Straight cut / dog box question

[robo1968]

Am I correct in thinking that with my straight cut gearbox (with syncro's) I shouldn't be attempting clutchless changes? and that's something that only a dog box can do? and that the straight cut / helical cut is a totally separate issue from being a dog box?

Just thought I'd ask to make sure as I've found some conflicting info on google on it, and I don't want to try a clutchless change without making sure first!

[SteveD]

basically yes

[7sRWild]

You're up early Steve.

[davidgh]

basically yes

Never said this before: I agree with Steve 

 

Muppet's explanation (all I'm capable of):

Straight cut vs. helical is about how the power is transferred when in gear (slightly stronger and more efficient vs. quiet).

Dogs and synchro rings are about engagement when changing gear (clunky, but quick and brutal good vs. smooth, but quick and brutal very bad indeed (got the T-shirt))

[Martyn Vann - Warwickshire AO]

Straight cut vs. helical is about how the power is transferred when in gear (slightly stronger and more efficient vs. quiet).

Dogs and synchro rings are about engagement when changing gear (clunky, but quick and brutal good vs. smooth, but quick and brutal very bad indeed (got the T-shirt))

 

Good description of the differences apart from helical gears are stronger and more durable than straight cut gears in addition to being quieter, hence why they are used in production gear boxes. The rolling friction of the straight cut gears reduces transmission power losses when compared to the sliding friction of helical gears, hence why they are used in racing applications.

 

You are absolutely correct about the gear engagement methods.

[Dave Eastwood (Gadgetman) - Club Chairman]

The only reason that the misconception about straight cut gearboxes superior strength has come about is that it's rare when comparing a sc version to the original that you're actually comparing only a change of tooth profile. You will often find that the sc version has many other areas of improved design, from material choice, through to bigger better bearing arrangements. It's these changes that happen to make the box "stronger" (as in able to cope with more torque, higher revs etc).

 

you can clutchlessly change with a synchro box, but it's not a fast way to change gear, probably slower than you'd manage with the clutch if you were doing it with any sort of mechanical sympathy to avoid destroying the synchros in double time.

 

A dog box on the other hand, while it does need a little technique in order to preserve the life of the dogs, does as the others have said, positively need a faster more brutal approach. 

[Martyn Vann - Warwickshire AO]

Absolutely correct Gadgetman, standard gearboxes are generally built just strong enough for their application.

 

As a reminder to all:-

 

During clutchless changes on a synchro box the inertia of the engine or the whole vehicle is loaded through the baulk ring as it tries to equalise gear speeds. Hence the warning that premature wear or failure WILL occur.....

[AdamR]

During clutchless changes on a synchro box the inertia of the engine or the whole vehicle is loaded through the baulk ring as it tries to equalise gear speeds. Hence the warning that premature wear or failure WILL occur.....

 

Is this still the case if there is zero 'lurch' during the gear change (ie. the revs are matched perfectly)? Just wondering as should I have a clutch fail mid-race, I would be tempted to continue - but not if it meant a gearbox rebuild afterwards!

[Norman Verona]

Whilst is may not be the quickest laps you drive without a clutch and not do that much damage. Just equal the revs and gently engage the next gear.

 

I've driven, on the road, without a clutch many times. The biggest problem is started from rest. But once moving it's just a matter of longer distance anticipation, like slowing down at red lights so far back that you don't have to stop before the lights change.

[Martyn Vann - Warwickshire AO]

I too have had to drive without the clutch on a number of occasions and it is about being very precise with gear change timing, engine and road speed but it should generally be avoided on synchromesh systems. Having seen many occasions of 'knowledgeable' driver's have gearbox problems because they think they a being expert by doing clutchless changes regularly!

 

Adam, you may need to make a value judgement on how much time or places you are likely lose against prolonging the life of your gearbox....clearly, leading on the last lap is going to be worth the risk, but back of the pack at the first corner it may be best to park it....!  

 

I must admit the light weight of a seven will ease the issues compared to a standard car with massive speakers, 2000W amp and five occupants etc etc.