Ride Height vs. Tire Diameter Question

john bush

New member
Sep 23, 1998
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We are signed up for a full wheel alignment this Friday at a shop in Santa
Rosa having good marks from a couple of netters. We hope to further reduce
road wander, which has already been significantly improved with new shocks
and a Caspro front stabilizer. The front end was serviced and a 6 wheel
alignment performed last year and the system is now pretty tight or "exact",
but it still "exactly" wanders around quite a bit. (I had Gene Fisher
drive it a couple of weeks ago and he emitted a yelp while recovering from a
local bump he had not noticed driving his own coach.) It's pretty clear a
body lean in one direction (say right) will produce a turn to the opposite
(left); the leaning caused by either a bump or deliberate turn (left).

One loose end remains to be resolved before having this shop tackle the
alignment. The rear end sits low as set in last year's alignment. If I
deliberately raise the rear by nearly 2 inches, all chassis measurement
points nicely fall into their respective windows and the body appears level.
In doing so, the air bags appear to be over inflated, the cones noticable
off horizontal. I have G159 LT225/75R16 tires on Alcoa rims, with 80 PSI
cold all around. The top of the tread is typically 28 3/4" off the
pavemen, cold, and measured in the dark. I keep hearing that the 225/75R15
essentially duplicates the original wheel and tire diameter which underlies
the chassis ride height measurement points on the frame. I keep wondering
if my current wheel and tire combination is really smaller than the OEM
configuration, and if not properly taken into account, will cause the height
adjustments (and consequently front end geometry) to be distorted when
forced to match the original height adjustment specs.

Can anyone verify that my 28 3/4" is close enought to the original loaded
tire diameter to pass for the original?

Am I correct in expecting the cones to be essentially vertical when properly
set?

Any suggestions?

TIA

John Bush
75 Palm Beach
Sunol, CA
 
Hi John
Goodyear has their tire specs located at
http://www.goodyear.com/us/tires/tirecatalog/G159LTSize.html

The static loaded radius (SLR) for the 8.75R16.5 and the 225/75R16 are both
13.6 inches.

I suspect the ride height was originally specified for bias ply tires which
would sit a bit higher than radials. The ride height at the frame should be
set to the SLR plus or minus a fixed offset. I for one would like to know
what that offset is.

Does anyone know the SLR of the 8.75/16.5 bias ply tire?

Can it be assumed the ride height on the front was to ensure the CV joint
was running in a straight line with no angle? If so, would the drive shafts
be sitting horizontal minus the camber angle? (i.e. about 1 degree off
level)

Dave Mumert

- ----- Original Message -----
From: "John Bush"
To: "GMC Net"
Sent: Tuesday, July 18, 2000 7:51 AM
Subject: GMC: Ride Height vs. Tire Diameter Question

> We are signed up for a full wheel alignment this Friday at a shop in Santa
> Rosa having good marks from a couple of netters. We hope to further
reduce
> road wander, which has already been significantly improved with new shocks
> and a Caspro front stabilizer. The front end was serviced and a 6 wheel
> alignment performed last year and the system is now pretty tight or
"exact",
> but it still "exactly" wanders around quite a bit. (I had Gene Fisher
> drive it a couple of weeks ago and he emitted a yelp while recovering from
a
> local bump he had not noticed driving his own coach.) It's pretty clear
a
> body lean in one direction (say right) will produce a turn to the opposite
> (left); the leaning caused by either a bump or deliberate turn (left).
>
> One loose end remains to be resolved before having this shop tackle the
> alignment. The rear end sits low as set in last year's alignment. If I
> deliberately raise the rear by nearly 2 inches, all chassis measurement
> points nicely fall into their respective windows and the body appears
level.
> In doing so, the air bags appear to be over inflated, the cones noticable
> off horizontal. I have G159 LT225/75R16 tires on Alcoa rims, with 80 PSI
> cold all around. The top of the tread is typically 28 3/4" off the
> pavemen, cold, and measured in the dark. I keep hearing that the
225/75R15
> essentially duplicates the original wheel and tire diameter which
underlies
> the chassis ride height measurement points on the frame. I keep wondering
> if my current wheel and tire combination is really smaller than the OEM
> configuration, and if not properly taken into account, will cause the
height
> adjustments (and consequently front end geometry) to be distorted when
> forced to match the original height adjustment specs.
>
> Can anyone verify that my 28 3/4" is close enought to the original loaded
> tire diameter to pass for the original?
>
> Am I correct in expecting the cones to be essentially vertical when
properly
> set?
>
> Any suggestions?
>
> TIA
>
> John Bush
> 75 Palm Beach
> Sunol, CA
>
 
Hi John,

>In doing so, the air bags appear to be over inflated, the cones noticable
>off horizontal.

The airbags will only be horizontal at one extension length (ride height).
How much is noticeable to you?

> I have G159 LT225/75R16 tires on Alcoa rims, with 80 PSI
>cold all around. The top of the tread is typically 28 3/4" off the
>pavemen, cold, and measured in the dark. I keep hearing that the 225/75R15
>essentially duplicates the original wheel and tire diameter which underlies
>the chassis ride height measurement points on the frame. I keep wondering
>if my current wheel and tire combination is really smaller than the OEM
>configuration, and if not properly taken into account, will cause the height
>adjustments (and consequently front end geometry) to be distorted when
>forced to match the original height adjustment specs.

Arch asked this question at some point because Wes Caughlin has it answered
in this month's newsletter. According to Cinnabar, the diameters "are
virtually identical" and you can use the OEM specs safely.

Henry
 
>
>
>Can anyone verify that my 28 3/4" is close enought to the original loaded
>tire diameter to pass for the original?
>
>Am I correct in expecting the cones to be essentially vertical when properly
>set?
>
>Any suggestions?
>
>TIA
>
>John Bush
>75 Palm Beach
>Sunol, CA

Most tire manufacturers have info on their web sites about their tire sizes.

Goodyear is at http://www.goodyear.com/us/tires/tirecatalog/G159LTSize.html

Their tire diameter for the G159LT 8.75x16.5 radial is 28.9 inches.
The static load radius is 13.6 inches (thats from the ground to the
center of the axle).

Their G159LT 225/75R16 has a radius of 29.3 inches and also shows a
static load radius of 13.6 inches.

If you have different manufacturer's tires it might vary a little but
will be fairly close.

You should have no problem just setting the ride height to GM's
factory specs with the 225/75R16 tires.

As to the cones, I don't think that they are vertical when at the
proper ride height. I believe that the tops tip outward a bit. I
can go out and look at mine if you need me to.

Emery Stora
77 Kingsley
Santa Fe, NM
 
On Tue, 18 Jul 2000 06:51:59 -0700 "John Bush"

> Can anyone verify that my 28 3/4" is close enought to the original
> loaded tire diameter to pass for the original?
> TIA
>
> John Bush
> 75 Palm Beach
> Sunol, CA
>
Hi John,

I was going to ask you how my old girl was doing but I think I will hold
off til you have this problem resolved!

I also use the same tire and in a 100 mile test I was under by 2% which
says we are pretty close as far as the odometer and tire matchup is
concerned. I got about the same with the OEM wheels and tires.


David Lee Greenberg
The GMC Motorhome Registry
Dedicated To The Preservation of the Classic GMC Motorhome
http://www.GMCss.com/Registry.htm
http://www.Buskirk-RushRV.com/GMC-Registry/online-registry.htm