| [GMCnet] Dave Lenzi's modified upper A arms to increase caster. [message #328281] |
Tue, 16 January 2018 09:15 |
glwgmc
Messages: 1014
Registered: June 2004
Karma: 10
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Senior Member |
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Look to the rear, Bob. If ride height is set to factory spec, as I suspect on your coach it is, then make sure all six wheels are frame parallel. The rear does steer the front if they are not. If one on a given side is not you can bend the arm until it is. If both on one side are out the same amount then shim the mount. Set rear camber to no more than one degree tire in at the top.
The front alignment is very straight forward once the rear is correct. With bias ply tires the factory spec in 73 was 3 degrees caster. That was quickly changed to 2 degrees in 74 and remained there even after GM offered radial ply tires. As you know from your racing experience, bias ply tires behave differently at speed than radial ply tires as the contact patch moves back providing more caster at speed than static. Radial ply tires do not so need more caster to begin with in order to have enough caster at speed to drive straight easily.
If the rear is set properly then 3 to 4 degrees is enough for most coaches with radial ply tires. Be careful going much over that as you could over load the front steering components. Zero toe (frame parallel) and zero camber on the front usually works well for most coaches with radial ply tires.
When new, with biased ply tires a GMC drove straight and true one hand on the steering wheel. 40 years later they still do with radial ply tires if set up properly (which is different from the factory settings) front and rear.
The thing I found most often missing from commercial alignment is they rarely drive the coach after jacking it up (as most do to make it easier to reach the adjusting cams). On most coaches it takes driving it a few miles to get the suspension to settle in to where it will be going down the road. If the front end alignment is set before that drive then caster, caster and toe may all change once it does settle in and the freshly “aligned” coach may wander all over the place.
Jerry
Jerry Work
The Dovetail Joint
Fine furniture designed & hand crafted
in the 1907 former Masonic Temple building
in historic Kerby, OR
http://jerrywork.com
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Date: Tue, 16 Jan 2018 03:59:25 +0000
From: Bob Dunahugh
To: "gmclist@list.gmcnet.org"
Subject: [GMCnet] Fw: Dave Lenzi's modified upper A arms to increase
caster.
Message-ID:
Content-Type: text/plain; charset="iso-8859-1"
I put a lot of Dave's parts in out first 78. Got it aligned. Did the same with a friend's 78. Both drove well. Just not as good as I thought they should. Bought this latest 78 with about 100,000 less miles that has all it's original parts. Drives better then the ones I fixed. It's got to be how the aligning is done. I know one other person that has had the same thing happen. Bob Dunahugh 78 Royale
________________________________
From: Bob Dunahugh
Sent: Monday, January 15, 2018 11:59 AM
To: gmclist@list.gmcnet.org
Subject: Dave Lenzi's modified upper A arms to increase caster.
This replacement 78 Royale drives better then our last one. I pulled Dave's knuckles, relay arm, idler arm, and the new steering wheel slip joint out of the burned Royale. Plus the upper A arms that had the offset rubber bushings installed. Already installed the knuckles. Not the relay, idler arm, or slip joint yet. Thinking of putting in the reworked A arms that Dave makes. Caster is extremely important for having the GMC drive perfectly straight. His pair of A frames are about $750. Thinking that I'll order some from his next run later this year. Any input out there? Bob Dunahugh. 78 Royale
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Jerry & Sharon Work
78 Royale
Kerby, OR
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