15 replies to this topic

[cheapracer]

Be careful on your answer!

Trying to establish info about the bias ratio if any of a mid to late 60's F1 car (GP car!).

I have tried many a place to get an answer including Hewland, Jack Knight and ZF but can't pin it down. I have even posed the question to Sir Jack and waiting for response as he used Hewlands exclusively then.

I am in the belief that the early ones were simple locked affairs and at some stage later some bias, around 80% was more common through machining different numbers of tracks/cams/slots in the outer and inner rotors and offsetting the pawls I'm guessing the action to be something in the way of a rotor oil pump.

1937 GP Benz used a ZF cam and pawl by the way.

But when?

There is a few Hewland parts manuals around the web but none give a clear view to how the rotors compare.

If you have a drawing or cutaway drawing side on that would be great.

There was a previous thread on it here but again, nothing conclusive ... http://forums.autosp...?threadid=51946

Hewland's suprisingly honest answer!..

Dear Cheapracer,

Congratulations on asking the question I've been dreading for a number of years now! Unfortunately I have no accurate answer for you. Our knowledge of the cam and pawl diffs doesn't even extend back as far as where they originated, let alone the mechanical operational properties of them. I believe that we must have copied the track and pawl profiles from elsewhere, without really understanding them ourselves.All we know about the operation is that this type of diff works on a very inefficient mechanical transmission which locks when the internal friction overcomes the motive forces. We understand from our customers that the characteristics of the diff change dramatically if the internals wear at all. I'm sorry I can't be more specific. I'll elevate your question on the (long) list of those currently unanswered. If we find an answer in the near future I'll be sure to let you know. In the meantime if you find out more yourself I'd appreciate being kept in the loop. Regards,

Andy Scott.

Design Director,

Hewland Engineering Ltd.

Tel: +44 (0)1628 827600

Fax: +44 (0)1628 829706

Please visit our website at WWW.HEWLAND.COM


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From: David mansfield [mailto:david.m@hewland.com]
Sent: 27 May 2008 08:08
To: 'Andy Scott'
Cc: 'Alan Ranscombe'
Subject: FW: 60's diffs

Can you answer this one please. Best regards Dave MansfieldHewland SalesTel +44 (0) 1628 827600Fax +44 (0) 1628 829706email david.m@hewland.com

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From: Cheapracer
Sent: 27 May 2008 04:38
To: enquiries@hewland.com
Subject: 60's diffs

Hi Guys :-)

Can i trouble you for some info please?

Around 1965 to 1967 the Hewland gearboxes used in GP racing (Brabham etc) used a cam and pawl LSD.

I am trying to understand if the diff when the pawls locked in was 100% locked or had some bias/slip between the wheels?

Information about this on the net is greatly varied with many different opinions and even a quote from Carol Smith saying that 75/80% bias was normal but what years unknown.

Can you clear this up, many would be appreciative!

Thanks for your valuble time.

[Fat Boy]

Originally posted by cheapracer

Andy Scott.

Design Director,

Hewland Engineering Ltd.

FWIW, you contacted the right guy. If he doesn't know the answer, though, you're in trouble. RDV might have a good stab at it, but the truth is that things just weren't measured in those days like they are now. The only way to get the answer might be to find one and test it.

[Bill Sherwood]

Talked to a mate of mine who worked for Repco-Brabham in the late 60's/early 70's and he reckoned that a fair few used open diffs, and most of the rest used pawl types, that locked as soon as there was a bit of slip happening.

[NRoshier]

tried Trans-X or Xtrac?

[murpia]

See below for analysis details of a Honda diff used in an ATV which is cam and pawl principle. The pawls shuffle laterally in this design, I think they shuffle radially in others.

Used in the 2004 Honda TRX350 4WD ATV, probably in others also.





From what I've heard, the characteristics of any cam and pawl 'diff' are very dependent on the amount of wear, as Andy Scott has stated above.

Regards, Ian

[phantom II]

I recently had a ground up restoration on my Ferrari 250 Lusso that got hit by Wilma a few years back. It has the identical mechanicals of the famed SWB 250 GTO. The differential has a limited slip and locking mechanism inside about the size of a base ball. It can be taken apart and assembled by hand. Not my hand though. I was shown how to do it 3 times and I still couldn't do it. A Rubik cube is easier. I just have a picture of the outer casing but I will see if I can locate the original part. You can buy any part of any Ferrari ever made. It has five ball bearings about 3/4" diameter. They each fit into a tapered slot machined into the surface inside the sphere which is made of two parts. To assemble the unit, you move these two separate parts into a certain position which accepts one of the balls. Then you move it another way to accept another and so on. It all locks together somehow and I'll be damned if I can take it apart or put it together. This fits into the crown wheel carrier without bolts. Italians make the most beautiful machines. It is a lovely thing that you could put on your mantel piece. If I weren't married, my house sure would be decorated differently. When torque is lost on one wheel, the balls jam in their respective slots which lock the spinning side shaft to the one half of this splined sphere. What is this called? It works flawlessly.

[jpf]

Murpia -- great diagrams, thanks. That looks like the same system described as AP's Suretrac unit on this page:
http://www.minimania...40/ArticleV.cfm

... which appears to be the source of some excerpted quotes in the other thread that cheapracer linked to in his opening post.

[scooperman]

that description of the Ferrari differential sounds like a Briggs and Stratton rope recoil starter mechanism.

[Greg Locock]

I wonder which is more reliable?

War story: certain Italian carburettor manufacturers used to religously buy B&S engines and reverse-engineer them.

[Ray Bell]

Hewland copied the ZF arrangement, as far as I know...

We had these in BMC centres when we raced Clubmans. At least when we were allowed to run them when we raced Clubmans.

[Engineguy]

Originally posted by phantom II
I recently had a ground up restoration on my Ferrari 250 Lusso that got hit by Wilma a few years back.

I'll bet Fred was pissed. After all, the low rates he got by switching to GEICO were the secret to him living beyond the salary he recieved from Mr. Slate. If GEICO raised his rates... no more big rocks for Wilma!

[McGuire]

Originally posted by phantom II
What is this called?

A ball-type diff, incredibly.

[RDV]

Ah, a trip down memory lane...
The cam and pawl diff allows a limited amount of slip between the wheels, in effect there is a slot difference between the inner and outer tracks, (or left and right in the case of the lateral shuffle type), and the pawls shuffled back and forwards ..er in and out really...cant remember the ratio now, but think in the range of 10%(..dont hold me to this...I'm trying to visualize 40 years later )
Relatively trouble free, as long as the pawls didnt get too worn...very quickly thrown out as soon as the ramp and plate lockers came in.

[cheapracer]

RDV I've only seen unclear pictures but imagined that they "walked" around as you describe to give some difference in wheel speeds.

I'm still not clear though, in Hewlands pictures the outer drum looks as though its secured to the crownwheel which would mean 1 wheel is fixed drive and the other walks at the different speed including going faster when its on the outside in a turn.

This could be an earlier box though which I am aware of being 100% locked on takeup.

You couldn't draw a bit of a sketch could you?

Thanks Phantom11, that probably explains what Fazza used for their 60's GP cars.

I'm also waiting for a promised reply from Jack Brabham himself. I even had a letter of apology for the delay (he's overseas apparently).

[britishtrident]

Jack Knight cam and pawl diff were used in racing Imps -- radial arangement of the pawls. When you stripped them down it looked really Heath-Robinson but they worked really well with nice soft action, as a result they gave a considerable improvement in handling on tight bumpy hairpins.

To clear things on how this worked it was the slotted cage the the pawls ran in that was attached to crown wheel, the inner cam drum was attached to one halfshaft the outer cam drum to the other. Really just a concentric version of the Honda arrangment.

[britishtrident]

URL http://www.minimania...ges/jk_pawl.jpg


http://www.minimania...40/ArticleV.cfm

Jack Knight also have info on their website