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| Why is the front higher than the rear on a GMC? [message #87673] |
Mon, 07 June 2010 14:10  |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
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I was reading the "Clutch fan experiment" thread and Gary Casey mentioned he lowered his front end about 1" to make the coach more level.
Does anyone know the reason why GM spec'd the front raised higher than the back? There must be a reason for it.
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: [GMCnet] Why is the front higher than the rear on a GMC? [message #87674 is a reply to message #87673] |
Mon, 07 June 2010 14:16  |
jimk
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Registered: July 2006
Location: Belmont, CA
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If nothing else,I have been to seminars since 1980 and they all told
us that they cannot suggest a different height that will make it
perform any better.
On Mon, Jun 7, 2010 at 12:10 PM, Bruce Hislop <bruce@perthcomm.com> wrote:
>
>
> I was reading the "Clutch fan experiment" thread and Gary Casey mentioned he lowered his front end about 1" to make the coach more level.
>
> Does anyone know the reason why GM spec'd the front raised higher than the back? There must be a reason for it.
>
> --
> Bruce Hislop,
> S. Ontario Canada
> 77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI and ESC.
> http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
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Jim Kanomata
Applied/GMC, Fremont,CA
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| Re: [GMCnet] Why is the front higher than the rear on a GMC? [message #87684 is a reply to message #87682] |
Mon, 07 June 2010 16:24 |
midlf
Messages: 2212
Registered: July 2007
Location: SE Wisc. (Palmyra)
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| Robert Mueller wrote on Mon, 07 June 2010 15:57 |
Emery,
Ken Frey sets the front end ride height by setting the axles level.
Regards,
Rob Mueller
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But then how does he set the rear ride height? Does he just set it to factory spec or does he adjust the rear to maintain the same differential from the front as factory spec. To explain: If adjusting the front axle to level gives a ride height spec say 1/2" (or whatever) lower than factory front spec, does he then set the rear also 1/2" lower or does he still set the rear to the specified dimension. I ask this because of the many many comments about proper ride height being so important to steering and handling.
Steve Southworth
1974 Glacier TZE064V100150 (for workin on)
1975 Transmode TZE365V100394 (parts & spares)
Palmyra WI
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| Re: [GMCnet] Why is the front higher than the rear on a GMC? [message #87688 is a reply to message #87684] |
Mon, 07 June 2010 16:56 |
Ken Coit
Messages: 151
Registered: November 2005
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I swear I read somewhere that the optimum "angle of attack" had to do with
the steering geometry and that if it wasn't set to "spec." that would cause
handling problems, especially in ruts. Did I dream this? I think it might
have been a presentation on travel height setting or torsion bar
adjustments.
On Mon, Jun 7, 2010 at 5:24 PM, Steve Southworth <midlf@centurytel.net>wrote:
>
>
> Robert Mueller wrote on Mon, 07 June 2010 15:57
> > Emery,
> >
> > Ken Frey sets the front end ride height by setting the axles level.
> >
> > Regards,
> > Rob Mueller
>
>
> But then how does he set the rear ride height? Does he just set it to
> factory spec or does he adjust the rear to maintain the same differential
> from the front as factory spec. To explain: If adjusting the front axle to
> level gives a ride height spec say 1/2" (or whatever) lower than factory
> front spec, does he then set the rear also 1/2" lower or does he still set
> the rear to the specified dimension. I ask this because of the many many
> comments about proper ride height being so important to steering and
> handling.
>
> --
> Steve Southworth
> 1974 Glacier TZE064V100150
> Palmyra WI
> _______________________________________________
> GMCnet mailing list
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Ken Coit, ND7N
Raleigh, NC
Parfait Royale
1978 Royale Rear Bath, 403, 3.07
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| Re: [GMCnet] Why is the front higher than the rear on a GMC? [message #87693 is a reply to message #87688] |
Mon, 07 June 2010 17:30 |
klassic kampers
Messages: 93
Registered: July 2008
Location: greer,s.c./ellijay,ga
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my answer would be "caster"...........the lower the rear is, the more positive caster you will have.......the more positive caster you have, the more stable the vehicle is when driving in a straight line......this is true up to a point and if you get too much positive caster, then when you make a slow turn you get "caster bind".........this is the experience and opinion of one person and should be taken as that.........good luck.. .....
Mike Stewart
1973 GMC 26' Canyonlands /
1973 B.S.A. B50 street tracker-----
Greer,S.C/Ellijay,Ga
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| Re: [GMCnet] Why is the front higher than the rear on a GMC? [message #87697 is a reply to message #87693] |
Mon, 07 June 2010 18:06 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
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| klassic kampers wrote on Mon, 07 June 2010 17:30 |
my answer would be "caster"...........the lower the rear is, the more positive caster you will have.......the more positive caster you have, the more stable the vehicle is when driving in a straight line......this is true up to a point and if you get too much positive caster, then when you make a slow turn you get "caster bind".........this is the experience and opinion of one person and should be taken as that.........good luck.. .....
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i have run across at least two guys that had no working level valves in the rear and their coach set high in the back and they said it didn't handle well. i suggested dropping the rear manually to the GM specs and it made a great difference in handling in both cases.
we did not look at the front axles but i think that if the front is set to make the axles level then the rear should be set lower by the same difference as in the GM spec.
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: Why is the front higher than the rear on a GMC? [message #87710 is a reply to message #87673] |
Mon, 07 June 2010 19:42 |
kingd
Messages: 592
Registered: June 2004
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As has been stated, if the rear is lower than the front there is more caster than if the rear was raised. BUT.... Suppose one had a coach that drove GREAT, but didn't like the nose high attitude.
If the axles weren't straight(I think this puts the least wear on the CVs)adjust the front till the axles are straight, then adjust the rear to get the "strance" that to you looks good, then and I guess this is the hard part, do whatever is necessary to get the caster back to where it was before any changes.
Report to the forum the results so everyone doesn't repeat something that MAY NOT work.
DAVE KING
Toronto(wait till I win the $50 million(tax free) Canadian LOTTO MAX !!!!)
DAVE KING
lurker, wannabe
Toronto, Ontario, Canada
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| Re: Why is the front higher than the rear on a GMC? [message #87715 is a reply to message #87710] |
Mon, 07 June 2010 20:18 |
C Boyd
Messages: 2629
Registered: April 2006
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| kingd wrote on Mon, 07 June 2010 20:42 |
As has been stated, if the rear is lower than the front there is more caster than if the rear was raised. BUT.... Suppose one had a coach that drove GREAT, but didn't like the nose high attitude.
If the axles weren't straight(I think this puts the least wear on the CVs)adjust the front till the axles are straight, then adjust the rear to get the "strance" that to you looks good, then and I guess this is the hard part, do whatever is necessary to get the caster back to where it was before any changes.
Report to the forum the results so everyone doesn't repeat something that MAY NOT work.
DAVE KING
Toronto(wait till I win the $50 million(tax free) Canadian LOTTO MAX !!!!)
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At Bean Station Ken Burton set the ride height on my Crestmont, there were scales under the front wheels. By raising my right rear 1/4" it added 50lb to the left front. Using this as a base, if I raised both sides 1.6" to level the coach, in theory it could add 700lb to the steering/drive axle.
C. Boyd
76 Crestmont
East Tennessee
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| Re: Why is the front higher than the rear on a GMC? [message #87716 is a reply to message #87673] |
Mon, 07 June 2010 20:32 |
kingd
Messages: 592
Registered: June 2004
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Raising the ride height on 1 rear corner will transfer weight to the opposite side on the front,(cross weighting) however if both rears are raised the same amount the change in weight on the front is probably negligible, yeah I know the CG would move a little to the front so in theory there would be would be more weight on the front but I would expect it to be very slight. However if the coach is stood vertically on the nose, ALL the weight is on the front.
DAVE KING
DAVE KING
lurker, wannabe
Toronto, Ontario, Canada
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| Re: Why is the front higher than the rear on a GMC? [message #87724 is a reply to message #87719] |
Mon, 07 June 2010 21:42 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
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| RF_Burns wrote on Mon, 07 June 2010 20:44 |
I don't buy the caster theory. If I could remember my trig from high school, I'd bet raising or lowering the back or front by an inch or so would affect the caster only by a small fraction of a degree. Remember there is about a 14ft lever arm between the front and back wheels.
160x2x3.14 = 1004.8 / 360 = 2.79" per degree of caster. Therefore by my calculations you would need to raise the rear by 2.8" to affect a 1 degree change in caster, or 1" = 0.35 degree change.
Straight front axles would make sense
What say yea?
|
when you start with 2 1/2 deg. of caster and add .35 to it that is a good size percentage. i would think a noticeable difference.
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: Why is the front higher than the rear on a GMC? [message #87732 is a reply to message #87724] |
Mon, 07 June 2010 23:05 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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| fred v wrote on Mon, 07 June 2010 21:42 |
| RF_Burns wrote on Mon, 07 June 2010 20:44 |
I don't buy the caster theory. If I could remember my trig from high school, I'd bet raising or lowering the back or front by an inch or so would affect the caster only by a small fraction of a degree. Remember there is about a 14ft lever arm between the front and back wheels.
160x2x3.14 = 1004.8 / 360 = 2.79" per degree of caster. Therefore by my calculations you would need to raise the rear by 2.8" to affect a 1 degree change in caster, or 1" = 0.35 degree change.
Straight front axles would make sense
What say yea?
|
when you start with 2 1/2 deg. of caster and add .35 to it that is a good size percentage. i would think a noticeable difference.
|
I have been playing a lot with ride heights and weight applied to the front wheels on my coach since I returned from the Bean Station and Hamilton rallies. I have found there is a a great deal of interaction between the rear height and the front heights. Also between there is a lot of interaction between the rear heights and the weight applied to the individual front wheels. I was just looking at it again tonight.
I have not fully figured out all I think I know but some of the observations I have made are:
1. A 1/4" inch difference in rear ride height on one wheel will apply (or remove) 150 pounds of weight on the diagonally opposite front wheel.
2. A 1" low setting on both rears will raise the front ride heights by about 1/2".
3. My coach shows a 3/8" inch difference (left to right) between the wheel well cut out and the frame. I have not checked the front for the same difference yet.
4. Adjusting the rear sensor statically while parked gets you in the ball park but is not totally accurate for running down the road. Every time I did it I found that the height would be too high after driving in auto for 5 miles or so. I resorted to setting them, driving it, reading the height, and readjusting them again until I got it right on the money going down the road.
My only conclusions so far:
1. You must have the rear ride heights set exactly correct before trying to adjust the fronts.
2. There is enough height variation in the rear switches that you must set them by driving the coach between each measurement.
3. When adjusting the fronts you must rest the rear coach on a couple of blocks in the rear to prevent them from moving up and down during the front torsion bar adjustment. (sorry Colonel) 50 or 100 pounds of downward force is all that is needed to keep it rigid. On every GMC (both 23 and 26) that we did this way at Hamilton and BS we found after adjusting the fronts by height that the weight applied to our scales was fairly close to equal. Further adjusting the front to balance the weights did not move the ride heights at all.
I do not know how anyone can expect to get the fronts anywhere near specs with out the leveling switches working or without blocking the rear while adjusting the front.
More to come
Ken B.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Why is the front higher than the rear on a GMC? [message #87736 is a reply to message #87734] |
Mon, 07 June 2010 23:14 |
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ljdavick
Messages: 3548
Registered: March 2007
Location: Fremont, CA
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I resemble that!
Larry Davick
On Jun 7, 2010, at 9:10 PM, crsalert@frontiernet.net wrote:
> Actually it is quite technical. They discussed it for weeks on end in the GM testing grounds and engineering studios.
>
> The final conclusion was the front needed a higher ride height to compensate for the fat old men driving them.
>
> If you also notice the driver side is a tad higher since the wives hopefully weighed less.
>
> See they were thinking wsy ahead of the curve.
>
> Gospel truth. And I have some prime beach front property on the Gulf to sell you. I'll even throw in the crude rights.
>
> Marcus
>
> ----- Original Message -----
> From: "fred veenschoten" <fredntoni@cox.net>
> To: gmclist@temp.gmcnet.org
> Sent: Monday, June 7, 2010 10:42:54 PM
> Subject: Re: [GMCnet] Why is the front higher than the rear on a GMC?
>
>
>
> RF_Burns wrote on Mon, 07 June 2010 20:44
>> I don't buy the caster theory. If I could remember my trig from high school, I'd bet raising or lowering the back or front by an inch or so would affect the caster only by a small fraction of a degree. Remember there is about a 14ft lever arm between the front and back wheels.
>>
>> 160x2x3.14 = 1004.8 / 360 = 2.79" per degree of caster. Therefore by my calculations you would need to raise the rear by 2.8" to affect a 1 degree change in caster, or 1" = 0.35 degree change.
>>
>> Straight front axles would make sense
>>
>> What say yea?
>
> when you start with 2 1/2 deg. of caster and add .35 to it that is a good size percentage. i would think a noticeable difference.
>
> --
> Fred V
> '77 Royale RB 455
> P'cola, Fl
> _______________________________________________
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Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, Ca
Howell EFI + EBL + Electronic Dizzy
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| Re: [GMCnet] Why is the front higher than the rear on a GMC? [message #87745 is a reply to message #87736] |
Tue, 08 June 2010 06:38 |
Jim Bounds
Messages: 842
Registered: January 2004
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Have not had a chance to read all of this thread but I will tell you what I have found why the front is higher at "ride height" which is BTW not where I think the coach should be set when it is in town driving around.
Sorry but it is true, you can feel a marked difference driving the coach on the highway in travel or in full raise. The rolled back caster you get when in ride height really does make straight ahead driving on a highway easier. In town the rolling forward of the coach does allow you to turn that much tighter and the coach is more responsive in turns.
Besides, where it was designed in the late 60's, jacking the back up was most popular-- I have shackles on my 65 Chey Belair! GM would not have designed it butt down, the roacker panels would have been set up at least level to the road if not a little high in the back! They would have not disreguarded the civil engineer formula for speed bumps and curbs to make the back of the coach in danger. No, I think the designers figured someone would crank the rear up in town to be safe then ONLY put the coach in ride height when cruising on the highway. I think the explaination of use came more from the attorneys than from the design team. It is a fact that there is better steering control with the butt lower but in town you have far more benifit with the rear up-- besides, I think the coach looks better that way.
Speculate on this all you want but the fact is when you showe off your coach, people ask whats wrong with it when the butt is lower. Then you explain "ride height" they listen but still think it looks odd. Come on, they would not have dressed their premier motorhome project funny by making the coach look like it's "packing"!
The air ride system on the GMC is an active driving aid and should be used as such-- remember, if you don;t use it you loose it! Try it, jack the coach up to at least level if not up a bit in the back, go drive in town and see if you don;t notice a difference. then go park the coach and see if folks don;t give you more compliments with it in that position-- they have with mine!
Jim Bounds
------------------------
----- Original Message ----
From: Larry Davick <ljdavick@comcast.net>
To: "gmclist@temp.gmcnet.org" <gmclist@temp.gmcnet.org>
Sent: Tue, June 8, 2010 12:14:37 AM
Subject: Re: [GMCnet] Why is the front higher than the rear on a GMC?
I resemble that!
Larry Davick
On Jun 7, 2010, at 9:10 PM, crsalert@frontiernet.net wrote:
> Actually it is quite technical. They discussed it for weeks on end in the GM testing grounds and engineering studios.
>
> The final conclusion was the front needed a higher ride height to compensate for the fat old men driving them.
>
> If you also notice the driver side is a tad higher since the wives hopefully weighed less.
>
> See they were thinking wsy ahead of the curve.
>
> Gospel truth. And I have some prime beach front property on the Gulf to sell you. I'll even throw in the crude rights.
>
> Marcus
>
> ----- Original Message -----
> From: "fred veenschoten" <fredntoni@cox.net>
> To: gmclist@temp.gmcnet.org
> Sent: Monday, June 7, 2010 10:42:54 PM
> Subject: Re: [GMCnet] Why is the front higher than the rear on a GMC?
>
>
>
> RF_Burns wrote on Mon, 07 June 2010 20:44
>> I don't buy the caster theory. If I could remember my trig from high school, I'd bet raising or lowering the back or front by an inch or so would affect the caster only by a small fraction of a degree. Remember there is about a 14ft lever arm between the front and back wheels.
>>
>> 160x2x3.14 = 1004.8 / 360 = 2.79" per degree of caster. Therefore by my calculations you would need to raise the rear by 2.8" to affect a 1 degree change in caster, or 1" = 0.35 degree change.
>>
>> Straight front axles would make sense
>>
>> What say yea?
>
> when you start with 2 1/2 deg. of caster and add .35 to it that is a good size percentage. i would think a noticeable difference.
>
> --
> Fred V
> '77 Royale RB 455
> P'cola, Fl
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
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_______________________________________________
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