Yet another thread on Overdrive

[John Murch]

My 3ltr has a high axle ratio (3.54) and an 'A' box so is great for fast country roads. Not so great in the mountains and hills where a lot of my driving is done and frequent changes into first are called for (not a problem).
However from time to time I'm faced with a very steep hill start and on occasions have thought that I would not get out of the situation, too much clutch smell for my liking.
So, what to do? more torque (in the form of 4.5ltr block); lower axle ratio, say 4.23 and an overdrive, or could one alter the 1st gear ratio in the existing gear box? Not keen on that as it would create a bigger gap between 1st and 2nd and slow the gear change.
I know that there are a considerable number of overdrive units out there working well, but I do worry a little about their power consumption, (ok fit an alternator or uprate the dynamo) and they are quite complex.
What are the thoughts of you clever engineers out there - could a small high/low gear box be made, one that you would need to stop the car in order to make the change? Is this what 4x4 vehicles have? The low ratio only being selected when appoaching the mountainous bits.
Could such a thing be considerably smaller than an overdrive and thus also have a market for MkVI and R types?

[John Robins]

There used to be a device called the Handa Underdrive, beloved of caravanners in the late 50s/early 60s. I know no more than that, but somebody might. Four wheel drive transfer boxes would be too big one would have thought.

http://pad.sagepub.com/content/9/1/71.abstract

[John Murch]

Thanks John, I can only find fleeting references to the HandA unit, usually associated with 100E Fords, but if they were offered as after market items for people towing caravans, one would assume that they offered a beefier version too.
I came across a unit made by Haystee, based on a 1970's Santana item.
Looking at the exploded parts diagram it seems just as complex as the Laylock od! However it has a mechanical operation rather than electrical. It would need modification to be fitted to our sort of cars.

http://www.heystee-automotive.com/webstore/index.php?route=product/product&product_id=97

John

[James O'Neil]

John,

As these and similar units are designed for a Land Rover, with its offset rear differential, matching them to a vehicle with a central rear diff would probably require major surgery. But I've bought several items from Paul Heystee over recent years and wouldn't hesitate to recommend his company.

Regards,

James

[Paul Spencer]

Hi John,

As you probably know, I have 4.23:1 plus overdrive and find it ideal. On tours I carry a short length of prop-shaft that can be slotted in in pace of the o/d, but I have never had to use it. I think it is only the more powerful cars that have a problem. Happy to take you out any time.

I once asked Brineton about changing (raising) the first gear ratio in my BS box and they said it was only possible to change the full set for some technical reason I didn't understand. Perhaps it is possible on an A box. You could ask them.

Paul

[Stephen Blakey]

John,

There's also the American Mitchell Model 1000 overdrive "Manufactured for open driveline vehicles built in the 20’s and 30’s." (They've got a website but it's a bit rubbish)

It would be a right old carry-on getting one shipped to Europe and finding someone to install it, but I thought I'd mention it for completeness.

It uses a pair of gears, variously described as GM or BorgWarner and a mechanical change so there should be no current consumption.

Nigel Stennet-Cox has a Mitchell on his flattie and speaks highly of it. He told me he is no longer a BDC member but he'll be in the book.

Sometimes the best bit of a project is the planning and dreaming. Enjoy.

[John Murch]

Stephen,

As you say, so much of the fun is in the dreaming and planning, I've always got plans for routes and modifations that will probably not happen, but I do so enjoy the research process and the information elicited may be of help to others.

Thank you for the information on the Mitchell unit, I have emailed them for more details.

The web site is http://www.mitchelloverdrivemfg.com/our-products/

The Heydtee unit that I mentioned in an earlier posting is totally unsuitable.(so the manfacturers tell me).
John

[Christopher Carnley]

There is a pdf picture of one on flatheadv8.org/mitchell, however I know that Mitchell ceased production of this unit in 2000, as truckers buy automatics.

I had a 1952 Ford F7 five speed gearbox in last year and an F5 4 speed overdrive gearbox in early this year, with the cast iron cases, and large gears, and an ex GM synchromesh system, much better than any of the old Bentleys.
Heavy man, it took too big lads to shift them!
Apart from an odd corroded ball bearing or two and one ropey Hyatt caged roller, they were good for another 60 years and another million miles.

I had a call from the US a couple of weeks ago regarding the suitability of the R Type manual for the Speed 6, just for the synchromesh system.
I explained that not only was the R Type not an independent unit, and would require an enormous amount of adaptation, but where would they find any replacement parts (Me!)?
(I have had an R-R 20/25 gearbox highly modified to suit a Lagonda LG45, the winner of the past 2 RAC 1000 mile thingys, for the ratios, in for a rebuild).
I suggested that they find a suitable firm (Tremec) to make them a new gear train incorporating the GM type synchros, but warned the caller that it would be difficult to impossible to fit the lot into the Speed 6 casing.

[John Murch]

[Christopher Carnley]

It is like learning a new language, the 6 speed Porsche type synchro box driver, can "snick" up and down, without all that dratted speed watching and double (de-)clutching in the Panhard devised box of delights(?).
The synchromesh system was only ever seen as a stop gap measure between the sliding gear/dog boxes and the fully automatic ones.

[Stephen Blakey]

John,
Please let us know what, if anything, the Mitchell family have to say. You will have spotted that they also do underdrives which is what you were originally asking about.
Good luck,
Stephen

[John Murch]

I'm sure that this unit would fit well in the prewar cars, I think that it is quite a bit shorter than the Laylock units, so could it fit a Mk VI?

John

[Paul Spencer]

I have a Laycock in the 3 Litre, and I would say that is marginal on length. Shorter would be better. As a result, my input shaft only has a single u/j and no spline, which I think is a bad engineering solution (someone more knowledgeable is welcome to chip in here). There is rubber in the o'd mounting, but not much movement.

[John Murch]

I think that I read somewhere that the laylock od is circa 17inches long, the mitchell is 10". However these measurements may be wrong or taken from different points, beware!
John

[Paul Spencer]

I have just been out to the garage in the rain. 13.5" between the two propshafts for the Laycock. I think there are two models, but I am not going out again to check which mine is.

Paul

[John Murch]

So not that much different in size.
The possible advantages of the Mitchell is
1/ ability to use it in any gear, so if you have a low 1st ratio, 1st od may be attractive.
2/ no electrical drain.
What would the disadvantages be?

I understand that you can also buy it set up as an underdrive, which is why it initally caught my eye.

John

[Paul Spencer]

I can only see two potential disadvantages. The first is just that you would be a pioneer. The other is the lever gearchange. Could you hide it?

My first gear would be better higher, but if you used the Mitchell unit in first, you can't really swich it out on your way into second. I wouldn't use the Laycock in first - too much torque. I have the cut-out to only allow it in top. Some people allow third as well.

[John Murch]

You don't have to have a lever gear change, a cable push/pull is also available.
There are lots of these units in use, dating back 20 years, so I would assume that the engineering is sound. But fitting one to a Bentley might be a first, although I don't see any reason why one couldn't use the same style of mounting brackets as the Laylock unit.
I suppose that you could switch it out between the first and second gear change (its got synchomesh!), but that change on a B bix would still be a bit slow.
John

[Christopher Carnley]

That push pull arrangement is similar to the overdrive fitted to the VDP 3 litre Princess,3 speed. Beware the load on the GM type synchro detent

As I mentioned previously, is this Mitchell device still being made, as I believe that it was discontinued in 2001?

The de Normanville type should only be used in top gear, for obvious reasons, ie it is not up to the torque in lower gears, boils the fluid and is an "overdrive" 5th gear substitute.

[John Murch]

Chris - I have been in telephone and email contact with Mitchell's and it is most definitely available, very helpful and informative people.
They offer quite a range of both overdrive and underdrive ratios. I think they stopped making the truck version in 2001.
Quote:
"Refer to as Model 1000, and it has always been rated at 10,000 pounds GCVW."
But I am not entirely sure how to interpret Gross Vehicle Weight Rating. As my 3ltr weighs circa 3200lbs I assume that the unit should cope with the available torque even in 1st gear.
John

[Christopher Carnley]

John,

Good to know that, now it is all a question of adaptation and practicalities. The underdrive facility would appear (on paper) to be a less costly exercise than a gearbox redesign and then being faced with a long wait for the parts to be made and tested. (Tell me about it!).