To:               Offroad@offroadlist.com
Date sent:        Tue, 17 Nov 1998 09:07:36 -0700
From:             "David Houk" 
Organization:     Motorola Semiconductor Products Sector
Subject:          [Fwd: Clutch Danger on Low Geared Vehicles]
Send reply to:    Offroad@offroadlist.com

Greetings: 
     I wanted to share with you an experience of mine with an off-road
vehicle recently.  I have not heard of this happening before, nor was I
aware of this potential situation.  But this is worth being aware of, I
believe.  I was wheeling my 84 CJ-7 a few weeks ago.  I had just descended
a rather steep downgrade and was in second gear, low range.  The terrain
flattened out to a wide sandy wash, with just a very slight downgrade. I
pushed in my clutch (but left the transmission in gear) and let the vehicle
gradually accelerate down the wash. This not something I normally do, but I
knew that at the end of this wash, (which was about 1/4 mile long), that I
would have to climb a steep upgrade and would have to shift back in second
gear away.  When I originally pushed in the clutch the vehicle was
traveling around 2 mph. As I kept the clutch in, the vehicle speed
gradually increased till about a minute later, towards the end of the wash,
the vehicle speed was approx. 20 mph.  Just as I was getting ready to apply
the brakes to slow the vehicle down, I heard a loud explosion.  This was
the sound of my clutch disc exploding.  The clutch disc basically flew
apart due to excessive RPM.  The clutch was disengaged, (pedal on the
floor) when this occurred. Engine RPM was at idle. (800 RPM)   Because the
clutch disc is splined to the input shaft of the transmission, it turns at
the input shaft speed of the tranny.  Because the transmission was in gear,
 the input shaft (and clutch) were being driven by the rear wheels of the
vehicle, thru the rear drivetrain, in coast mode.  My second gear tranny
ratio is 2.37. My Dana 300 low range reduction is 4.0 (terra-low gears) My
rear end reduction is 4.1  This gives my total gearing ratio at the time of
approx. 39 to 1.  At 20 mph, my 33" tires are turning at approx. 211 RPM. 
(20/60x5280=1760 FPM) 100" is static loaded circumference of 33". (So
100/12=8.33 and 1760/8.33=211)  So through the gear reduction, (in this
case gear amplification because the tranny is being driven by the rear
wheels,  211 x 39 = 8229 RPM.  The clutch disc was spinning at over 8000
RPM when it exploded.  Stock clutch discs are usually only rated to 6000 or
7000 RPM.  Even racing clutch discs are usually only rated to 10,000 RPM. 
The clutch disc is the widest part of the drivetrain where the most
centrifugal force is developed.  The vehicle exhibited little or no noise
before the explosion.  Even the whine of low range transfer case gears was
noticeably absent. (I assume because the gears were not loaded)  There was
no way to tell that excessive speed was developing in the clutch disc,
other that being aware of how the vehicle was being operated. Now the
kicker.  If the vehicle had still had stock gearing, this destruction of
the clutch never would have happened.  Stock gearing on this vehicle is
2.37 tranny, 2.61 transfer case, and 2.73 rear end for a total reduction of
approx. 17 to 1.  Clutch speed under this scenario would have been 211 x 17
= 3587 RPM, well within the range of normal clutch speed RPM.  
     When I discussed this situation with other mechanics and wheelers,
they had never heard of this situation occurring.  All thought I must have
released the clutch before the explosion. (Clutches are also rated for the
maximum RPM mismatch during engagement.) But the clutch pedal was fully
down when this occurred.
     The moral of this story is to be very careful when coasting a manual
transmission vehicle with considerably lower than stock gearing in either
the tranny, transfer case, rear end, or some combination thereof.  Always
shift to Neutral and coast with the clutch out. (engaged)  Any manual
transmission vehicle with gearing more than 1.5x to 2.0x lower than stock
is a potential candidate.  If you are not thinking about what you are
doing,  your vehicle could leave your stranded in the middle of nowhere. 
As usual, the biggest variable is the driver.  If anyone else has heard of
this happening, (or has had it happen to them), I would appreciate hearing
about it.

David Houk

R17706@email.sps.mot.com




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To:               Offroad@offroadlist.com
Date sent:        Tue, 17 Nov 1998 14:06:51 -0700
From:             Brett Davis 
Subject:          Re: [Fwd: Clutch Danger on Low Geared Vehicles]
Send reply to:    Offroad@offroadlist.com

>      The moral of this story is to be very careful when coasting a manual
> transmission vehicle 

Period.  

It doesn't matter if the gearing is lower or not and here is why:  If
your gearing would have been stock, then you would have been in 1st
gear, low range, with the ratio in the 30 to 40 range, and the same
thing would have happened.  Ditto for me.  I might have been in granny
gear, high range (30:1) or 2nd gear, low range (33.2:1).  In this
situation, with my aired down swampers at 25 mph, the disc would be
turning 7600 rpm.  I usually don't "ride the clutch" and with this info,
I definitely won't start now.

> Always shift to Neutral and coast with the clutch out. (engaged)  
> If you are not thinking about what you are doing,  your vehicle could leave 
> your stranded in the middle of nowhere.
> As usual, the biggest variable is the driver.  If anyone else has heard of
> this happening, (or has had it happen to them), I would appreciate hearing
> about it.

Never heard of it, but it makes total sense.  Thanks for the heads up.
-- 
Brett Davis 
bdavis@enviropax.com

"...BACKCOUNTRY VEHICLE USE IS AVAILABLE ONLY TO ABLE-BODIED AND
EXPERIENCED OFF-ROAD DRIVERS WHO CAN AFFORD TO OWN AND MAINTAIN 
EXPENSIVE VEHICLES"    - SUWA
 Salt Creek closure, Canyonlands National Park, UT 1998

CJ8 with 99% stock parts (stock, not original)  
RTI 30 deg 971

Utah4WDA, SM4x4, RR4W       http://www.off-road.com/~utah4wda/
LUN -   Land Use Network    http://www.off-road.com/landuse/landuse.htm
ORML -  Offroad mailinglist http://www.offroadlist.com
jeep-slc                    http://circle17.com/jeep/
USALL - Utah Shared Access Land List
USA-ALL - Utah Shared Access Alliance http://www.usa-all.com/





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To:               Offroad@offroadlist.com
From:             "smith" 
Subject:          Re: [Fwd: Clutch Danger on Low Geared Vehicles]
Date sent:        Tue, 17 Nov 1998 18:36:40 -0700
Send reply to:    Offroad@offroadlist.com

Brett,
     I had the same thing happen to me back in 1977 with my dad's brand new
1/2 ton Chevy truck.  I had been trying to climb a long sand hill in the
snow.  After several attempts I got tired of waiting for the long slow back
up procedure down the hill in low range, so just pushed the clutch in with
the transmission still in reverse and the transfer case still in low.  I
estimate I reached 20 mph coasting backwards down the hill with the clutch
depressed, when just before I got to the bottom of the hill:  Ka Boom.  You
guessed it, an exploded clutch disc.  Luckily the truck had less than 1000
miles on it and the clutch was replaced under warranty.  And all this time I
just figured dad had gotten a faulty clutch from GM.  Thanks for clearing up
the mystery after 21 years, but don't tell my dad.

Phil Smith.






FROM: Roger Brown <4crawler@home.com>
SUBJECT: Re: Centerforce Dual Friction Clutches
DATE: Mon, 11 Jun 2001 05:39:25 GMT
ORGANIZATION: Excite@Home - The Leader in Broadband http://home.com/faster
NEWSGROUPS: rec.autos.4x4

Willem-Jan Markerink wrote:
>    Roger Brown <4crawler@home.com> wrote:
> >I've had one for nearly 3 years on my 4Runner.  Still going strong,
> >really smooth at low speeds and grab like crazy at the top end.  Only
> >drawback I can see is you have to shift/clutch just as careful in
> >4th/5th gear as you do in 1st/2nd.
> 
> And be very careful when you are driving/coasting down a steep hill in low
> gear, fast, with the clutch disengaged....the clutch will then rotate at
> much higher rpm's than it was designed for, and possibly explode:
> 
> http://www.a1.nl/phomepag/markerink/coasting.htm

I have a similar warning on my Marlin Crawler page:
	http://reality.sgi.com/rogerb/4x4/CheapTricks/Marlin.html#Caveats











From: Miller.JF@forces.gc.ca
To: w.j.markerink@a1.nl
Subject: Coasting article..
Date: Tue, 28 Sep 2004 06:48:41 -0400

I read the articles about coasting with the clutch in causing the clutch
disc to explode due to over speeding.(too high rpm)
I was told this by an old Sgt Major when I was driving my ten ton
MRT.(mobile repair team) Since it topped out around 95km's per hour,
whenever I hit a big down hill I would shove in the clutch and use "mexican
overdrive". He told me of trucks blowing clutches "back in the day" due to
this. I always thought it was a wives tale. He said it was because the
difference in rpm from the clutch disc and the flywheel, I see now it was
from actual overspeeding the disc.
And no it never blew up on me even though I went as fast as 140km's per
hour! Lol!
Jim

M/cpl Miller
Inf Det
Maint
J-1

If you have any question, remark, comment, want to share some philosophy or just want to express your opinion about these pages, feel free to send email to: w.j.markerink @ a1.nl