| Why do we still measure ride height to the frame slots? [message #359172] |
Sun, 04 October 2020 06:51  |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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So, my thinking is the slots were used in production, but are not the greatest idea in the real world now.
The rear slots are several feet behind the bogie, which is the suspension point of the rear wheels. Unless you are checking the ride height on a perfectly flat level piece of concrete, you could have severe errors in your measurement. Not to mention you can't access the slot under the generator anyway.
I measure from the ground to the bogie Zerks. This is the pivot point for the rear suspension and mid-way between the two rear wheels. At this point the rear measurement is not affected by the front height or need to be level surface front to back. Also the area between the two wheels is most likely to be reasonably flat if not on a concrete surface.
I believe the measurement is 13.25" on my coach to the Zerks.
I know some measure to the top of the rear wheel-wells. I believe this measurement can introduce some errors and uncertanties as well. That measurement is on the body, not to the frame. The body is sitting on rubber pads and the weights and distributions can change significantly with fresh and black water quantities and also propane load introducing some amounts of errors.
What say yeh?
Just my farm-boy mechanic way of seeing things.
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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| Re: Why do we still measure ride height to the frame slots? [message #359173 is a reply to message #359172] |
Sun, 04 October 2020 07:04  |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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Senior Member |
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Also, forgot to mention the tetter-totter effect. The rear slots are nearly 5 feet behind the bogies, which are the pivot point for the rear suspension. So if the front wheels are not level with the rear wheels, the rear measurement will be thrown out. Also, if you are doing a height adjustment, you do the back first then the front. If you had to adjust the fronts, then the rear needs to be adjusted again because the measurement has changed. If it was adjusted using an accepted measurement to the Zerks, the measurement should not change.
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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| Re: Why do we still measure ride height to the frame slots? [message #359177 is a reply to message #359173] |
Sun, 04 October 2020 08:14 |
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Richard RV
Messages: 631
Registered: July 2012
Location: Full-timer for 12 years, ...
Karma: -17
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Senior Member |
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Bruce,
I agree with you about measuring from the bogey box location for the reasons you mentioned, but the isolation pads aren't a short term factor and is a very low order variable.
Back of the phone calculation:
Worst case scenario of variable weight on the body of liquids/propane
~1000# (that's totally empty to totally full which rarely happens if the coach is in service)
Isolation pad area
6x3"x3" (pads on back half of coach only)
54 in2
So ~20psi delta from variable liquid loads
The ride height measurement spec is +/- 1/4" and I'd be surprised if the pads compressed more than 1/16" with the change in load from full to empty.
Measuring to the belt line won't throw off the validity.
Richard
'77 Birchaven TZE...777;
'76 Palm Beach under construction;
‘76 Edgemont waiting its turn
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| Re: Why do we still measure ride height to the frame slots? [message #359178 is a reply to message #359173] |
Sun, 04 October 2020 08:28 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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Senior Member |
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In my vast collection of nearly useless information is the fact that I am a distant relation to Christian von Nagel.
In case you don't recognize that name, he wrote an early 19th century treatise on the effects of experimental error. In this, he allowed that there are often unknown variables (like the front ride height) that, if can be maintained as stable (even if you don't know they exsist), will not effect the validity of the result. Unfortunately, this was in later years translated as "Finnagle" and intended to mean you can do what ever you want to get the answer that suits.
Why does this apply?
Because as long as Bruce measures his rear ride height the same way, his result will be valid. This is also very true as most coaches have the rear registry point hidden by the generator mount.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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