore might not be needed.
BenT
sent from my electronic leash
On Oct 30, 2011, at 1:32 PM, "Ken Lewis" <kdlewis@northstate.net> wrote:
> Two wheel drive DOKA.
>
> Offset is 42. I arrived at the back spacing by dividing the width, 7.5"(190.5mm) by two and adding the offset. A formula I found in the Vanagon archives. I "assumed" it to be correct.
> Ken
>
>
> ----- Original Message ----- From: "BenT Syncro" <syncro@gmail.com>
> To: "Ken Lewis" <kdlewis@NORTHSTATE.NET>
> Cc: <vanagon@GERRY.VANAGON.COM>
> Sent: Saturday, October 29, 2011 3:32 PM
> Subject: Re: An interesting article on gas mileage vs wheel size
>
>
> Putting on a 2wd? Do you have the offset rather than a measured or calculated backspace? It's the figure that says 'ET' followed by a couple of numbers. I'd like to help but don't want to rely on the expression of offset as backspace because accuracy can be affected by how you arrived at that number IMHO.
>
>
>
> BenT
>
> sent from my electronic leash
>
> On Oct 29, 2011, at 9:43 AM, Ken Lewis <kdlewis@NORTHSTATE.NET> wrote:
>
>> I found this article while researching what it would take to get 17"x7.5"
>> Audi A4 wheels to work on my crewcab.
>>
>> This line caught my eye;" What's immediately apparent from the results is
>> that as the wheel-and-tire packages get larger and heavier, acceleration and
>> fuel economy suffer. Neither is a huge surprise, but we measured a
>> 10-percent drop in fuel economy and a four-percent degradation in
>> 0-to-60-mph acceleration from the 15s to the 19s".
>>
>> I looked at these this morning.
>> http://winstonsalem.craigslist.org/pts/2618213904.html . Nice looking BIG
>> wheels. Some road rash. He also has the longer lug bolts.
>>
>> The bolt hole size is good but it appears the center bore (~57mm) would need
>> to be opened up. I need to actualy take a front wheel off my truck and do
>> some measuring . I have read the grease cap can be shortened to get rid of
>> the lip.
>>
>> I arrived at a back spacing of 137.25mm on the Audi wheels . By my
>> calculations I would need a 14mm spacer? Anyone?
>>
>> Thanks Ken
>
>
