Ride height

Caster is increased by moving the upper control arm rear adjuster closer to
the frame of the coach. If the maximum adjustment has been used, then an
eccentric or offset bushing can be fitted into the rearmost arm of the
upper control arm.
Jim Hupy
Salem, OR
78 GMC Royale 403

On Wed, Sep 7, 2016 at 2:48 PM, D C _Mac_ Macdonald
wrote:

> Doesn't a lower rear increase caster which makes for more stable
> "tracking" of the coach?
>
> Mac Macdonald in OKC
> '76 ex P.B.
>
> Sent from my iPad
>

> >
> > I also use 2x2 cut to length and measure from ground to bottom of frame.
> >
> > On the 76 Eleganza the problem is the front height. it is too high by at
> least an inch on both sides- more so no the driver side.
> >
> > The adjustment screws on the pork chop are as LOW as they will go. NO
> further lowering adjustment available.
> >
> > How can I get the front lower?
> >
> > I believe the torsion bars and pork chops came with the coach from the
> factory.
> >
> > The dinette and seats as wells as the 2 original overhead cabinets are
> out of the coach so it may be 200-300 pounds light in front.
> > I wouldn't think that would make much difference.
> >
> > I guess I could load a couple of sacks of feed in the front and see if
> it makes a difference.
> >
> > Any suggestions?
> >
> >
> > --
> > Gene Barrow
> > Lake Almanor, Ca.
> > 1976 Palm Beach
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
At every second or third GMCMI rally I drag my scales out and we weigh coaches for anyone that is interested. I have doen the last 2 and might bring
them to Mansfield. We have done the same at Bean Station, Hamilton, and some Eastern States rallys. As part of that operation we measure ride
heights. I pre-cut sets have sets of 4 blocks that are cut with the correct rear and front ride heights. I make the rear blocks out of scrap 2x4s
and the fronts out of scrap 1x2' or 2x2's. This size difference accomplishes 2 things.

1. It keeps the volunteers and me from confusing what blocks go to the front or rear.

2. The 2x4's can be used to rest the rear frame on while adjusting the front ride heights. By resting the frame on the 2x4's there will be no
movement due to weight changes as the front bars are being adjusted. At several rallys I have taken a bunch of extra extra blocks cut out out of
scraps and handed them out to anyone that wanted them. No matter how many I bring they are always all gone by the time we finish weighing coaches.

The blocks are a simple and easy to use and it take less than a minute to measure each coach before pulling them on to the scales.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Jim H. posted about our experience measuring ride heigh of coaches entering=
the Casa de Fruita rally, =E2=80=9C=E2=80=A6...There was only one coach me=
asured upon arrival that was correct according to factory specs. It was m=
y own coach, a 78 Royale with a 403 and all stock suspension, no extra batt=
eries or toads or carriers on the rear. Two people aboard without a lot of =
spare parts=E2=80=A6.=E2=80=9D Mine was, too, Jim (grin). See you thi=
s weekend. For all of you who want to free lance ride height and think=
only the relative ride height is important, I would respectfully disagree.=
There is far more to chassis design than just absolute ride height, but i=
t all starts there. Anti-dive geometry, bump steer and lots of other thing=
s are also affected if you monkey around with the factory designed absolute=
ride height. The wild card here is that our coaches were designed around =
bias ply tires where the tire goes egg shaped at speed and the contact patc=
h moves back producing more caster at speed than at rest. The design and s=
pecs were never changed even when the factory started also offering radial =
ply tires. We all now run radial ply tires where the contact patch does no=
t move at speed so we need to dial in as much caster as we can get (not a l=
ot given the limited adjustment range available) just to get enough caster =
at speed to go down the road without wandering. If you fiddle around with =
the ride height you are also changing everything else and most likely will =
make your coach handle less well rather than better. Yes, while going down=
the road if you want more caster lower the rear a bit and most will find t=
hat helps at speed but hurts a lot at low speed and at rest as it may over =
stresses the steering components. My suggestion is to follow proper pr=
ocedure to block the rear at the correct factory ride height as the rear wi=
ll effect the front. Then set the front to factory ride height after you d=
rive the coach a few miles to settle the suspension properly. Recheck the =
front again with the rear set to the correct ride height and repeat until b=
oth are correct. Then and only then can you begin adjusting camber, caster=
and toe on both the front and the rear tires. Jerry Jerry Work Th=
e Dovetail Joint Fine furniture designed and hand crafted in the 1907 for=
mer Masonic Temple building in historic Kerby, OR glwork ht=
tp://jerrywork.com
 
Jerry - FWIW, the ride height tool that is part of your front end alignment=
set is terrific. I cut a hole on the bracket for the generator so I c=
an check both sides for the back. Dolph DE N8JPC Wheeling=
, West Virginia 1977 26=E2=80=99 ex-PalmBeach 1-Ton, Sullybilt Bags,=
Reaction Arms, 3.70 LSD, Manny Transmission, EV-6010 =E2=80=9CThe Alu=
minum and Fiberglass Mistress" > On Sep 7, 2016, at 9:27 P=
M, Gerald Work wrote: > > Jim H. posted about our exp=
erience measuring ride heigh of coaches entering the Casa de Fruita rally, =
=E2=80=9C=E2=80=A6...There was only one coach measured upon arrival that wa=
s correct according > to factory specs. It was my own coach, a 78 Royale =
with a 403 and all stock suspension, no extra batteries or toads or carrier=
s on the rear. Two people aboard without a lot of spare parts=E2=80=A6.=
=E2=80=9D > > Mine was, too, Jim (grin). See you this weekend. > =
> For all of you who want to free lance ride height and think only the r=
elative ride height is important, I would respectfully disagree. There is =
far more to chassis design than just absolute ride height, but it all start=
s there. Anti-dive geometry, bump steer and lots of other things are also =
affected if you monkey around with the factory designed absolute ride heigh=
t. The wild card here is that our coaches were designed around bias ply ti=
res where the tire goes egg shaped at speed and the contact patch moves bac=
k producing more caster at speed than at rest. The design and specs were n=
ever changed even when the factory started also offering radial ply tires. =
We all now run radial ply tires where the contact patch does not move at s=
peed so we need to dial in as much caster as we can get (not a lot given th=
e limited adjustment range available) just to get enough caster at speed to=
go down the road without wandering. If you fiddle around with the ride he=
ight you are also changing everything else and most likely will make your c=
oach handle less well rather than better. Yes, while going down the road i=
f you want more caster lower the rear a bit and most will find that helps a=
t speed but hurts a lot at low speed and at rest as it may over stresses th=
e steering components. > > My suggestion is to follow proper procedure=
to block the rear at the correct factory ride height as the rear will effe=
ct the front. Then set the front to factory ride height after you drive th=
e coach a few miles to settle the suspension properly. Recheck the front a=
gain with the rear set to the correct ride height and repeat until both are=
correct. Then and only then can you begin adjusting camber, caster and to=
e on both the front and the rear tires. > > Jerry > Jerry Work > T=
he Dovetail Joint > Fine furniture designed and hand crafted in the 1907 =
former Masonic Temple building in historic Kerby, OR > > glwork=
m > http://jerrywork.com > > > > > > > > > ____=
___________________________________________ > GMCnet mailing list > Uns=
ubscribe or Change List Options: > http://list.gmcnet.org/mailman/listinf=
o/gmclist_list.gmcnet.org
 
> Caster is increased by moving the upper control arm rear adjuster closer to
> the frame of the coach. If the maximum adjustment has been used, then an
> eccentric or offset bushing can be fitted into the rearmost arm of the
> upper control arm.
> Jim Hupy
> Salem, OR
> 78 GMC Royale 403

A pic of the "upper control arm rear adjuster" adjusted to it's maximum caster position might help.

Thanks

--
John Shotwell
Ridgeville Corners, OH
78 Royale Center Kitchen
Web Site: GMCmhRegistry.com
Email: john at gmcmhregistry dot com
 
As someone mentioned, I use a length of 4x4 as a ride height gauge.
Overall, the block 9-3/4" high. One half of the block is cut down to
8-7/16" high. The high part is labelled "Front", the lower, "Rear". Those
are the heights of the bottom of the frame directly below the front and
rear measurement ovals when the ride height is correct. (Fact is, I thing
the 8-7/16" is 1/16" in error).

But what I really want to get across is that, despite its importance to
proper handling, we need to realize just what the effect of ride height is
on caster change. Remember that the wheelbase of a 26' GMC is 160". From
simple trigonometry, a 1" change in front vs rear ride height will yield
only 0.358* of caster change (arctan (1/160)). So, to achieve 1* more
caster, we'd have to ride around with the rear airbags almost all the way
down (it rides hard there). :-)

Ken H.

> Hi all, if I may indulge the collective this evening.
> I got all the way to the shop and somehow left my service Manuals at
> home😮😔. I sure could use the front and rear ride heights if anyone has
> them handy.
>
 
> As someone mentioned, I use a length of 4x4 as a ride height gauge.
> Overall, the block 9-3/4" high. One half of the block is cut down to
> 8-7/16" high. The high part is labelled "Front", the lower, "Rear". Those
> are the heights of the bottom of the frame directly below the front and
> rear measurement ovals when the ride height is correct. (Fact is, I thing
> the 8-7/16" is 1/16" in error).
>
> But what I really want to get across is that, despite its importance to
> proper handling, we need to realize just what the effect of ride height is
> on caster change. Remember that the wheelbase of a 26' GMC is 160". From
> simple trigonometry, a 1" change in front vs rear ride height will yield
> only 0.358* of caster change (arctan (1/160)). So, to achieve 1* more
> caster, we'd have to ride around with the rear airbags almost all the way
> down (it rides hard there). :)
>
> Ken H.

Ken,

I know the 9-3/4" value is correct and the same as mine. I "think" the other 8-7/16" is also the same as mine but I do not have the cut blocks here
at the moment to see if mine match yours. I'll try to remember to look the next time I'm at the airport. I thought we compared the numbers one time
in the past and agreed that yours and mine were the same.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Ken,

Man I never realized how much a change of camber of 0.358 degrees could make! WOW - NOT!

:-)

Maybe you have put this GMC myth to bed!

Regards,
Rob M.
USAussie - Downunder
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of Ken Henderson
Sent: Thursday, September 08, 2016 1:48 PM
To: gmclist
Subject: Re: [GMCnet] Ride height

As someone mentioned, I use a length of 4x4 as a ride height gauge.
Overall, the block 9-3/4" high. One half of the block is cut down to
8-7/16" high. The high part is labelled "Front", the lower, "Rear". Those
are the heights of the bottom of the frame directly below the front and
rear measurement ovals when the ride height is correct. (Fact is, I thing
the 8-7/16" is 1/16" in error).

But what I really want to get across is that, despite its importance to
proper handling, we need to realize just what the effect of ride height is
on caster change. Remember that the wheelbase of a 26' GMC is 160". From
simple trigonometry, a 1" change in front vs rear ride height will yield
only 0.358* of caster change (arctan (1/160)). So, to achieve 1* more
caster, we'd have to ride around with the rear airbags almost all the way
down (it rides hard there). :-)

Ken H.
 
> Ken,
> Man I never realized how much a change of camber of 0.358 degrees could make! WOW - NOT!
>
> :)
>
> Maybe you have put this GMC myth to bed!
>
> Regards,
> Rob M.
> USAussie - Downunder
> AUS '75 Avion - The Blue Streak TZE365V100428
> USA '75 Avion - Double Trouble TZE365V100426
I believe Ken is probably right about the caster not being significantly affected by ride height settings, and not contributing to handling issues.
But there is also probably a valid reason that the GM engineers spec'd the front to be an inch and a half higher than the rear. It might be that
somehow it affects the airflow and that's what makes the steering squirrely when that difference is not right. I dunno.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."
 
I so want to be a cranky old man, but I'll be nice...

I don't know anyone who has really disputed that the correct ride height improves handling, it's more of a matter of degree.
I'm sure we all agree that screwed up ride height will make the coach handle weird, like if the left front is really low.

All I was talking about in the original message was an easier way to determine ride height since the rear slot is dependent on the front height which is dependent on the rear height.
I'm pretty convinced that measuring ride height at the bogies would take the front height errors out of the equation and simplify my life as I am adjusting ride height a lot lately.
I also think it would reduce confusion for the average GMCer as the rear height could be set without having to have the front height perfect.

But this is turning out to be another wireless air discussion

----------------------------------------

>

>> Ken,
>> Man I never realized how much a change of camber of 0.358 degrees could make! WOW - NOT!
>>
>> :)
>>
>> Maybe you have put this GMC myth to bed!
>>
>> Regards,
>> Rob M.
>> USAussie - Downunder
>> AUS '75 Avion - The Blue Streak TZE365V100428
>> USA '75 Avion - Double Trouble TZE365V100426
> I believe Ken is probably right about the caster not being significantly affected by ride height settings, and not contributing to handling issues.
> But there is also probably a valid reason that the GM engineers spec'd the front to be an inch and a half higher than the rear. It might be that
> somehow it affects the airflow and that's what makes the steering squirrely when that difference is not right. I dunno.
> --
> 73 23' Sequoia 4 Sale
> 73 23' CanyonLands Parts Unit 4 Sale
> Upper Alabama
> "Highest price does not guarantee highest quality."
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Hey Keith, it matters not one whit what the air supply is for the bags. If
it has taken that turn, it has really gone sideways for sure.
I think this discussion is almost always promoted by someone who has
a difficult time getting down and looking under a coach, and then getting
back up again. There is nothing inherently wrong with any method of
measurement, including knotted string. Hey, it worked for the Pyramids I
think.
What I have a difficult time with is the opinion that the "GMC looks
funny with the front end higher than the rear" gang. Regardless of
appearances, the Engineers who developed the coach had sound fundamental
engineering reasons for doing it the way that they did. I know what they
were, and so do a big bunch of other people well schooled in making a GMC
Motor Home behave itself driving down the road.
If it is easier to cut a chunk of curly maple to the correct length
and set it under the frame slot at the rear of the coach to hold that
height while making adjustments to the torsion bars, GO FOR IT. But the
bogie box is a pivot point. I wouldn't use it myself. I use Jerry Work's
alignment kit, and follow his instructions. But I use a camber/caster gage
instead of computing from the turning plates. It is faster for me to do it
that way. I have a precision gage that was very expensive. Not every GMC
guy will have that in his shop, but I work on these coaches every day.
If the rear of the coach is too low, it is difficult to adjust the
upper control arm snail cams to get enough caster with stock bushings. Some
coaches vary a whole bunch as far as the amount of caster you can obtain,
with the 73 and 74 coaches leading the way. But if ANYTHING in the front
suspension and steering gear is worn out, save your money and time and
don't bother with alignment or ride height for that matter. Spend your
money on solar panels or GPS units. It will have more effect. (rant off)
Jim Hupy
Salem, OR
78 GMC Royale 403

> I so want to be a cranky old man, but I'll be nice...
>
> I don't know anyone who has really disputed that the correct ride height
> improves handling, it's more of a matter of degree.
> I'm sure we all agree that screwed up ride height will make the coach
> handle weird, like if the left front is really low.
>
> All I was talking about in the original message was an easier way to
> determine ride height since the rear slot is dependent on the front height
> which is dependent on the rear height.
> I'm pretty convinced that measuring ride height at the bogies would take
> the front height errors out of the equation and simplify my life as I am
> adjusting ride height a lot lately.
> I also think it would reduce confusion for the average GMCer as the rear
> height could be set without having to have the front height perfect.
>
> But this is turning out to be another wireless air discussion
>
> ----------------------------------------
>
> >

> >> Ken,
> >> Man I never realized how much a change of camber of 0.358 degrees could
> make! WOW - NOT!
> >>
> >> :)
> >>
> >> Maybe you have put this GMC myth to bed!
> >>
> >> Regards,
> >> Rob M.
> >> USAussie - Downunder
> >> AUS '75 Avion - The Blue Streak TZE365V100428
> >> USA '75 Avion - Double Trouble TZE365V100426
> > I believe Ken is probably right about the caster not being significantly
> affected by ride height settings, and not contributing to handling issues.
> > But there is also probably a valid reason that the GM engineers spec'd
> the front to be an inch and a half higher than the rear. It might be that
> > somehow it affects the airflow and that's what makes the steering
> squirrely when that difference is not right. I dunno.
> > --
> > 73 23' Sequoia 4 Sale
> > 73 23' CanyonLands Parts Unit 4 Sale
> > Upper Alabama
> > "Highest price does not guarantee highest quality."
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
> ...There is nothing inherently wrong with any method of measurement, including knotted string. Hey, it worked for the Pyramids I think. ...
Having trouble remembering stuff from that long ago? If you are that old, you are entitled.
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."
 
And this is the heart of the question.
Why would you NOT use the pivot point to measure ride height?

----------------------------------------
snip---
. But the
> bogie box is a pivot point. I wouldn't use it myself.

snip---

 
I would not use it because it is a fulcrum point. The rear of the coach can
still lower when you raise the front torsion bars. I think it is best to
prevent that from occurring, not to facilitate it. But, that is my opinion,
yours might vary and you are entitled to it. I have done dozens of these
coaches, and my system works well for me. Just my stubborn old man ways, I
guess.
Jim Hupy

> And this is the heart of the question.
> Why would you NOT use the pivot point to measure ride height?
>
> ----------------------------------------
> snip---
> . But the
> > bogie box is a pivot point. I wouldn't use it myself.
>
> snip---
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
> And this is the heart of the question.
> Why would you NOT use the pivot point to measure ride height?
>
> ----------------------------------------
> snip---
> . But the
> > bogie box is a pivot point. I wouldn't use it myself.
>
> snip---
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

Because, if you pivot off the bogie, as you raise or lower the front measure slot you will be moving the rear slot the opposite way. (think
teeter-totter) You want to set the rear and front slot heights, then set the bogie manual air adjustment to maintain that measurement.

--
Patti & Jerry Burt
73 Gmc 26' Canyon Lands -
Photos - http://jburt.smugmug.com/GMC-Motorhome
Lots of upgrades but lots to do to make it ours.
Members: FMCA - GMCMI - GMCWS - Pacific Cruisers
 
Yeah, but the slot position doesn't matter, and the teeter totter effect gives a false indication untill everything is right.
When focusing solely on the rear suspension the thing that matters is the angle of the rear suspension arms. You want them to be level with the ground.
Looking at it that way, the bogie pivot points are exactly where you wan to measure.

Well if you wanted to be perfect you's measure the rear spindle height and then set the bogie pins level with that.
But you shouldn't be running the wrong sized tires anyway so the spindle height can me optimized out

----------------------------------------
> Date: Thu, 8 Sep 2016 15:46:24 -0600
> To: gmclist
> From: jburt
> Subject: Re: [GMCnet] Ride height
>

>> And this is the heart of the question.
>> Why would you NOT use the pivot point to measure ride height?
>>
>> ----------------------------------------
>> snip---
>> . But the
>>> bogie box is a pivot point. I wouldn't use it myself.
>>
>> snip---
>>
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>
> Because, if you pivot off the bogie, as you raise or lower the front measure slot you will be moving the rear slot the opposite way. (think
> teeter-totter) You want to set the rear and front slot heights, then set the bogie manual air adjustment to maintain that measurement.
>
> --
> Patti & Jerry Burt
> 73 Gmc 26' Canyon Lands -
> Photos - http://jburt.smugmug.com/GMC-Motorhome
> Lots of upgrades but lots to do to make it ours.
> Members: FMCA - GMCMI - GMCWS - Pacific Cruisers
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Why do you want the bogie arms to be parallel to the ground? Is there some
benefit for them to be parallel? No more than there is any sound reasoning
that the lower control arms are parallel to the ground, or that the half
shafts are parallel to the ground. None that I was ever taught when I went
to G.M. training schools in the 1980's. This is slightly off the subject,
but, propeller shaft universal joints in automotive applications are
engineered to be a half degree out of parallel and slightly out of phase
with each other to minimize sympathetic harmonic vibrations. Not arguing
with you Keith, or anyone else here for that matter, just that my training
and experience tell me what I know. I could be wrong, and frequently am.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> Yeah, but the slot position doesn't matter, and the teeter totter effect
> gives a false indication untill everything is right.
> When focusing solely on the rear suspension the thing that matters is the
> angle of the rear suspension arms. You want them to be level with the
> ground.
> Looking at it that way, the bogie pivot points are exactly where you wan
> to measure.
>
> Well if you wanted to be perfect you's measure the rear spindle height and
> then set the bogie pins level with that.
> But you shouldn't be running the wrong sized tires anyway so the spindle
> height can me optimized out
>
> ----------------------------------------
> > Date: Thu, 8 Sep 2016 15:46:24 -0600
> > To: gmclist
> > From: jburt
> > Subject: Re: [GMCnet] Ride height
> >

> >> And this is the heart of the question.
> >> Why would you NOT use the pivot point to measure ride height?
> >>
> >> ----------------------------------------
> >> snip---
> >> . But the
> >>> bogie box is a pivot point. I wouldn't use it myself.
> >>
> >> snip---
> >>
> >>
> >> _______________________________________________
> >> GMCnet mailing list
> >> Unsubscribe or Change List Options:
> >> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> >
> >
> > Because, if you pivot off the bogie, as you raise or lower the front
> measure slot you will be moving the rear slot the opposite way. (think
> > teeter-totter) You want to set the rear and front slot heights, then set
> the bogie manual air adjustment to maintain that measurement.
> >
> > --
> > Patti & Jerry Burt
> > 73 Gmc 26' Canyon Lands -
> > Photos - http://jburt.smugmug.com/GMC-Motorhome
> > Lots of upgrades but lots to do to make it ours.
> > Members: FMCA - GMCMI - GMCWS - Pacific Cruisers
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
Well, you know I'm in total agreement with you.

Gary Kosier
77PB w/500Cad
Newark, Ohio

--------------------------------------------------
From: "Keith V"
Sent: Thursday, September 08, 2016 5:56 PM
To:
Subject: Re: [GMCnet] Ride height

> Yeah, but the slot position doesn't matter, and the teeter totter effect
> gives a false indication untill everything is right.
> When focusing solely on the rear suspension the thing that matters is the
> angle of the rear suspension arms. You want them to be level with the
> ground.
> Looking at it that way, the bogie pivot points are exactly where you wan
> to measure.
>
> Well if you wanted to be perfect you's measure the rear spindle height and
> then set the bogie pins level with that.
> But you shouldn't be running the wrong sized tires anyway so the spindle
> height can me optimized out
>
> ----------------------------------------
>> Date: Thu, 8 Sep 2016 15:46:24 -0600
>> To: gmclist
>> From: jburt
>> Subject: Re: [GMCnet] Ride height
>>

>>> And this is the heart of the question.
>>> Why would you NOT use the pivot point to measure ride height?
>>>
>>> ----------------------------------------
>>> snip---
>>> . But the
>>>> bogie box is a pivot point. I wouldn't use it myself.
>>>
>>> snip---
>>>
>>>
>>> _______________________________________________
>>> GMCnet mailing list
>>> Unsubscribe or Change List Options:
>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>>
>> Because, if you pivot off the bogie, as you raise or lower the front
>> measure slot you will be moving the rear slot the opposite way. (think
>> teeter-totter) You want to set the rear and front slot heights, then set
>> the bogie manual air adjustment to maintain that measurement.
>>
>> --
>> Patti & Jerry Burt
>> 73 Gmc 26' Canyon Lands -
>> Photos - http://jburt.smugmug.com/GMC-Motorhome
>> Lots of upgrades but lots to do to make it ours.
>> Members: FMCA - GMCMI - GMCWS - Pacific Cruisers
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Thanks Gary, I think!! (Grin)
Jim Hupy

> Well, you know I'm in total agreement with you.
>
> Gary Kosier
> 77PB w/500Cad
> Newark, Ohio
>
> --------------------------------------------------
> From: "Keith V"
> Sent: Thursday, September 08, 2016 5:56 PM
> To:
> Subject: Re: [GMCnet] Ride height
>
> Yeah, but the slot position doesn't matter, and the teeter totter effect
>> gives a false indication untill everything is right.
>> When focusing solely on the rear suspension the thing that matters is the
>> angle of the rear suspension arms. You want them to be level with the
>> ground.
>> Looking at it that way, the bogie pivot points are exactly where you wan
>> to measure.
>>
>> Well if you wanted to be perfect you's measure the rear spindle height
>> and then set the bogie pins level with that.
>> But you shouldn't be running the wrong sized tires anyway so the spindle
>> height can me optimized out
>>
>> ----------------------------------------
>>
>>> Date: Thu, 8 Sep 2016 15:46:24 -0600
>>> To: gmclist
>>> From: jburt
>>> Subject: Re: [GMCnet] Ride height
>>>

>>>
>>>> And this is the heart of the question.
>>>> Why would you NOT use the pivot point to measure ride height?
>>>>
>>>> ----------------------------------------
>>>> snip---
>>>> . But the
>>>>
>>>>> bogie box is a pivot point. I wouldn't use it myself.
>>>>>
>>>>
>>>> snip---
>>>>
>>>>
>>>> _______________________________________________
>>>> GMCnet mailing list
>>>> Unsubscribe or Change List Options:
>>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>>>
>>>
>>>
>>> Because, if you pivot off the bogie, as you raise or lower the front
>>> measure slot you will be moving the rear slot the opposite way. (think
>>> teeter-totter) You want to set the rear and front slot heights, then set
>>> the bogie manual air adjustment to maintain that measurement.
>>>
>>> --
>>> Patti & Jerry Burt
>>> 73 Gmc 26' Canyon Lands -
>>> Photos - http://jburt.smugmug.com/GMC-Motorhome
>>> Lots of upgrades but lots to do to make it ours.
>>> Members: FMCA - GMCMI - GMCWS - Pacific Cruisers
>>>
>>> _______________________________________________
>>> GMCnet mailing list
>>> Unsubscribe or Change List Options:
>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>>
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
I'm pretty sure that at the factory ride height the rear suspension arms are parallel to the ground, but I need to verify.

The reasoning is all about controlling the camber/caster/toe curve. If you start with the lower arm ( front suspension ) parallel is things get better as the suspension compresses, more camber, less toe in, causing the car to behave nicely, mostly the toe change making the car under steer not over steer.
If you start below level the curves tend to get worse then better as the suspension compresses, and you get over steer then under steer as the wheels toe in and out

This is less important on the GMC rear as there isn't a camber or toe change on bump, but keeping the forward arm level will reduce pole vaulting on braking.
But you ask a good question. I'll have to research that better.

GM, and most manufacturers, have to design the suspension height for a variety of loads and situations, so they tend to set it too high for best performance. Thats why lowering a car tends to improve cornering performance. It improves bump steer and roll center. It does sacrifice load capability and suspension travel however.
Also modern cars have gotten a LOT better about this compromise, I doubt lower a Corvette C6 would improve anything.

A comment was made a while ago that a 1" height change front to rear would result in a ,3 degree caster change in the front. Considering that I can only get 1.5 degrees caster, loosing .3 degrees becomes significant.

As far as ujoint misalignment, my understanding is that the slight misalignment is there to keep the joint moving to distribute the grease, if just a little. And yes the joints need to be 90 degrees out of phase with equal and opposite angles. Ive spent way too much time chasing vibrations on my C3 Vette that were due to a side to side misalignment of the diff. I had to slot the trans mount and move it over 3/4" then smoooth.
I could have just rotated the diff, but dropping the diff is a royal PITA on those things.

----------------------------------------

>
> Why do you want the bogie arms to be parallel to the ground? Is there some
> benefit for them to be parallel? No more than there is any sound reasoning
> that the lower control arms are parallel to the ground, or that the half
> shafts are parallel to the ground. None that I was ever taught when I went
> to G.M. training schools in the 1980's. This is slightly off the subject,
> but, propeller shaft universal joints in automotive applications are
> engineered to be a half degree out of parallel and slightly out of phase
> with each other to minimize sympathetic harmonic vibrations. Not arguing
> with you Keith, or anyone else here for that matter, just that my training
> and experience tell me what I know. I could be wrong, and frequently am.
> Jim Hupy
> Salem, Or
> 78 GMC ROYALE 403

>
>> Yeah, but the slot position doesn't matter, and the teeter totter effect
>> gives a false indication untill everything is right.
>> When focusing solely on the rear suspension the thing that matters is the
>> angle of the rear suspension arms. You want them to be level with the
>> ground.
>> Looking at it that way, the bogie pivot points are exactly where you wan
>> to measure.
>>
>> Well if you wanted to be perfect you's measure the rear spindle height and
>> then set the bogie pins level with that.
>> But you shouldn't be running the wrong sized tires anyway so the spindle
>> height can me optimized out
>>
>> ----------------------------------------
>>> Date: Thu, 8 Sep 2016 15:46:24 -0600
>>> To: gmclist
>>> From: jburt
>>> Subject: Re: [GMCnet] Ride height
>>>

>>>> And this is the heart of the question.
>>>> Why would you NOT use the pivot point to measure ride height?
>>>>
>>>> ----------------------------------------
>>>> snip---
>>>> . But the
>>>>> bogie box is a pivot point. I wouldn't use it myself.
>>>>
>>>> snip---
>>>>
>>>>
>>>> _______________________________________________
>>>> GMCnet mailing list
>>>> Unsubscribe or Change List Options:
>>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>>
>>>
>>> Because, if you pivot off the bogie, as you raise or lower the front
>> measure slot you will be moving the rear slot the opposite way. (think
>>> teeter-totter) You want to set the rear and front slot heights, then set
>> the bogie manual air adjustment to maintain that measurement.
>>>
>>> --
>>> Patti & Jerry Burt
>>> 73 Gmc 26' Canyon Lands -
>>> Photos - http://jburt.smugmug.com/GMC-Motorhome
>>> Lots of upgrades but lots to do to make it ours.
>>> Members: FMCA - GMCMI - GMCWS - Pacific Cruisers
>>>
>>> _______________________________________________
>>> GMCnet mailing list
>>> Unsubscribe or Change List Options:
>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
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