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Cam & Pawl Differential

Cam & Pawl Differential

Cam & Pawl Differential

I hope it would be OK if I asked a few questions about cam & pawl differentials. I've always sort of fancied them, but am not sure I fully understand how those infernal contraptions work... And not much in the way of resources is available for them- an odd mention here or there, and the best one can hope for is description of their workings (but those often also include discrepancies, even conflicting 'findings', &c). While Stokes is rather complimentary about them, the rest of sources describe them with almost anything short of brutish, unreliable, nuisance, and so forth... But if they had lasted in Grand Prix and Formula 1 racing for almost 60 years (they were AFAIK introduced by Mercedes or Auto Union in early '30ies, and Stokes reports them still being used in F1 in '90ies) would suggest some merit in the design which the rest of authors usualy fail to register.

First of all, I came to think of them as not being differentials, in the purest sense of the word, but rather as spools which allow a certain amount of slippage on the inner wheel when neccessary. In a sense that pawls that engage the lobes will transmit torque to both sided equally (and the way I see it, with minimum to no loss at all), but if excessive 'drag' occures on one cam, it will be allowed to slip (with increased friction and other losses on that side alone). Sort of like spool which can act as a differential when forced to. (This in a way may be in consistency with one of the earliest descriptions I found, by Pomeroy, which reports them having great losses in low speed corners- which did not matter much with overpowered beasts of the day, and was even supposed to be beneficial to handling by making cars less throttle sensitive, and hence less prone to spin out when applying 600BHP through skinny tyres in hairpins).

Now, this does make sense to me, but is also a source of confusion. That would mean the diff will have understeering influence on the car, up to the point where differential action occurs and it switches to contributing to the oversteer. Which should be, first of all, opposite of how I percieve other LSDs behave- being neutral/oversteery in normal operation and progressing to understeering influence as they lock-up. It seems to big an issue not to be picked up on by somebody- from Stokes who is transmission guy, or say Carroll Smith whose drivers might have noticed it, esp. when authors speak of LSD more or less abruptly coming into action. (Sorry for introducing handling and stuff to the proceedings, but I feel the differentials are usually 'underprivileged' components- chassis books rarely make a mention of their influence, while transmission books reduce them to the perception of few gears attached to the bits that matter. I choose to take interdisciplinary approach, with one inseparable from other.)

Another thing that I would be keen to learn more about is the lobe design. Almost no mention of it is to be found in literature, IIRC save one source which implied black art being used in conjuction with trial and error method evolving to the current design (again, no shape of lobes mentioned- only 13 lobes on outter cam, 11 on inner and 8 pawls)...

Any info, or your thoughts on the subject would be most welcome. Thanks in advance.


RE: Cam & Pawl Differential

Is this what is otherwise known as a "Detroit locker"?

RE: Cam & Pawl Differential

Brian- I don't think so... Haven't seen Detroit Locker, but Carroll Smith makes a distinction between the two- and in his unmistakeable style, while not being very flattering about ZF ie cam and pawl diff, calls Detroit Locker an abortion and better only than an open diff, adding that the only way to run is to remove a cam and run it locked.

Whatg I also failed to mention in my long-winded opening post is that it seems to me that cam and pawl will allow slipping of cams only by having eccentricity or 'play' between cams. Otherwise I see no way of diff not locking fully when two 'peaks' get in contact with a pawl. Am I correct in assuming so?

RE: Cam & Pawl Differential

I suspect that it operates similarly to a Detroit locker even if the details of it are a little different.

Basically the torque gets sent to the slowest-turning wheel, and a wheel that wants to turn faster, overruns the mechanism and is allowed to spin faster but does not have any torque sent to it.

In cornering, this has the effect of applying torque to the inside wheel and none to the outside wheel, which indeed applies a straightening torque to the chassis (feels like understeer). If the inside wheel breaks traction and starts spinning, only then will the outside wheel be driven.

(A spool would mechanically link the two, and in this situation the inside wheel would be applying drive force up to the traction limit while the outside wheel would be applying braking force up to the traction limit - at the same time.)

The Detroit lockers indeed seem rather coarse and primitive in their operation. I've only ever seen them used for off-roading and drag racing.

RE: Cam & Pawl Differential

Mr. Petersen, I do hope you don't mind me saying so- but I do hate you. Not only does your post make mine look like a load of gibberish, but it makes me wonder why are you so right and I so wrong- the description you give is fairly consistent with observations, so now I'm supposed to figure out why the differential letting the outer wheel 'overrun' is more 'in tune with reality' than letting the inner wheel 'drag', as I *ass*umed. Pls. let me sleep on that and see if I can find a reason for error of my ways. :)

P.S. if one can be glad of making a fool of oneself, this seems as a good example as any :D

RE: Cam & Pawl Differential

After the apex, under steer might be an advantage. You need to generate polar moment moment to turn in and reduce it to line up with the exit.

Without looking it up, I thought a Detroit Locker had a dog clutch action and if there was a set difference in wheel speed or a set difference in amount of relative rotation independent of speed, some ramps or cams locked the dogs and the rear axle halves locked with zero differential action. They where certainly used extensively in group C circuit cars here years ago. They may now be replaced by viscous couplings or worm gear types for more sensitive control. Their main advantage was a reputation for durability and consistency and the fact the axle was locked which gave a handling characteristic that suited a flamboyant tail out driving style. Tail out, both wheels driving, full throttle was a fairly fast turn exit strategy if somewhat unkind to rear tyres. They where either fully open or locked with no transition action.

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RE: Cam & Pawl Differential

Seems like (and this is from the-days-of-my-youth) the "detroit locker" rear end had a tendency to "hunt" in & out of locking mode in the turns, depending on power applied. ie, back off the throttle in a turn (lose your nerve, whatever)and the car would swap ends with you. I think the esteemed C. Smith would call this power oversteer?

Worked good at the street light gran prix, and was definitely a bragging point at the Dairy Queen later.  

RE: Cam & Pawl Differential

TTF, it cannot be 'power oversteer' if you 'back off the throttle'. Roll oversteer, perhaps and that speaks a lot to the chassis setup and not necessarily to the type differential being used.

I've used Detroit Lockers since the mid 60's and, I suppose I've become accustomed to their quirks, not all bad either. Won a lot of races, set five lap records at various tracks and we are still using the very first one I bought in our vintage Lotus Cortina.  However, my son owns and races the car now and he does NOT like the Detroit...Different strokes, etc. He much prefers a 'spool'.

I use a Tran-X clutch type in my fwd Mini and a 'spool' in the Frankland quickchange of the streetroadster. I've driven a race car with a Salisbury and found it "normal".  I'm not sure what I was supposed to perceive as "much superior" as Mr. Smith put it, to the Detroit.



RE: Cam & Pawl Differential

Thanks for clearing that up, Rod, and thanks for the link. I'm familiar with Bill Sherwood from his Toyota 4AGE stuff.

RE: Cam & Pawl Differential

Sorry for not *yet* having fathomed why I got the things 'other way around' regarding slipping/overrun of the diff.

As for Detroit Locker, if it's as this picure depicts, I would say that is seems quite similar to axial cam and pawl differential (have seen the picture somewhere, might've been '42 Automobiltechniches Handbuch, but there I seem to observe a slight, albeit not at all insignificant, difference. Namely, that Detroit Locker would go from no lock to full lock almost instanteniously (sort of step curve), courtesy of 'lobe' vs 'tooth' design, whereas ZF types, both depicted in my post and axial ones, provide a gentle, continous characteristic*. The way I see it, in ZF type overreving wheel will still get torque, but reduced, because of friction on lobe shapes (as well as considering those would be the key to understanding the fundamental mistake I made earlier that Brian made obvious).

The reason I'm troubling you with all this is that a member whose opinion I respect immensely once said that they work very nicely (or something to that effect), and even Carroll Smith admits they work 'pretty darned well' when 'working properly'. What detriments their performance is constant wear, and I believe new fancy materials could address that issue. But before considering improving something (or even hoping to do so), I'd think thorough understanding of the operation would be in order...

* OK, I might as well admit why I fancy them so much- they seem much smaller and more compact than other designs (a racing cam & pawl diff is reputed to be the size of tuna fish can), purely mechanical operation, and that it seems sooo deceptively simple,yet delicate design is apparently needed. The other reason may not be so 'fancy'- in a way I percieve them as exact opposite to Torsen/Gleason diffs which try to operate as open diffs yet on account of high worm gear losses provide locking effect; whereas cam & pawl tries to work like a spool and looses efficiency when differential action occurs... actually, if I was to use strong words like Smith, to which I've not earned the right, I'd call Torsen method of operation as abominable, as evidenced by their swithch from MkI to MkII.

Another difference to Detroit Locker seems to be that it Locker may have more evenly spread torque transmission (should be a good thing) among its 'teeh', whereas with cam & pawl no more than 3 or 4 pawls transmit the torque, which makes even the analysys ever more so complex, but OTOH allows some pawls to pick up torque transfer when others slip, giving much smoother characteristic...

(sorry for another long-winded post)

RE: Cam & Pawl Differential

I believe Mr. Petersen's description is correct.

I recently had a detroit locker apart and I took some photos of the mechanisms and tested its operation when it was installed.

The only thing I didn't understand was that once the overspeed wheel unlocks it freespins until it falls down to the speed of the driver.  Looking at the design I expected it to constantly break, relock, and break as it climbs over each cam.

This one came out of a truck that the area police used when chasing people in the woods.

Perhaps these photos will help.


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