Fatman

...as per title.

madmoog

iTrader: (1)

Straight cut gears with no syncro, as they used to be and as they still are in alot of race cars and bikes.

Fatman

So why are they called "dog" gearboxes? A dog to use, what?

What's the benefit of running without synchromesh?

greasemonkey

This is one of those questions where it'd be easier to show you than tell you! Pictures truly are worth a thousand words in the right circumstances!

Unfortunately though, ain't got any!

The basic difference between "normal" road car gearboxes and dog boxes is that dog boxes don't have synchromesh hubs. These are frictional devices that match the speed of the gear you're about to change to to the speed of the rotating shaft, thus making gearchanges smoother, and more tolerant of bad technique with the stick.

The only form of gear engagement in a dog box are castellated "dog" rings (imagine the top of a castle wall for a rough idea of what they look like) that lock together, joining the gear to the shaft.

Because there's no need to wait for the synchro hub to match the rotation of the gear and shaft when shifting, dog box gearchanges are faster, although the downside is that gearchange technique (in particular timing, and matching engine speeds) is important to avoid missing gears, or indeed damaging the box.

Also, because the synchros aren't there, there is much more space inside the box to make the gears and dogs bigger and stronger. This in turn makes them more resistant to shock loadings - such as driving a powerful car over rough gravel, or changing gear without the use of the clutch.

greasemonkey

Madmoog - whether the gears are straight or helical cut is a separate issue. You can get helical dog boxes and straight cut synchro boxes!

The important difference is the gear engagement method.

[Edited by greasemonkey - 7/15/2003 10:16:16 PM]

Fatman

OK - gotcha. I guess, therefore, that if they're straight-edged teeth (castellations) the gears would be potentially stronger but with the downside of requiring greater precision for a clean change. Reasonable assumption?

Why would anyone use a dog box on a road car? A tad overkill, no?

Still - why the "dog" name?

madmoog

iTrader: (1)

Thanks greasemonkey I wasn't aware ot that!

greasemonkey

Yes, very reasonable assumption. Bear in mind that most regular synchro boxes still use a dog-type design for the actual selection. The difference is that the synchro won't let them engage until the shaft speeds have matched. Without the synchro, and with a large mismatch in shaft/gear speeds, the tops of the dogs will "buzz" off each other, damaging them if the pressure is kept on the gearstick for long enough and hard enough.

To change gear properly with a dog box, you've got to be positive with the stick, but also sympathetic enough to get the timing and shaft rotation speeds right first!

If you want a rough idea of what it's like, don't forget that reverse gear on most road cars (and all Subarus) is non-synchro. You know what happens when you change to reverse a bit quick and you can hear and feel the "buzz" through the stick before it goes in? That's a lot like it...

For most people, most of the time, yes, but maybe not for the reasons you're thinking.

For a driver with fundamentally good gearchange technique, a good dog box gives a sweeter, more direct, and more rewarding gearchange than any synchro unit. If your technique is bad however (as most people's is!), the results could be ugly!

The main reason to do it is with a highly modified engine. Most of the dog gear kits that are around are designed with motorsport in mind and are thus much more robust than the standard box, which tends not to like large amounts of torque for very long.

The big problem with most dog kits is that they're bloody noisy. Roadcar gearboxes are designed for refinement and low noise rather than outright efficiency. Motorsport boxes tend to use different tooth profiles, and, as Madmoog mentioned, are often straight cut instead of helical. This give more efficient power transfer, but makes them whine like f***.

There must be a reason...!

[Edited by greasemonkey - 7/15/2003 10:53:55 PM]

Fatman

Thanks, greasemonkey - I appreciate the responses.

ChristianR

iTrader: (1)

Picture of the Unbreakable Gearset (dog box) that I had in my impreza , before I changed to the LP Uprated Syncro kit.



More info here: http://www.mrtrally.com.au/performance/gearsets.htm

and here :

http://www.mrtrally.com.au/performance/gearsets2.htm



[Edited by ChristianR - 7/15/2003 11:23:18 PM]

hypoluxa

You also need to double clutch if you want any sort of long term reliability.

5 straight cut dog gears would be a pain in the **** for every day road use, you would be better of with 1st, 2nd & maybe 3rd straight cut dogs and then 4th and 5th helical or semi-helical with synchros.

I think an uprated synchro box would be the wiser choice.

submannz

Either have to double clutch or match engine revs, eg if you change fast enough before the revs drop too much then you don't need clutch.

Fatman

@ hypoluxa - my question wasn't in relation to an upgrade to my car. I've read references to dog boxes here on S/N and I didn't know what they were, that's all.

KungFuMonkey

Are they not called "dogs" because the engagement teeth are Dog leg shaped?

Leslie

As usual Greasemonkey has given you excellent information. The dog rings are one half of the "Dog Clutch" which actually connects that gearwheel to the shaft for use. The gear is free to revolve in relation to the shaft until the dog ring is engaged. On competition gearboxes there is usually extra backlash in the dogs so that the gear is easier to engage,ie there is more space in between the dogs. I used to use the Hewland Volkswagen based gearbox as well the Hewland FG box. No synchro of course but the boxes were easy to use and there was rarely any difficulty in changing gear. It was not necessary to double de-clutch. That might have been smoother but there was not the time to do it. Hewland boxes were very well designed and built. being at the back of the car you couldn't hear it either

Les

[Edited by Leslie - 7/16/2003 2:13:54 PM]

greasemonkey

...And still are. In fact, both the Impreza WR Cars and Prodrive's Group N dog kits use Hewland internals.

Unfortunately not the case with either of the above! [img]images/smilies/mad.gif[/img]

Fuzz

nobody seem to have touched on the fact that helical cut gears are much quieter running that straight cut , the reason for their use in production cars. The downside being lack of strength !
straight cut for strength
helical for peace and harmony... for a while

a straight cut Jack Knight dog box in a mini is music to my ears

Andy

greasemonkey

I believe I (and indirectly Mr Moog) did touch on this issue earlier in the proceedings Your Honour

submannz

As well as a little clonky going into first and reverse

Fuzz

I reckon you edited it after I read it

lol.


Andy

greasemonkey

Lol! Not guilty Your Honour, as the evidence clearly shows