Commercial Vehicles Rear Ends

Posted by Joe on July 09, 1998 at 07:47:01

In Reply to: Re: AA ring and pinion gear change
posted by Dean West on July 08, 1998 at 13:46:07

: I have a 1929 Roadster pickup that has been restored. I'm not sure what the gear raio is. Does any one know what the stock gear ratio was? Did the trucks come with a lower ratio that the cars? It seems to like about 40 MPH and does have a tremendous amount of pulling power or lots of torque.
Dean
The "standard" Model A rear end has gears that result in ratio approximately 3.78 to one. (some variations slightly here asFord varied the gear ratios within a tooth or two during production)A lot of restorers are satisfied with this ratio as it yields a car with a reasonable pickup but does not require an excessive engine rpm to maintain 50+ MPH.
Some restorers who find their rear ends worn or require gear replacement will use a replacement gearset now available that is 3.54 ratio.This yields a slightly higher top end speed without excessive engine RPM but at a slight sacrifice to acceleration.I have this gearset now in my CC (Closed Cab) pickup and it's a little slow on the uptake but I can comfortably cruise at 55 mph.
Some Ford "commercial" vehicles (including my CC pickup prior to its rear end rebuild) have a 4.11 rear end.This was done for the purposes of improving it's ability to carry a load.Pickup is great, even with a load, but top end is about 45 MPH and that's pushing it.I would include your Roadster Pickup in this category as well as the CC pickup, some station wagons and the Special Delivery Bodies.
I have been told you can determine which rear ends have the 4.11 gears by looking at the "torque tube" which connects the banjo housing to the rear of the transmission: In the standard rear end the torque tube is straight sided for the rear 2/3rds and a rather abrupt taper at the front 1/3rd to the diameter where it connects to the transmission.In the 4.11 rear end, the torque tube tapers uniformly the whole length from the banjo housing connection to the connection diameter at the rear of the transmission.This differentiation may not be absolute since otherwise the rear ends are identical and a gear swap or torque tube swap is always possible.
You can positively determine your rear end ratio with the car stationary and one wheel blocked up.With the transmission in 3rd gear, use the starter crank to turn over the engine several or many turns (with the ignition off!) moving the rear wheel as well.You can then keep track somehow of the number of turns the upended rear wheel turns around.A little arithmetic gives you the ratio.
The bugaboo for any Model A on modern roads is the limited car speed before your engine rpm limit is exceeded.The Model A engine was good for its day but today is somewhat restrictive in that with only 3 main bearings, a rather limber (thin) crankshaft, and babbit bearings, the engine does not want to go especially fast due to a semi-harmonic bending of the crankshaft in operation that tends to "wipe out" the center main bearing.
Some of us get around this problem by substituting a Model B engine for the stock Model A engine: the engine overall is beefier with a higher top end and with only minor external appearance changes will substitute directly. (Downside is cost unless it was done by a previous owner years ago when a "B" engine was $15 from a junkyard)
Some of us get around this problem during an A engine rebuild by substituting a Model B crankshaft for the Model A.This gains the advantage and beefiness of the B engine with the external appearance of the A.Downside again is cost (perhaps $700 not to include the rest of the engine rebuild).
Some of us get around this problem by fitting our cars with accessory overdrives which give the car the additional speed for highway driving.Downside once again is cost (about $1500).
Some of us (including myself) get around this problem by retrofitting our cars with the high ratio (3.54) rear end gears. ($254 plus labor plus the acceleration downside I mentioned)
I have often suspected that the best solution would be to maintain the 4.11 rear end with a rather "long" overdrive which are available and by this means retain the best of both worlds.If you wanted to carry a load or have a reasonably quick acceleration from a stoplight, you could do direct drive.On highways the overdrive could be brought into play.
Wherever you end up, even to keep it stock, just watch your engine speed: you'll know instinctively when you've pushed your luck too far with the engine.The "semi-harmonic" bending of the crankshaft once the center main is wiped out is instantly recognizeable and you'll know when you're getting close if it isn't.Keeping the engine "tight" by removing shims on the main bearings periodically will improve your high end response too.
Hope all this helps,
Best regards,
Joe

Follow Ups:
Re: Commercial Vehicles Rear Ends Dean West July 10, 1998 at 14:48:15(2)
Re: Commercial Vehicles Rear Ends John Icenhower July 11, 1998 at 01:05:07(1)
Any and all information gratefully accepted. Joe July 11, 1998 at 21:56:55(0)

E-Mail Comment to Webmaster