| [GMCnet] Over steer [message #304326] |
Wed, 27 July 2016 09:04 |
glwgmc
Messages: 1014
Registered: June 2004
Karma:
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Senior Member |
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Hi Rob,
At a Western States rally a while back I gave a seminar on alignment and why it is difficult for many to achieve anything like the proper amount of caster on a GMC with the factory supplied parts and chassis mounting points. Hopefully, that presentation was done in an easy to understand way and was devoid of techy talk. That whole presentation can be downloaded from the GMCWS site or from my web site and may be helpful to many desiring more stable driving from their coaches.
The long and the short of the issue is that our coaches were designed around bias ply tires which allow the tire contact patch to move backward at speed resulting in more caster at speed than at rest. Radial ply tires behave very differently and the contact patch does not move at speed which results in the same caster at speed as you have at rest. In 73 the spec for caster at rest was 3 degrees and the designers apparently felt the bias ply tires would create another one to two degrees at speed so the coaches would go down the road with four to five degrees of caster so they would drive straight and true with one hand on the wheel. They quickly changed that spec to two degrees at rest in 74, likely because at three degrees static there was too much stress on the front end components. Two degrees remained the spec through the rest of production even though the coaches were offered with either radial or bias ply tires from about 75 on.
There is so very little adjustment range designed in to the stock configuration that it is hard to get much more caster while maintaining zero camber. Offset bushings help most, but not all, coaches achieve something near three or more degrees static caster but the only real way to get that or more is to change the location of the mounting ears, or the design of the A arms or both. Neither is practical for most owners to do. DaveL chose to alter the geometry via a redesign of the upper A arm and those apparently work very well, when he is able to produce them. As late as a couple of days ago he said he was still swamped with trying to catch up on 8.1L engine conversion kits he promised before experiencing health issues so is not making any of those re-designed A arms now.
The mounting ear locations were most likely welded on by hand using some sort of locating jig. Far from the precision we have come to expect in a day of high volume, robotically welded components. If they are just a little off fore-aft it will have a dramatic effect on how much caster you can achieve on that coach whether using offset bushings or stock ones. I can find no record of the factory ever changing the location of those mounting ears over the years, but I suppose they might have in an attempt to gain more caster for those using radial ply tires.
These and several other issues are why it is so important to set our coaches at proper ride height before attempting to do anything re the alignment. Raise up the rear or let the front sink and you loose caster you don’t have enough of anyway. Throw the front up relative to the rear and you gain caster but will upset the anti-dive geometry, bump steer and a host of other dynamics as well. Same with moving the front wheel spacing. The designers knew what they were doing far more than any of us I would surmise, so the best starting point is bang on the factory ride height and then go from there to try to compensate for 40 year old (worn out?) components, radial ply tires and manufacturing variations one coach to another.
Jerry
Jerry Work
The Dovetail Joint
Fine furniture designed and hand crafted in the 1907 former Masonic Temple building in historic Kerby, OR
glwork@mac.com
http://jerrywork.com
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Message: 3
Date: Wed, 27 Jul 2016 16:00:39 +1000
From: "Rob Mueller"
To:
Subject: Re: [GMCnet] Over steer
Message-ID:
Content-Type: text/plain; charset="us-ascii"
Jerry,
While I agree with what you have noted below as to the front alignment specs I am going to respectfully disagree as to how much
caster can achieved at zero camber.
It is my experience that coaches up to and including 1975 you are lucky if you can get two degrees caster at zero camber. In fact
the best I could get with Double Trouble was two degrees WITH Moog offset bushings. After I installed a pair of Dave Lenzi's offset
upper control arms I got five degrees and the way the coach drove was amazing! From 1976 on something happened and it is possible to
get two degrees and a bit more without having offset bushings.
Also up to 1/8 inch toe out is acceptable because when you apply power to a FWD vehicle it tends to tow in. Zero camber is important
as radial tires like to run flat (parallel to) on the road surface.
Regards,
Rob M.
USAussie - Downunder
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
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Jerry & Sharon Work
78 Royale
Kerby, OR
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