AIR ACTUATED CLUTCH

[JohnEd]

Like the air throttle, is there such a thing as an air "clutch"?

I recently had a demonstration of the use of the clutch in a 4106.  The driver didn't touch the throttle, just switched to fast idle and let out the clutch.  Bus pulled away smoothly and throttle was applied as it reached fast idle speed.  Was smooth as silk....no muss....no fuss.   
He seemed to just let the clutch out smoothly without regard to rpm.  I was told that that method would work on hills also and to use the throttle and slip the clutch was to invite a fried clutch.  That being the case, why can't I actuate the clutch with a switch and release it when I want the clutch engaged.  You would still have the "air" pedal on the floor for "close in work".

Thoughts?

John

[RJ]

John -

Something doesn't seem right here.

My 4106's fast idle will not work if the parking brake's released, which is the way most are integrated.  Safety issue, actually, as it locks the throttle pedal (well, actually the throttle arm on the governor housing).  If this guy's fast idle works with out the parking brake being set. . . well, I'd be concerned.  But then again, I'm safety-oriented.

It's pretty easy to walk any GMC coach away from a dead stop by feathering the clutch thru the take-up point w/o any throttle.  Trainers call this a "dead throttle start".  Notice I said "feather the clutch", not "dump the clutch".  Big difference.

A while back on one of the two major bus boards there was some photos of a fellow's "air assist" clutch mechanism.  A slave cylinder mounted in the exterior compartment below the driver, IIRC.  It may still be in the archives. . . time to go searching!

FWIW & HTH. . .

[zimtok]

I have air assist on my 4104 clutch. It does not take the place of the original linkage, it is attached at the rear in the engine compartment. A valve of sorts detects the pressure on the linkage and applies air to a cylinder that helps push the linkage.

.

[JohnEd]

Zimtok,

Thank you

Russ,

He may have been just demonstrating the "fast idle" take off to me but I don't think so.  Whatever safety mech is supposed to be there was disabled on that coach.  It worked really well and he gave me a measure of confidence that I could take off on a grade without frying the clutch by letting the gov control the throttle.  I have feathered more than my share of clutchs in my day and I am familiar with the procedure.  Heard so many horror stories about bus clutches, though, I am "almost" turned off on a stick unless I have a granny for insurance.  It has been an issue with me and maybe out of ignorance on my part.

What is the safty issue you refer to?

I will do that search you mentioned.

Thank you for your comments and advice.

John

[pipes]

John.......... I have an 05 Eagle. When I raised the drivers floor (and the windshield) 16" That, raised a problem with the mechanical CLUTCH and THROTTLE.
                                   (I had already swapped the 4 speed with a 10 speed trans)

I took out the clutch & throttle rods and left ONE of the stick shift rods for the trans............

For the clutch I used a foot throttle valve plumbed directly to the slave cylinder (Master cyl now in the scrap heap)
using a pressure regulator and 14 psi I found that it was not easy to control movement of the clutch IE too fast,so I plumbed from the foot pedal EXHAUST  to a petcock mounted in the dash area to retard the the air leaving the air cylinder.    I now have the petcock adjusted where it does not dump the clutch so fast and is easyier to control.         > air brakes, air throttle, air clutch <

                                            PS NOT tested on the Hiway yet............ works good around the farm !

BTW I have a hidden valve to to shut off air to the clutch & throttle. Thiefs = SLOW getaway!

[gus]

When you guys talk about feathering the clutch I assume you mean releasing it smoothly?

I never dump the clutch all at once. In some cases this can break a universal joint, more likely to happen in a heavily loaded truck than a bus. Dumping at idle speed will usually stall a DD2S.

I never, ever slip a clutch on a diesel because, except for DDs, there is so much torque it will burn the clutch. For a DD I let the clutch out smoothly as usual and if it begins to die I release it and start over. On a steep grade or in a hole this can take a few trys but usually works. Same thing maneuvering in tight spots.

There is no doubt an AT is far superior because of the too high gearing in the manuals. However, the cost of conversion is too much for me - it could cost almost as much as my bus is worth.

[Fredward]

The MC-5 has an air cylinder integral with the clutch mechanism back at the transmission. As the driver pushes the clutch pedal down, the air cylinder begins extending itself thereby reducing the amount of force the driver has to us to depress the pedal. Once you get the pedal about half way down, the cylinder does about half the pushing for you. The cylinder always has air pressure supply and it only provides assistance when the pedal is partially depressed.

[RJ]

JohnEd -

The safety issue I'm referring to is the fact that with the fast idle on, the throttle pedal is "locked" - you cannot move it at all.  (If you have an air throttle, all the pedal will do is flop around.)

The problem here is that because you cannot control the throttle, you could easily get into a situation where the bus takes off on you, and if you don't have sufficient air pressure already built up, you might not have any braking available to stop a fourteen-ton runaway.  Think "Audi unintended acceleration" from back in the '70s'.

That's why fast idles are wired into the parking brake circuit - they're not supposed to operate when the brake's released, only when applied.

Whoever showed you this "great technique" is playing Russian Roulette with his coach, IMHO.



Remember that the buses that most of us convert into RVs were never designed with stick shift powertrains geared to do what many RVer's do with their rigs.  Highway coaches are set up to run down the Interstate, pull into a depot, back out of the level stall, and hit the highway again.  No wiggling into campgrounds, or trying to chase a cable car up any number of San Francisco's hills.  So, yes, you have to be careful when you're doing something that the coach was never designed for.  And be smart enough to not get into a situation in the first place where you run the risk of smoking the clutch.

Obviously, automatics have a distinct advantage in this regard, which explains their popularity.  But that doesn't mean that you should pass on a good coach just because it's a manual, unless your intended use will be hard on a clutch.


I also think that the air assist clutch mechanism on Zimtok's 4104 is one that's available from HB Industries  (www.hbindustries.com) out of Southern California.


Gus -

Yes, when I talk about feathering the clutch, I'm talking about using it smoothly to get the coach rolling.  Your description of dumping the clutch is also correct, and not recommended at all.


FWIW & HTH. . .

[Len Silva]

A dead throttle start works fine on a hard level surface but be careful on a hill.  Trying to move my 4104 in the yard on a hill without giving it enough fuel caused a bit of exitement.  I let out the clutch as usual but the bus didn't move, sort of felt like the clutch was slipping, then it started rolling backwards.  I gave it more fuel and it rolled faster, huge clouds of white smoke billowing out of the intakes and my wife screaming something unintelligible at me.

I had stalled it and restarted backwards.  It only went about five feet and it was all over in just a couple of seconds but my poor brain had a hard time wrapping itself around that one.

Len

[JohnEd]

Russ,

Your concerns are all valid.  I would build it that way too if I were giving it to God knows who with a "bus drivers license".  Really I would!  For me I would put an alarm and light on the dash to indicate when it was on.  Isn't the hand/emergency brake engaged when the air is low?  Isn't the fast idle activated by the same air that is used for the brakes?  I am not being cute with these questions, I don't know for sure.  I read that the fast idle was activated by a lower pressure than the main system ran at....60lbs?

Thinking on the safety issue it seems that the danger would be if you took your foot off the gas to slow down in traffic and the coach speed regulated at the fast idle speed and overpowered your braking.  Like the speed control won't disengage.  An "Audi like" problem, for sure.

Len,

Write Rippley!  That would have left me with a palpatating heart. 

That is the exact situation that the guy with the fast idle was talking about.  The grade is too steep for a dead idle take off so you engage the fast idle and let the "govenor" take care of the throttle and spare the clutch.

Holy crap!  What high pucker factor experience.

Thanks for sharing....I think.

John

[gus]

The last time I cheked HB Industries air clutch setup it was $750 without installation and is probably more by now.

[Len Silva]

The one they are selling looks just like the optional air assist on some Eagles.  I sold a take out for $100 a few months ago.  There are probably some laying around some old eagle barns.

Len

[Dallas]

The one they are selling looks just like the optional air assist on some Eagles.  I sold a take out for $100 a few months ago.  There are probably some laying around some old eagle barns.

Len

I thought Eagles lived in Aeries, or is that "Rustoleums" 

Sorry, I could have resisted, but I didn't!

[Stan]

There was a major discussion on this subject about a year ago. Try searching for 'air clutch', 'air assisted clutch' or something similar.

[JohnEd]

Stan,

I started this and I have already apologized for not having done that search.  I found 19 threads and some are 2005.  I have read that search data for over an hour and am still not finished.  This board is a great resource.  Still, I have picked up things on this thread that I haven't encountered on those other threads.  For me it was worth it and the contributors haven't wasted their time and breath.  I suspect others have gotten useful info from this and the references to the "history".

Thanks for bringing up the past threads as there is a whole pile of data distributed in them.

Thank you,

John