<VV> Flat tow

Wed Mar 12 17:08:58 EDT 2008

/At 09:37 PM 3/11/2008, you wrote:
I towed a 4 spd from Buffalo to Va Beach behind a U-haul on a tow bar. No 
Problem!  But, I believe I read in Richard Finch's bookI(or was told about 
20 years ago) that you should start the engine and run the gears through the 
pattern a few times at gas stops.
      Could be imagining that, but I think it had something to do with the 
straight drive diff and trans sharing fluid.   

Actually no, the lube share has no function as far as the transmission's towed longevity is concerned.  

It has to do with the fact that the gearbox mainshaft will spin as long as the rear wheels turn.   When in neutral with the mainshaft spinning as in being flat towed, the idlers on the mainshaft will require some lube because they do *not* spin with the mainshaft unless the transmission is in gear whereupon one of them (depending on which gear you're in) will spin with the mainshaft which in turn will spin the cluster gear which will pick up lube and sling it all over the mainshaft and everything else inside the gearbox.   When in neutral while towed, the stopped engine with the clutch engaged keeps the input shaft still which makes the input and idler gears just sit there motionless and the only lube between their bushings and the mainshaft is what was left there by the cluster gear splash the last time it was spinning.     This is where flat-towing a gearbox car can give you some trouble... if the car is inop and has been sitting for 7 years without running and most of the gear lube has long since weeped out from between the idlers and the mainshaft...  THAT is a recipe for burning up bushings if you tow it 100 miles without some sort of trickery to lube things up.   

Putting the "dead" car in gear and rolling it around a bit with the clutch pedal pressed, will spin the input shaft via the mainshaft's turning while in gear and thus the cluster gear will pick up lube and wash the mainshaft with it, oiling things up nicely.   You have a LOT better chance of doing a 100 mile flat-tow without any damage than if you just take off cold-turkey in a car with dry bushings in the gearbox.     Works for me...  :)   Of course, one should make sure there's adequate lube in the transaxle...  ;)

Now, with a fresh splash of gear lube soaking the mainshaft and oozing onto the idler gear bushings, there's plenty of lube for a pretty fair stretch of flat-towing, particularly if you stop every 30-40-50 miles or so, for insurance, and start  the towed vehicle's engine while in neutral  (or push it around in gear, clutch disengaged as per above) which will spin the cluster gear and splash more gear lube everywhere.      

Now, if the mainshaft has some lube on it already, you may well get away with towing several hundred miles without "splashing" the transmission, but I'd not wanna make a habit of it...  I *have* seen a gearbox with one of its idler gears welded to the mainshaft which demonstrated a skidding of the back wheels when it finally seized up, which ended up causing my buddy to have to wedge a length of board between the clutch pedal and the bottom of the dash to disengage the non-running engine from the transmission to get the car the rest of the way home seeing as how the transmission was artificially "in gear".       

That was when I first began looking into just what had happened and why...   ;)   

Last time I flat-towed a gearbox vehicle (Corvair "dunebuggy" chassis), it was with an inop engine attached to it.    I had access to the top cover of the diff and we popped it off, and simply poured a few quarts of cheap motor oil into the diff to overfill it, which raised the level of lube in the transmission high enough to keep the mainshaft greased without having to rely on splash.   Over 100 miles of towing on I-81 and no problems, no issues although I did stop and check along the way, transmission/differential was warm to the touch but that's to be expected.   I was gonna drain the transaxle in any event afterwards, so the motor oil in the diff was no big deal.   Again, it was cheap insurance... although it did blow out a fair amount of excess lube from the diff's top vent as well as oozing some out of the less than outstanding axle seals...  but not a lot.   It was still pretty much full when I got everything back home.    The end result was that I got everything transported with no damage and no problems without worry.   

Powerglides can be flat towed anywhere forever without any real problems as long as you DO NOT SPEED.   They have two pumps and as long as the back wheels turn, so does that other pump which keeps ATF flowing through the transmission, greasing things nicely.   I've seen Powerglide 'Vairs prudently flat-towed over a thousand miles without issue, on the Interstates.   No overheating, no problems... and those who would mention that the ATF would overheat because the converter (cooler) isn't spinning should remember that the majority of heat in a Powerglide is generated inside the converter in the first place as the engine pulls against the converter and churns the ATF which generates the heat.     This doesn't mean it won't generate heat while being towed...  just don't keep the throttle mashed, it's not a race, keep the speed down...  and *Check* the transmission now and again.   Look for overheating, can't hurt.    

Heat generation in a towed Powerglide is (relatively speaking) negligible IF it's towed at moderate common-sense speeds.    Just make sure there's ATF in the transmission before you decide to flat-tow it across two states.    

Note:   All this is strictly my opinion and observation.   

tony.. 