Home » Public Forums » GMCnet » Why not fix one ton scrub radius with wheel offset?
| Re: [GMCnet] Why not fix one ton scrub radius with wheel offset? [message #361945 is a reply to message #361908] |
Fri, 22 January 2021 22:12   |
Zandr Milewski
Messages: 12
Registered: September 2020
Karma: -1
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Junior Member |
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Lots of things can contribute to torque steer, sure. but the GMCMH
starts from a pretty good position, since it has a fixed intermediate
shaft that equalized the halfshaft length. Minimizing scrub radius is
the next big lever, as you describe. Spindle length would be next, and
that's rather harder to deal with.
On 1/22/21 20:00, James Hupy via Gmclist wrote:
> If you put 2" wheel spacers on a stock GMC front end, it will also torque
> steer. I have done that very thing on my coach, and installed another set
> on a different coach, with the same results. Torque steer that will yank
> the steering wheel out of your grip.
> So, I do not equate torque steer with the 1-ton, but with front wheel
> drive vehicles. I have a P.T. Dream Cruiser that has 220 horsepower in it's
> turbocharged 2.4 L. engine. It weighs just over 2000 pounds, and is a real
> rocket ship. If you turn the front wheels slightly and turn the traction
> control off, it becomes a real handful if you jump on the throttle. I have
> torn out the engine mounts twice doing stupid stuff like burnouts with it.
> So, front wheel drives with surplus torque are more guilty than the 1-ton
> in my opinion. But, opinions vary.
> Jim Hupy
> Salem, Oregon
>
> On Fri, Jan 22, 2021, 7:37 PM Zandr Milewski via Gmclist gmclist@list.gmcnet.org> wrote:
>
>> Jim-
>>
>> At first approximation, no argument. These are six-ton beasts, and you
>> have to scale your expectations accordingly.
>>
>> On the other hand, if we can understand the suspension geometry at play,
>> we can make the best of our lot. Certainly, the concerns about scrub
>> radius have a basis in minimizing torque steer. The "non-parallel
>> control arms" question is more interesting, as that relates to camber
>> gain, and depending on other factors, bump steer.
>>
>> Zero camber gain is not always the answer. The only doc I've seen with a
>> discussion of suspension geometry didn't really understand how it
>> worked, as it concluded incorrectly that every one-ton coach should be
>> sitting on the bump stops.
>>
>> -Zandr
>>
>> On 1/22/21 17:18, James Hupy via Gmclist wrote:
>>> The GMC Motorhome is NOT a performance vehicle. It wallows in tight
>> curves,
>>> barely stays straight on good roads. It is at least 40 years old, and
>>> designed around parts plucked from production lines of various
>> automobiles
>>> of the era. Nothing state of the art about the suspension and drive
>> train.
>>> It does a fair job of point to point transportation of a heavy
>> vehicle
>>> that has a great deal of its weight centered above the tops of the tires.
>>> A race car, it definitely is NOT! If you do not expect sports car
>>> handling, and maintain the drive train/suspension parts well, it is safe
>>> enough to drive in traffic if you don't follow too closely and drive
>> "eyes
>>> way down the road ahead", you won't get in too much trouble.
>>> There are upgrades that improve braking quite a bit, and that is
>> one
>>> huge reason for the 1-ton conversion. That and much stronger lower
>> control
>>> arms and front wheel bearings as well as upper and lower ball joints.
>>> Handling? Very subjective at best. It still is a top heavy front wheel
>>> drive vehicle. Not much will ever change those facts.
>>> Do I love them? Yep! But, don't put lipstick on them. Underneath,
>> they
>>> are still a very good looking pig.(grin)
>>> Jim Hupy
>>> Salem, Oregon
>>>
>>> On Fri, Jan 22, 2021, 4:55 PM Joe Weir via Gmclist > gmclist@list.gmcnet.org>
>>> wrote:
>>>
>>>> Yes, My bad. The point was that while not completely fixing the issue,
>>>> it certainly moves it closer to correct.
>>>>
>>>> Zandr Milewski wrote on Fri, 22 January 2021 16:15
>>>> > On 1/22/21 06:55, John R. Lebetski via Gmclist wrote:
>>>> >> Also moving tire inboard may be able to fix the steering axis or
>> some
>>>> of the error, but not sure that totally fixes scrub radius.
>>>> > Scrub radius is literally defined as the distance between the center of
>>>> > the contact patch and the point where the steering axis touches the
>>>> ground.
>>>> > _______________________________________________
>>>> > GMCnet mailing list
>>>> > Unsubscribe or Change List Options:
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>>>> --
>>>> 76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator,
>>>> brakes, Sully airbags, fuel lines, seats, adult beverage center... those
>>>> Coachmen
>>>> guys were really thinking about us second hand owners by including that
>>>> beverage center...
>>>> Columbia, SC.
>>>>
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| Re: Why not fix one ton scrub radius with wheel offset? [message #361947 is a reply to message #361905] |
Sat, 23 January 2021 08:58 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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I’m not going to look it up (you can do that) but guessing that the steering axis is the intersection of the imaginary line through ball joints to where that intersects the tread width at the road. A passive measurement. The scrub radius is the distance from that imaginary point to the arc the tire creates when turning lock to lock. For example on a stock coach, adding spacers would increase that arc length. Instead of pivoting about the steering axis, you take on an open “c” shaped pattern. Therefore when hitting a pothole squarely instead of just having suspension deflection the tire wants to also turn outboard, causing an unwanted steering effect.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] Why not fix one ton scrub radius with wheel offset? [message #361948 is a reply to message #361936] |
Sat, 23 January 2021 09:50 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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James Hupy wrote on Fri, 22 January 2021 20:18The GMC Motorhome is NOT a performance vehicle. It wallows in tight curves, barely stays straight on good roads. It is at least 40 years old, and designed around parts plucked from production lines of various automobiles of the era. Nothing state of the art about the suspension and drive train.
It does a fair job of point to point transportation of a heavy vehicle that has a great deal of its weight centered above the tops of the tires.
A race car, it definitely is NOT! If you do not expect sports car handling, and maintain the drive train/suspension parts well, it is safe enough to drive in traffic if you don't follow too closely and drive "eyes way down the road ahead", you won't get in too much trouble.
There are upgrades that improve braking quite a bit, and that is one huge reason for the 1-ton conversion. That and much stronger lower control arms and front wheel bearings as well as upper and lower ball joints. Handling? Very subjective at best. It still is a top heavy front wheel drive vehicle. Not much will ever change those facts.
Do I love them? Yep! But, don't put lipstick on them. Underneath, they are still a very good looking pig.(grin)
Jim Hupy
Salem, Oregon
Jim,
If I did not know better, I would believe that our two coaches are no relation.
Chaumière is older and about one ton lighter (and three feet shorter), but until the recent crash (I will get it back as soon as I can) she would stay straight on a straight flat road (these are very rare in Michigan and getting rarer). My limit to exit cornering was keeping the things on the shelves on said shelves and not a vehicle handling situation. It has been my pleasure to watch the PITA tailgating pickup be forced to fall back in an during an exit loop.
In vehicle front end design, the camber should stay close during the anticipated body roll experienced in cornering. If one worked in that corner of the industry and/or drove race cars, when experienced it is well known alert that the driver should learn he cannot ignore. As it is a harbinger of bad things to come, it is unsettling to those of us that know this steering wheel reaction. It is not (despite the common remarks here) a result of the control arms not being parallel, because in the best handling vehicles those arms do not stay parallel under all conditions. As the friends at an OE (then American Motors) that had a computer program (at that time, it had to be a big expensive computer) to emulate this activity in about 1975 found out, the four bar linkage that is the front end of most cars and can make or break the handling. I don't know if you remember the late 70's AMC Eagle Wagon. They had it right and they were just an amazing vehicle to put into a corner. This was only true of the 4WD wagon as the other retained the original design.
Anyone with first hand experience will refuse to concede the knuckle bearing issue. As Dave Lenzi has so clearly established, it isn't the bearing that is the issue, but it is a single manufacturing shortcut that is the real issue here. If one mounts the bearing in accordance with the best practices, the life is them long enough to now be an unknown factor.
One will have to concede the lower control arm issue in the early coaches. Even reinforced per the GMC directive, they still had issues. Fortunately, those most often fail at low speeds in hard cornering situations. Even at low speeds, this can to severe damage to the coach.
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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| Re: [GMCnet] Why not fix one ton scrub radius with wheel offset? [message #361949 is a reply to message #361948] |
Sat, 23 January 2021 11:01 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Matt, I edited my comments on purpose for the "political correctness"
factor. When I drive my 78 Royale 403 equipped 3:70 final drive, my long
suffering wife, Judy is constantly reminding me of things like speed
limits, (I frequently exceed them, sometimes a lot), things falling off of
counters, drawers flying open and closed, etc. Caused by my (ahem)
aggressive driving style. (Grin) When my coach is stowed for rough seas, I
use the cornering sign speed as a guide for my entry speed into a curve. I
always add 20+mph to the posted cornering speed, and I add throttle
throughout the curve because 1. It's fun, and 2. It's a front wheel drive
vehicle.
But, it's a "Do as I say, not as I do" kind of deal. I don't want my
comments here to be interpreted as how everyone should drive their coach.
My "Sin in haste, Repent at leisure " behavior does not work for
everyone. Just me, I guess.
Yes, a well maintained GMC will handle fairly well when compared to
other motorhomes of that era, but not when compared to modern automobiles.
All I was saying, really.
Jim Hupy
Salem, Oregon
On Sat, Jan 23, 2021, 7:51 AM Matt Colie via Gmclist <
gmclist@list.gmcnet.org> wrote:
> James Hupy wrote on Fri, 22 January 2021 20:18
>> The GMC Motorhome is NOT a performance vehicle. It wallows in tight
> curves, barely stays straight on good roads. It is at least 40 years old,
> and
>> designed around parts plucked from production lines of various
> automobiles of the era. Nothing state of the art about the suspension and
> drive
>> train.
>> It does a fair job of point to point transportation of a heavy
> vehicle that has a great deal of its weight centered above the tops of the
>> tires.
>> A race car, it definitely is NOT! If you do not expect sports car
> handling, and maintain the drive train/suspension parts well, it is safe
>> enough to drive in traffic if you don't follow too closely and drive
> "eyes way down the road ahead", you won't get in too much trouble.
>> There are upgrades that improve braking quite a bit, and that is
> one huge reason for the 1-ton conversion. That and much stronger lower
>> control arms and front wheel bearings as well as upper and lower ball
> joints. Handling? Very subjective at best. It still is a top heavy front
> wheel
>> drive vehicle. Not much will ever change those facts.
>> Do I love them? Yep! But, don't put lipstick on them. Underneath,
> they are still a very good looking pig.(grin)
>> Jim Hupy
>> Salem, Oregon
>
> Jim,
>
> If I did not know better, I would believe that our two coaches are no
> relation.
>
> Chaumière is older and about one ton lighter (and three feet shorter), but
> until the recent crash (I will get it back as soon as I can) she would
> stay straight on a straight flat road (these are very rare in Michigan and
> getting rarer). My limit to exit cornering was keeping the things on the
> shelves on said shelves and not a vehicle handling situation. It has
> been my pleasure to watch the PITA tailgating pickup be forced to fall back
> in
> an during an exit loop.
>
> In vehicle front end design, the camber should stay close during the
> anticipated body roll experienced in cornering. If one worked in that
> corner of
> the industry and/or drove race cars, when experienced it is well known
> alert that the driver should learn he cannot ignore. As it is a harbinger
> of
> bad things to come, it is unsettling to those of us that know this
> steering wheel reaction. It is not (despite the common remarks here) a
> result of
> the control arms not being parallel, because in the best handling vehicles
> those arms do not stay parallel under all conditions. As the friends at an
> OE (then American Motors) that had a computer program (at that time, it
> had to be a big expensive computer) to emulate this activity in about 1975
> found out, the four bar linkage that is the front end of most cars and can
> make or break the handling. I don't know if you remember the late 70's AMC
> Eagle Wagon. They had it right and they were just an amazing vehicle to
> put into a corner. This was only true of the 4WD wagon as the other
> retained
> the original design.
>
> Anyone with first hand experience will refuse to concede the knuckle
> bearing issue. As Dave Lenzi has so clearly established, it isn't the
> bearing
> that is the issue, but it is a single manufacturing shortcut that is the
> real issue here. If one mounts the bearing in accordance with the best
> practices, the life is them long enough to now be an unknown factor.
>
> One will have to concede the lower control arm issue in the early
> coaches. Even reinforced per the GMC directive, they still had issues.
> Fortunately, those most often fail at low speeds in hard cornering
> situations. Even at low speeds, this can to severe damage to the coach.
>
> 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
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
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| Re: [GMCnet] Why not fix one ton scrub radius with wheel offset? [message #361952 is a reply to message #361949] |
Sat, 23 January 2021 15:49 |
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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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James Hupy wrote on Sat, 23 January 2021 12:01Matt, I edited my comments on purpose for the "political correctness"
factor.
<snip>
Yes, a well maintained GMC will handle fairly well when compared to
other motorhomes of that era, but not when compared to modern automobiles.
All I was saying, really.
Jim Hupy
Salem, Oregon
Jim,
I will concede that there are many modern passcar that handle better than a forty year old motorhome, and yes it was better than it's contemporaries in that regard. But I would have to guess that you have not driven many modern motorhomes of the same size class.
First, there are very few A's in the same size class as yours and Zero in mine....
The larger A's are buses and drive just like buses. (Surprise?)
Second, most of those in our size class are C's assembled on truck chassis and those that I have had the (mis)fortune to drive are as rewarding as a U-Haul. (This is not intended as a slam of U-Haul, after all, their target is not to be a comfortable travel vehicle.)
So, I think we really agree for the larger part.
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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| Re: [GMCnet] Why not fix one ton scrub radius with wheel offset? [message #361954 is a reply to message #361952] |
Sat, 23 January 2021 16:55 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Matt, When i was in the military, after basic training, I went to Drivers
School. I learned how to drive various military vehicles, M-88 Tracked
recovery vehicle, Tanks both light and medium, 5 Ton and 10 Ton Wreckers,
Trucks of all sizes and shapes, and Military transport busses. Those had
the driver located in front of the steering axle. You had to drive
completely into an intersection before you even thought about turning the
corner. Tricky buggers, they were. About the size of a Greyhound
Senicruiser. 46 passengers or so. Small round military rear view mirrors. a
seriously difficult vehicle to back up and park. But, nothing that compares
to today's Prevost Chassis motorhomes. And like you said, there is really
nothing else out there to compare a GMC coach to. Before or since our
coaches were built. So, yes, we agree.
Jim Hupy
Salem, Oregon
On Sat, Jan 23, 2021 at 1:50 PM Matt Colie via Gmclist <
gmclist@list.gmcnet.org> wrote:
> James Hupy wrote on Sat, 23 January 2021 12:01
>> Matt, I edited my comments on purpose for the "political correctness"
>> factor.
>>
>> Yes, a well maintained GMC will handle fairly well when compared to
>> other motorhomes of that era, but not when compared to modern
> automobiles.
>> All I was saying, really.
>> Jim Hupy
>> Salem, Oregon
>
> Jim,
>
> I will concede that there are many modern passcar that handle better than
> a forty year old motorhome, and yes it was better than it's contemporaries
> in that regard. But I would have to guess that you have not driven many
> modern motorhomes of the same size class.
>
> First, there are very few A's in the same size class as yours and Zero in
> mine....
> The larger A's are buses and drive just like buses. (Surprise?)
>
> Second, most of those in our size class are C's assembled on truck chassis
> and those that I have had the (mis)fortune to drive are as rewarding as a
> U-Haul. (This is not intended as a slam of U-Haul, after all, their
> target is not to be a comfortable travel vehicle.)
>
> So, I think we really agree for the larger part.
>
> 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
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
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| Re: [GMCnet] Why not fix one ton scrub radius with wheel offset? [message #361961 is a reply to message #361954] |
Sat, 23 January 2021 22:42 |
dsmithy
Messages: 210
Registered: July 2012
Location: Lincoln Nebraska
Karma: 0
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Senior Member |
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Matt and Jim,
This discussion is exactly what makes the forum so valuable to me. Discussion that spreads well beyond what I have experienced or will ever experience, and yet still applies to my coach and my experience. Thanks for (in your almost disagreement) showing me more sides of the puzzle/equation/motorhome. Nicely done.
Douglas & Virginia Smith,
dsmithy18 at gmail,
Lincoln Nebraska,
’73 “Sequoia” since ‘95: "Wanabizo";
Quadrabag/6 wheel disks/3:70 final/Paterson QuadraJet/Thorley’s/Alloy wheels/Sundry other
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| Re: Why not fix one ton scrub radius with wheel offset? [message #361964 is a reply to message #361905] |
Sun, 24 January 2021 08:18 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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As well as spreading knowledge/experience differences of opinion make for good horse races and dog shows.
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] Why not fix one ton scrub radius with wheel offset? [message #361974 is a reply to message #361964] |
Sun, 24 January 2021 15:15 |
Keith V
Messages: 2337
Registered: March 2008
Location: Mounds View,MN
Karma: 0
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Senior Member |
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IMHO the GMC drives pretty damn well for a big ol motorhome. Not as good as my Mustang, but probably better that the mustang I totaled in 2004.
I did have the mis-pleasure of owning a 22 foot class C on a GM chassis and a 454. Driving that thing was more like sailing than driving, every crosswind or passing vehicle caused a panicked correction. I took it to a local motorhome repair shop, they drove it and said it's a lot better than most!
The GMC is tight, predictable and rides like a minivan.
I do not have the 1 ton conversion.
Keith Vasilakes
Mounds View MN
76 exRoyale
MicroLevel
________________________________
From: Gmclist on behalf of Johnny Bridges via Gmclist
Sent: Sunday, January 24, 2021 8:18 AM
To: gmclist@list.gmcnet.org
Cc: Johnny Bridges
Subject: Re: [GMCnet] Why not fix one ton scrub radius with wheel offset?
As well as spreading knowledge/experience differences of opinion make for good horse races and dog shows.
--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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Keith Vasilakes
Mounds View. MN
75 ex Royale GMC
ask me about MicroLevel
Cell, 763-732-3419
My427v8@hotmail.com
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