Why was GMC setup to ride low in the rear?

I have a theory on why the ride height is set low in the back - or rather, why it is high in the front.  It all starts with the radiator core support, or rather the lack of one.  One thing that core supports do that I don't think was well appreciated back then is block the air flow around the radiator.  The GMC doesn't have one, I expect because of the air compressor, steering, batteries and other stuff that would get in the way.  Then they found it tended to overheat.  What to do?  Easiest and cheapest (not necessarily the cheapest in the long run, but the cheapest that afternoon) thing to do was to put in a larger radiator.  Couldn't up or out, so they went down.  Oops, not enough ground clearance.  What to do?  Turn the height adjusters a few turns and magic - problem solved.

I don't like the looks of it set that high so I lowered the front about an inch.  I'll have to watch the curbs.  I don't buy that it had anything to do with ride or handling - there's just nothing in the front suspension that changes much with height.  Although the mischief the rear suspension gets into can be reduced by lowering the ride height in back.  Okay, there's an outside chance that they lowered the rear at the last minute to reduce rear wheel lock-up.

No one else might buy my theory, but I'm stickin' to it.  I was around Detroit  in those days and That would be how they would think.
Gary Casey
'73 23'

I recently adjusted my ride height to the factory specs, both front
and back. The rear air system always adjusts the rear height based
on load weight and it sets it dead nuts to spec.

The rear is a tad lower, but not much in my opinion. I know it was
designed that way for handling and steering, but I think the rear
being a little lower gives the coach a nice good soft look too.

Ray
 
Just wondered. I know trailers in Arkansas and Alabama are put up on
blocks. :>)

-----Original Message-----
From: Ray Erspamer
Reply-To:
Date: Fri, 19 Aug 2011 10:45:20 -0700 (PDT)
To:
Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?

>I "think" I have a motorhome.....
>
> Ray
>
>
>Ray & Lisa Erspamer
>78 Royale "Great Lakes Eagle"
>Center Kitchen TZE368V101144
>Wauwatosa, Wisconsin 53226
>Email: 78GMC-Royale
>414-745-3188
>Web Site: http://ray-lisa.page.tl/
>
>
>
>----- Original Message ----
>From: Byron Songer
>To: gmclist
>Sent: Fri, August 19, 2011 11:55:20 AM
>Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?
>
>Ray and Rob,
>
>Do you guys have trailers or motorhomes?
>
>Byron
>
>
>
>_______________________________________________
>GMCnet mailing list
>Unsubscribe or Change List Options:
>http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
>_______________________________________________
>GMCnet mailing list
>Unsubscribe or Change List Options:
>http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
 
IMO the reason the rear ride height on this and other GM vehicles of the era was set lower at the rear was to create an amount of stability under hard braking (anti dive).

With the vehicle level the front end dives under braking,transfering weight to the front wheels, but lessening weight on the rears, which would decrease stability and increase the possibility of the rear brakes locking (especially in wet conditions).

Lowering the rear in relationship to the front would decrease the amount of dive in the front under braking.
http://www.circletrack.com/techarticles/antidive_suspension_tech_parameters/index.html


--
1976 23' GMCII By Explorer
 
Front end dive is centered around the front spindles when the coach is under
braking. The GMC is set up to brake about 60 to 70 percent of the total
effort on the disc brakes on the front. Whether the rear is lower or higher
makes little difference. Weight transfer will occur a greater or lesser
amount partly depending on how much the front suspension compresses during
braking. Shocks and the strength of the torsion bars determine that rate,
not how high the rear is. The center of the front wheels is the center of
the forces.
Jim Hupy
Salem, OR
78 GMC Royale 403

>
>
> IMO the reason the rear ride height on this and other GM vehicles of the
> era was set lower at the rear was to create an amount of stability under
> hard braking (anti dive).
>
> With the vehicle level the front end dives under braking,transfering weight
> to the front wheels, but lessening weight on the rears, which would decrease
> stability and increase the possibility of the rear brakes locking
> (especially in wet conditions).
>
> Lowering the rear in relationship to the front would decrease the amount of
> dive in the front under braking.
>
> http://www.circletrack.com/techarticles/antidive_suspension_tech_parameters/index.html
>
>
> --
> 1976 23' GMCII By Explorer
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
 
Riding buddies taught Judy to ride while I was doing
a remote tour at a radar station in SE Iceland in 1977.
She's now 71 and I'm 73.

Mac

----------------------------------------
> Date: Fri, 19 Aug 2011 09:04:01 -0700
> From: 78gmc-royale
> To: gmclist
> Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?
>
> Nice Bikes Mac !!!
>
> Lisa doesn't want a bike of her own, she just likes to sit on the back and look
> at the scenery and take pictures. Works for me !
>
> Ray
>
>
> Ray & Lisa Erspamer
> 78 Royale "Great Lakes Eagle"
> Center Kitchen TZE368V101144
> Wauwatosa, Wisconsin 53226
> Email: 78GMC-Royale
> 414-745-3188
> Web Site: http://ray-lisa.page.tl/
>
>
>
> ----- Original Message ----
> From: D C *Mac* Macdonald
> To: gmclist
> Sent: Fri, August 19, 2011 10:44:56 AM
> Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?
>
>
> Just wondered. I ride a 2000 FLTRSEI 95 CID Screamin' Eagle Roadglide
> and wife Judy rides a 2007 FXDL 96 CID Dyna Low Rider.
>
> ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
> ~~ ~ D C "Mac" Macdonald ~ ~~
> ~ ~ Amateur Radio - K2GKK ~ ~
> ~ ~ USAF and FAA, Retired ~ ~
> ~ ~ ~ Oklahoma City, OK ~ ~ ~
> ~~ ~ ~ "The Money Pit" ~ ~ ~~
> ~~ ~ ~ (TZE166V101966) ~ ~ ~~
> ~ ~ ~ ex-Palm Beach, 76 ~ ~ ~
> ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
>
> www.gmcmhphotos.com/okclb
>
>
>
>
>
> > Date: Fri, 19 Aug 2011 05:47:55 -0700
> > From: 78gmc-royale
> > To: gmclist
> > Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?
> >
> > Nope, not a screaming eagle in the trailer, just a standard Ultra Classic. We
> > just like eagles and we liked the logo.
> >
> > Ray
> >
> > Ray & Lisa Erspamer
> > 78 Royale "Great Lakes Eagle"
> > Center Kitchen TZE368V101144
> > Wauwatosa, Wisconsin 53226
> > Email: 78GMC-Royale
> > 414-745-3188
> > Web Site: http://ray-lisa.page.tl/
> >
> >
> >
> >
> > ----- Original Message ----
> > From: D C *Mac* Macdonald
> > To: gmclist
> > Sent: Thu, August 18, 2011 11:34:04 PM
> > Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?
> >
> > I like the color scheme. I have one question, though.
> >
> > Is there actually a Screamin' Eagle Harley in the trailer
> > that has what appears to be a dead ringer for Harley's
> > Screamin' Eagle logo/insignia?
> >
> > It surely looks very much like what is on each side of
> > the fuel tank on _MY_ Harley!
> >
> > ~~ ~ D C "Mac" Macdonald ~ ~~
> > ~~ ~ ~ "The Money Pit" ~ ~ ~~
> > ~~ ~ ~ (TZE166V101966) ~ ~ ~~
> >
> >
> >
> >
> > From: Ray Erspamer
> > Reply-To:
> > Date: Thu, 18 Aug 2011 07:15:10 -0700 (PDT)
> > To:
> > Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?
> >
> > I recently adjusted my ride height to the factory specs, both front
> > and back. The rear air system always adjusts the rear height based
> > on load weight and it sets it dead nuts to spec.
> >
> > The rear is a tad lower, but not much in my opinion. I know it was
> > designed that way for handling and steering, but I think the rear
> > being a little lower gives the coach a nice good soft look too.
> >
> > Pix link follows.
> >
> > http://tinyurl.com/3rwm3oe
> >
> > Ray
> >
> > Ray & Lisa Erspamer
> > 78 Royale "Great Lakes Eagle"
> > Center Kitchen TZE368V101144
> > Wauwatosa, Wisconsin 53226
> > Email: 78GMC-Royale
> > 414-745-3188
> > Web Site: http://ray-lisa.page.tl/
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
 
No, mine is definitely UP ON BLOCKS. I built a set of these wood jack stands
designed by Steve Southworth. If you ever want to feel totally comfortable
working under your coach, this is the way to go. The coach frame bites into the
wood, no chance of slipping at all and solid as a rock. Steve tested these in a
press and they held up to 12 ton. I highly recommend these, cost me $14 to make
them Wednesday night..

http://tinyurl.com/44agr2u

Ray

Ray & Lisa Erspamer
78 Royale "Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC-Royale
414-745-3188
Web Site: http://ray-lisa.page.tl/

----- Original Message ----
From: Byron Songer
To: gmclist
Sent: Fri, August 19, 2011 4:17:50 PM
Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?

Just wondered. I know trailers in Arkansas and Alabama are put up on
blocks. :>)

-----Original Message-----
From: Ray Erspamer
Reply-To:
Date: Fri, 19 Aug 2011 10:45:20 -0700 (PDT)
To:
Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?

>I "think" I have a motorhome.....
>
> Ray
>
>
>Ray & Lisa Erspamer
>78 Royale "Great Lakes Eagle"
>Center Kitchen TZE368V101144
>Wauwatosa, Wisconsin 53226
>Email: 78GMC-Royale
>414-745-3188
>Web Site: http://ray-lisa.page.tl/
>
>
>
>----- Original Message ----
>From: Byron Songer
>To: gmclist
>Sent: Fri, August 19, 2011 11:55:20 AM
>Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?
>
>Ray and Rob,
>
>Do you guys have trailers or motorhomes?
>
>Byron
>
>
>
>_______________________________________________
>GMCnet mailing list
>Unsubscribe or Change List Options:
>http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
>_______________________________________________
>GMCnet mailing list
>Unsubscribe or Change List Options:
>http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist

_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
 
Bruce,

While it is a fun thing to think through issues like ride height and all, I will tell you where the rubber meats the road is where theories are proven or busted and I will tell you without a doubt that from ride height-- it makes a noticable difference in steerability and the feel of the wheel when you raise the rear.  Absolutlely and without a doubt.  When we align a coach, we use the "proper ride height" at set up the specs at that point, when lifted, the steering stiffens up, startes to hunt a bit  and when turning you find it easier to make tight turns.  This is not theory, it is true.

I still feel, based on real world driving, that "travel" mode is for highway driving and lifting the rear makes to coach look better and aids in tight cornering.  It seems very logical to use the suspension for this expressed purpose so I stand behind this explaination.  Opinions vary and I guess thats good but try it, lift the rear & I think you will find in town driving better then hit a highway in that condition and press travel, as the rear drops you will feel a marked straight ahead steering pickup.

Jim Bounds
-------------------------

----- Original Message -----
From: Bruce Hislop
To: gmclist
Cc:
Sent: Thursday, August 18, 2011 9:31 AM
Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?

Rob,
I've ponder this before as well.

- I also don't think it has anything to do with caster.  Thanks for measuring the distance from the center of the bogie to the rear height adjust. That means the radius from the front axle is actually 160+56 = 216" so ~ 1/4° per inch of ride height adjustment.

- Gas tank... The gas tanks sit side by side so the 1 7/16" difference over 216" isn't going to amount to much over the say 48" between the tanks (3/10") inch.

- Weight radio... Again 1 7/16" difference isn't going to shift any meaning amount of weight.

- Angle of attack??  I know AoA as it applies to airplanes but I don't see where this would make any difference on the GMC.

One thing I was wondering about was the position of the rear bogie arms with relationship to the bogie arm pivot point.  Raising the rear would put the pivot point above the bogie arm axle. The forces on the front bogie arm would cause it to tend to lift the back just as in braking?  especially goving over potholes... Maybe.. maybe not...

Or maybe they thought it looked cool...

or maybe someone made an error in the specs and no one questioned it!

--
Bruce Hislop,
S. Ontario Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI and ESC.
Hubler front end installed and WOW! !:)
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
 
> I think you missed one. This allow two BIG people to sit up front and the coach will level out. If they sit in the rear the airbags automatically adjust. If they sit up front front the coach rides a little lower.


Not only the weight differential due to occupants, but also fuel and other items loaded into the coach. I think loaded vs unloaded weight differential is as likely a reason as any for the nose high attitude in the specifications.

As far as aesthetics, I couldn't care less what is deemed to conventionaly "look better", I only car what is most appropriate and most functional for THIS particular vehicle. And it's not like there have not been other precedents for this type of specification. Many car designs have one end or the other set high to take into account potential loads in the trunk and passenger areas (mostly lighter cars where loaded weight differentials can make a more significant change in the ride height).
--
Pete Smay
Long Beach, CA
1977 Kingsley
 
Pete,

I'm sorry but there's one problem with your theory, as Ken noted fuel, water, and LPG are all in the rear of the coach and the
automatic ride height will compensate for it.

Regards,
Rob M.

-----Original Message-----
From: Pete

> I think you missed one. This allow two BIG people to sit up front and the coach will level out. If they sit in the rear the
airbags automatically adjust. If they sit up front front the coach rides a little lower.

Not only the weight differential due to occupants, but also fuel and other items loaded into the coach. I think loaded vs unloaded
weight differential is as likely a reason as any for the nose high attitude in the specifications.
--
Pete
 
> Pete,
>
> I'm sorry but there's one problem with your theory, as Ken noted fuel, water, and LPG are all in the rear of the coach and the
> automatic ride height will compensate for it.
>
> Regards,
> Rob M.


Fuel is located ahead of the rear wheels, the variable weight of which also contributing somewhat to the loading on the front suspension (as the rear self-levels). Passengers, supplies, carryons, etc. are usually located ahead of the rear wheels, and likewise would variably load the front suspension while the rear self-levels.

So, I still believe this is why the non-leveling front suspension is set a little higher to account for additional variable load on it.
--
Pete Smay
Long Beach, CA
1977 Kingsley
 
Pete,

You are correct - somewhat and I stand corrected - somewhat.

Below is a photograph of the gas tank locations vis-à-vis the rear suspension.

It appears that the center of the rear tank is inline with the middle axle and the front tank is in front of that.

http://en.wikipedia.org/wiki/File:GMC_Motorhome_Chassis.jpg

Therefore you are correct in noting that the fuel weight is supported somewhat by the front axles.

As noted on other occasions my math skills are no longer what they were and I don't have a snowball's chance in hell calculating the
percentage of fuel weight carried by the front / rear suspension. :-)

Regards,
Rob M.

-----Original Message-----
From: gmclist-bounces [mailto:gmclist-bounces] On Behalf Of Pete
Sent: Friday, 26 August 2011 6:20 AM
To: gmclist
Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?

> Pete,
>
> I'm sorry but there's one problem with your theory, as Ken noted fuel, water, and LPG are all in the rear of the coach and the
> automatic ride height will compensate for it.
>
> Regards,
> Rob M.

Fuel is located ahead of the rear wheels, the variable weight of which also contributing somewhat to the loading on the front
suspension (as the rear self-levels). Passengers, supplies, carryons, etc. are usually located ahead of the rear wheels, and
likewise would variably load the front suspension while the rear self-levels.

So, I still believe this is why the non-leveling front suspension is set a little higher to account for additional variable load on
it.
--
Pete Smay
Long Beach, CA
1977 Kingsley
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
 
> ...
> It appears that the center of the rear tank is inline with the middle axle and the front tank is in front of that.
>
> http://en.wikipedia.org/wiki/File:GMC_Motorhome_Chassis.jpg
>
> Therefore you are correct in noting that the fuel weight is supported somewhat by the front axles. ...


Just keep in mind that the chassis pictured is of a 23 foot coach. A 26 foot coach would have about 2 feet more space between the gas tanks and the mufflers.

I am not sure how much it would effect how much of the fuel weight was on what axle....

But I would guess that more of the fuel would be carried by the rear suspension.... as compared to a 23 foot coach.
--
Mike Miller -- Hillsboro, OR -- on the Black list
(#1)'73 26' exPainted D. -- (#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
http://m000035.blogspot.com
 
I believe Pete may have the best answer.

Fresh and black water are the only really heavy variables behind the rear axle that would remove weight from the front, however like fuel they are close to the bogie mount which is the pivot point for the rear weight.

The GMC was advertized as a cruising vehicle to "see America" accommodating upto 6 people. The people plus provisions would be mostly at the front so while the back would stay at a constant height the front would be weighing down.

With the one-ton kit installed and wheels set out further, I find my front end suspension less stiff, even though I had spacers before. Also the much improved front braking results in a bit more front end dive. I'll also get a bit more front suspension "give" with weight due to the wider wheel spacing.

I still love my one ton though!


--
Bruce Hislop,
S. Ontario Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI and ESC.
Hubler front end installed and WOW! !:)
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
 
Mike,

By George, you're right! I thought the mufflers were a bit close to the tanks but it NEVER dawned on me that it was a '23!

I agree the added distance from the front suspension would mean more of the weight would be carried by the rear suspension on a '26.

Regards,
Rob M.

-----Original Message-----
From: Of Mike Miller

Just keep in mind that the chassis pictured is of a 23 foot coach. A 26 foot coach would have about 2 feet more space between the
gas tanks and the mufflers.

I am not sure how much it would effect how much of the fuel weight was on what axle....

But I would guess that more of the fuel would be carried by the rear suspension.... as compared to a 23 foot coach.
--
Mike
 
Bruce,

The myriad of responses has been great Brainstorming.

I will summarize all the ideas and publish them when I get the measurements that confirm the 2" front to rear beltline trim
calculated drop.

I reckon that it was done for a number of reasons not just one.

Regards,
Rob M.

-----Original Message-----
From: Bruce Hislop

I believe Pete may have the best answer.

Fresh and black water are the only really heavy variables behind the rear axle that would remove weight from the front, however like
fuel they are close to the bogie mount which is the pivot point for the rear weight.

The GMC was advertized as a cruising vehicle to "see America" accommodating up to 6 people. The people plus provisions would be
mostly at the front so while the back would stay at a constant height the front would be weighing down.

With the one-ton kit installed and wheels set out further, I find my front end suspension less stiff, even though I had spacers
before. Also the much improved front braking results in a bit more front end dive. I'll also get a bit more front suspension "give"
with weight due to the wider wheel spacing.

I still love my one ton though!

Bruce
 
Naw, here's the real deal.

Early in its development one of the GMC slide rule guys determined that a coach with a positive angle of incidence would create lift as it moved through the air. This would have two positive results. It would reduce the effective weight of the coach which would have a positive impact on fuel economy.

How well does it work? I'll bet Ken henderson could do the math.

Now you know.

Glenn
 
Rob,

Good on you, Mate!

Byron

-----Original Message-----
From: Rob Mueller
Reply-To:
Date: Sat, 27 Aug 2011 00:30:15 +1000
To:
Subject: Re: [GMCnet] Why was GMC setup to ride low in the rear?

>Bruce,
>
>The myriad of responses has been great Brainstorming.
>
>I will summarize all the ideas and publish them when I get the
>measurements that confirm the 2" front to rear beltline trim
>calculated drop.
>
>I reckon that it was done for a number of reasons not just one.
>
>Regards,
>Rob M.
>
>-----Original Message-----
>From: Bruce Hislop
>
>I believe Pete may have the best answer.
>
>Fresh and black water are the only really heavy variables behind the rear
>axle that would remove weight from the front, however like
>fuel they are close to the bogie mount which is the pivot point for the
>rear weight.
>
>The GMC was advertized as a cruising vehicle to "see America"
>accommodating up to 6 people. The people plus provisions would be
>mostly at the front so while the back would stay at a constant height the
>front would be weighing down.
>
>With the one-ton kit installed and wheels set out further, I find my
>front end suspension less stiff, even though I had spacers
>before. Also the much improved front braking results in a bit more front
>end dive. I'll also get a bit more front suspension "give"
>with weight due to the wider wheel spacing.
>
>I still love my one ton though!
>
>Bruce
>
>_______________________________________________
>GMCnet mailing list
>Unsubscribe or Change List Options:
>http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist