The Stovebolt Site Forums The Shop Area Driveline Redneck "overdrive"

I'm playing around with an idea, and I'd welcome some input, pro or con. It probably won't get beyond the "drawing board" stage, but it sounds like something that just might work.

We purchased a mini-motorhome recently made on a 1-ton van chassis with a raised fiberglass roof, small kitchen, shower and toilet facilities, and a couch that folds out to become a doulbe bed. No, it's not a Chevy, but bear with me- - - -I'd like this post to remain in the "driveline" forum if possible. It gets 8.5 MPG on the interstate, and a lot lower MPG in town! I'm thinking of mounting a 3-speed standard transmission on crossmembers immediately behind the C-6 automatic transmission, facing backwards where 3rd. gear will be direct drive, and a shift to 2nd., either by vacuum or solenoid linkage will become an "overdrive". Fabricating the necessary drive flanges and crossmembers to make the driveline hook up won't be a terribly difficult project, but I'm concerned about geartrain noise from driving the cluster gear backwards to its normal rotation.

Comments, anyone?
Jerry

I've heard it being attempted but never heard any results...Can't believe it would be a long-lived set-up running on the back sides of the gears and loading the bearings the opposite way, but who knows until it's been done,
Scott

I agree with Scott, running the cluster backwards (in any gear besides 3rd) would probably create some issues. What about a Gear Vendor OD or a Ford E4OD tranny? Also I often see some "Brownie" boxes on ebay. Those would be super stout for your application.

SimS

How are you gonna clutch it to make the shift? I see it coming out of third but getting stuck in neutral for lack of synchronization.

How does a 2-speed rear axle work? I've made thousands of air-operated shifts with big rig transmissions of all types without clutching. Just select the gear, back off the throttle momentarily, and it shifts. I'm thinking of using a vacuum chamber and a stout spring to the shifter linkage. Preselecting the gear stretches the spring, and when the power is interrupted for a moment the shift gets made by the spring. Ditto for the downshift, with a vacuum chamber and a spring pulling the other way.

On the subject of reverse-loading gears, lots of Volkswagen transaxles got the ring gear flipped over for Corvair engine conversions, and ran many thousands of miles afterward.
Jerry

HRL;

Two speed axles work with a planetary gear set with a sliding "clutch" gear. So there are different set of circumstances vs. a synchromesh/cluster gear set up in a transmission.

I'm thinking that your idea of shifting will probably work. We all know that you can shift without a clutch if you match the RPMs, reduce torque, etc. The thing that I don't know about is running the helical cut cluster gear backwards with a vehicle load.

You are correct about the VW/Corvair setup but that gearset is a hypoid design and the GVW is/was a lot less than an RV.

I think your idea will work in theory but my only concern is the longevity when the unit is in "redneck overdrive" under load for long periods of time. You should make your conversion "bolt in", keep the original driveshaft and "GIT-R-DONE"! It may work just fine.

SimS

Most of the OD transmissions of the 50's were .70. I'm afraid a 3 speed running backwards in second would be too much overdrive. It might turn out to be a real dog behind an automatic. The old BW OD units were really tough. If your are thinking of something like a divorced auxiliary, I think mounting something like a T85, T86 or T89 behind an automatic would work and you could shift into OD electrically. There used to be a feed mill that used a transmission for a speed reducer. They bolted a home made adapter to the front of the transmission and cut the input shaft to accept a Torrington bearing. They then put a pulley on the end of the input shaft and ran a feed mixer with it. It ran for years and may still be running. There are a lot of old Ford OD transmissions around. You might even be able to put a straight shaft through the front part of the box and support it where the input bearing goes. Any way, I think a BW overdrive unit would be the way to reduce rpm by 70%. It might even be possible to build an adapter to totally eliminate the front box and use the OD unit as a divorced unit.

Exactly like this
http://www.gearvendors.com/

Have you checked the price on that gear vendors unit lately? I've got another option, adapting a 4L-80e GM transmission to the 460 engine. It will cost about $1500.00 by the time I buy the trans, get the dedicated computer for it, and do the adapting. The Ford AOD transmission just isn't up to handling the torque of a 460- - - -back when I maintained the police cars for the Pulaski, TN PD, the 351W engines ate up AOD's all the time.
Jerry

You'll need an E4OD behind a 460, Jerry-the powerstroke diesels are torquey enough to turn the torque convertors blue, but they still hold up. It's based on a C6, instead of the C4 like the AOD.

Thanks for the heads-up. What vehicles came stock with the E4OD?

I've got another idea, which might be simpler than a transmission swap. The suggestion about using the B-W OD as a divorced unit reminded me that I've got the remains of a Volvo 4-speed transmission that was married to a Laccock OD unit. The OD is gone, but there's a few of them on Epay worth the money, and a bunch less than a Gear Vendors unit. The Laycock was used behind the Austin-Healey I6, the Jag, and a few other torquey engines, and it's got its own built-in hydraulic pump. All it takes to make it work is an electrical hookup. I can use the Volvo output shaft to build an input for the Laycock unit and mount it on a crossmember behind the C-6. It would take a little fabrication work, but nothing that a lathe and a welder can't handle!
Jerry

They came behind diesel & 460 rigs(3/4 ton-up), prolly 88-96 or so. Prolly all over in used small van-based buses/U-haul trucks.

Can't wait for the pictures, sounds good on paper. Anybody have a Laycock od picture?

Cosmo;

This is a Volvo Laycock OD for their 700 & 900 series cars. I think early on the Gear Vendors units used something similar.

http://media.skandix.de/pimg/1004377.jpg

SimS

Here ya go- - - - -I think Gear Vendors uses the same unit with some adapters for their setup, and bumps the price up 4 or 5 times.

http://www.hunt101.com/data/500/laycock_OD.jpg

I'll need to use the output shaft from the Volvo transmission with the pump drive eccentric built into it, and make an input flange and bearing carrier for it.
Jerry