| [GMCnet] Summary of the Align it Right! presentation made at GMCWS Casa de Fruita rally [message #167888] |
Sat, 28 April 2012 12:41  |
Sharon Work
Messages: 9
Registered: April 2012
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Junior Member |
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Summary of the Align it Right! presentation made at GMCWS Casa de Fruita rally
Your GMC was designed around using bias ply tires. Bias ply tires are very different from radial ply tires. A bias ply tire goes egg shaped at speed and the contact patch area moves back the equivalent of up to 2 degrees of caster (the angle that makes your GMC go straight down the road). Radial ply tires do not. The initial caster angle specced in 73 was +3 degrees. They expected the tread distortion in bias ply tires to add up to another +2 agrees of caster for a total of around +5 degrees at speed. They apparently found that much caster put undue strain on the steering gear so by the 75/76 manual they changed the caster spec at rest to +2 degrees and it stayed that way through the end of production.
Even when they switched to radial ply tires in 76 they did not alter the front end geometry to compensate for the different behavior of the radials so your coach set to factory spec is going down the road with too little caster to make it stay straight. You need a total of +4 to no more than +5 degrees of caster.
Absolute ride height is critical to maintaining the proper front end geometry. Some speculate that as long as the ride height is higher in front than rear absolute ride height is not important. That is false. If you ride height is lower than the factory spec the anti-dive geometry built into the design goes away and your coach will pitch downward way too much under heavy braking. If your ride height is lower than the factory spec your front axles and CV joints run at an angle instead of straight which can cause the axle to whip - a feeling some associate with torque steer. Restore the ride height and the "torque steer" feeling may well go away.
A second characteristic of bias ply tires is they tend to lift the inside of the tire contact patch area when under side load, like when going around corners. As a result, your GMC was designed with a bit of tire top in camber on the front tires to offset this condition but at the expense of increased tire wear. Radial ply tires do not exhibit this characteristic so they should stand straight up (zero camber) for best tire wear and to eliminate the tendency of a radial ply tire to dive right or left when they hit a bump if they are leaned in at the top.
Again, even when they switched to radial ply tires in 76, GM did not alter the caster angle to compensate for the different behavior of the two kinds of tires. They also continued to offer bias ply tires as an option, especially on the transmodes so they were stuck between alignment specs optimized for bias ply tires or those that would be optimum for radial ply tires. They choose the former so now that most all of you are running radial ply tires, you now need to compensate yourself. That can only be done correctly with your coach at the factory specced ride height front and rear.
Anytime you jack up any part of your GMC you have to drive it a few miles to get the suspension to settle into the attitude it will take going down the road. No amount of wiggling or jostling will settle it fully. If your alignment is set when the suspension is not fully settled, ride height will drop as you drive which will reduce caster, increase camber (tilt) and also effect frame parallelism in addition to eliminating the anti-dive geometry and perhaps causing the drive axles to whip as they rotate.
Very few alignment businesses have experience that is relevant to properly aligning your GMC. Years of car and light truck experience is not directly transferable to your GMC. Years of heavy truck and large diesel pusher experience is not relevant at all to your GMC. Neither is years of experience aligning race cars. So, you may never have had a proper alignment no matter how many times you paid for one. Few shops are willing to drive the coach every time they jack it up as they frequently do while setting the alignment to spec and few of you are willing to pay them to do that which would more than double, or even tripple, the cost of an alignment.
Few shops have relevant experience with a vehicle where both the front and the rear ride height are adjustable so after a "professional" alignment you coach is probably going down the road lower than it should. In fact, over 90% of the coaches we measured at the GMCWS Casa de Fruita rally were set at the wrong ride height. Most were low in front and many were high in back, the very worst of all possible conditions if you want your coach to drive properly.
Fortunately, you now can check ride height and alignment yourself right on your own parking pad with the DIY alignment kit owned by GMCWS. It only takes 30 to 60 minutes. Find out how and view the entire presentation I made at the Casa rally by going to my web site, http://jerrywork.com > GMC motorhome page and scroll down to find the direct link to the presentation - Align it right! Presentation made to GMCWS Casa de Fruita rally, spring 2012 It will soon also be available from the GMCWS web site.
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
541-592-5360
www.southernoregonguild.org
www.siskiyouguild.org
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