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| [GMCnet] disk brake upgrade [message #235135] |
Sun, 05 January 2014 09:59  |
glwgmc
Messages: 1014
Registered: June 2004
Karma:
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In all of these "bad" design choices keep in mind that the coaches were designed around bias ply tires which exhibit unique characteristics at speed which results in increased caster as speed increases. We now all run radial ply tires which do not, so we need to dial in more static caster to make up for that difference. When GM added the option for radial ply tires, bias ply tires were still also used on some coach versions so whatever they decided for static caster had to be a compromise. If they increased static caster for the radial ply tired coaches, it would increase the load on the steering components for the bias ply tired coaches. They apparently decided to leave the static caster spec at 2 degrees as that is where it stayed throughout the rest of production.
Rut wander became a real issue when in the 80s some suggested changing over to steel sidewall load range E radial ply tires run at 80 psi - supposedly for greater load carrying capacity in a condition when one front tire might carry higher than designed loads for a short period of time. With a fabric sidewall tires (load range D or E) set at the correct 55-65 PSI pressure for the loads our coach tires carry, rut wander even at lower caster angles speced for bias ply tires, is not too much of an issue if the rest of the front end components are tight.
Everything goes to hell in a hand bag when the front ride height is allowed to drop whether from age on the torsion bars, or because a PO thought it "looked better", or because the rear ride height adjusters are out of whack, or because a commercial alignment shop jacked up the front to make their adjustments and did not drive the coach to allow the front end to settle down to where it would be going down the road. Ride height low in front decreases whatever amount of caster we can get whether with the stock front end or with the one ton, and decreased caster will lead to the coach not wanting to drive straight no matter what else might be going on. Virtually all the coaches we measured coming into the GMCWS rally at Casa de Fruita, CA, a couple of years ago were ride height low in front - hence all came in with way too little caster for optimum cruise performance.
One final "interesting" note. This from the 1936 Cord publication, "Information for Salesmen" in the Questions and Answers section:
"Q - What is the width of the tread?
A - 56" front and 61" rear
Q - Why is the tread wider in the rear?
A - To allow more room in the body and to make the car wider and therefore safer, because it will not tip so easily"
Cord was a ground up design by Gordon Behrig as part of a design competition while he worked at GM. When he was hired to design a baby Duesenberg based on that design he was apparently under no constraints as to components from other vehicles that had to be used. He selected a wider rear track than front track for a front wheel drive car anyway. The car name was changed to Cord just before production commenced.
Jerry
Jerry Work
The Dovetail Joint
Fine furniture designed and hand crafted in the 1907 former Masonic Temple building in historic Kerby, OR
Visitors always welcome!
glwork@mac.com
http://jerrywork.com
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Message: 8
Date: Sat, 04 Jan 2014 16:51:25 -0600
From: Kerry Pinkerton <Pinkertonk@MCHSI.com>
Subject: Re: [GMCnet] disk brake upgrade
To: gmclist@temp.gmcnet.org
Message-ID: <3964a.52c8906c@gmc.mybirdfeeder.net>
Content-Type: text/plain; charset="ISO-8859-15"
snip+++++That said, I'd tend to think all of the items he mentioned are certainly not optimal. Imo, the front end, while it has indeed lasted for decades and works ok, is not a good design. The difference in track width was just cheaping out, same with the caster issues, all so they could use as much of the Toro clip as possible. Perhaps roads weren't as bad back then or perhaps a new coach didn't rut run like today.
I'd expect that by 1975, they knew about all these things that could be improved but didn't because they:
1- Didn't think the additional expenditures would increase revenue/profit
2- Bean counters (and I are one.. :) ) wouldn't let them spend the money.
Not slamming our coaches at all so I hope no one gets their knickers in a twist. I love my coach...and I love to try and make it better.
That said, If I can come across an extra set of bogies, I'm going to do two trailing arms on the Art Deco Car Hauler....and a 1 ton front end. With a blank slate chassis and no interior limitations, adding in the additional forward bogie mount and only using the trailing arms will be pretty straight forward++++snip.
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Jerry & Sharon Work
78 Royale
Kerby, OR
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