Straight cut boxes and double declutching

[RobM77]

Forgive this basic question, but I don't know much about gearboxes at all

Does anyone here know why racing drivers in times past always had to double declutch, and even rally drivers in the 80s did it, but these days with a modern straight cut gearbox double declutching isn't ever used?

Obviously, the driver of a modern racing gearbox is braking right on the limit of adhesion, as were the drivers in the 40s and 50s, so the smoothest gearchange possible is required, but the technique used has changed over the years - so what's changed inside the gearbox?

This page of gearchange advice was written by My Hewland himself:

http://www.hewland.com/svga/help.htm

[Hooli]

I think it's more to do with better brakes tbh. I'm sure others now better but in ye olden days engine braking was used to help slow the cars but with modern brakes you can brake to the point of locking wheels anyway.

[GavinPearson]

RobM77 said:

Forgive this basic question, but I don't know much about gearboxes at all

Does anyone here know why racing drivers in times past always had to double declutch, and even rally drivers in the 80s did it, but these days with a modern straight cut gearbox double declutching isn't ever used?

Obviously, the driver of a modern racing gearbox is braking right on the limit of adhesion, as were the drivers in the 40s and 50s, so the smoothest gearchange possible is required, but the technique used has changed over the years - so what's changed inside the gearbox?

This page of gearchange advice was written by My Hewland himself:

http://www.hewland.com/svga/help.htm

Firstly, straight cut only means that there is no helix angle on the gears, which makes them mechanically more efficient, but with the penalty of noise.

I think you are really trying to find out why gearboxes with dog tooth engagement used to often require double de-clutching and nowadays you barely hear of it.

Those with a bit of experience driving a Land Rover Series 2 or older will know that they had a non-synchro firt and if you changed gear fast enough that you didn't need synchromesh.

Additionally those who have ridden a bike or driven a gearbox kart will know they use non-synchro dog boxes (that also in most cases happen to be straight cut) and know that they rarely miss a change.

I think the improvement is mostly perceived and through good marketing, rather than any any particular technology advances.

[RobM77]

Sorry, yes, I meant a dog box. I know very little about gearboxes :-)

So essentially thus is just down to speed? If that's the case, why didn't Fangio just change gear quick enough to avoid DDC then?

[Holst]

RobM77 said:

Sorry, yes, I meant a dog box. I know very little about gearboxes :-)

So essentially thus is just down to speed? If that's the case, why didn't Fangio just change gear quick enough to avoid DDC then?

I guess that reliability had a part to play as well.
DDC will be kinder on the box and clutch. In fangios day gearboxes would have been more fragile than modern examples.

[RobM77]

Holst said:

RobM77 said:

Sorry, yes, I meant a dog box. I know very little about gearboxes :-)

So essentially thus is just down to speed? If that's the case, why didn't Fangio just change gear quick enough to avoid DDC then?

I guess that reliability had a part to play as well.
DDC will be kinder on the box and clutch. In fangios day gearboxes would have been more fragile than modern examples.

Thanks. So I guess the answer to my question is purely down to fragility? Does that explain why rally drivers in the 80s were DDC but single seater driver's weren't? Was it the length of the stages and the power of the engines that meant longevity was of more concern in rallying than in single seater racing (max 1hr30min between gearbox changes)?

The Hewland link in my original post is worth reading - Mr Hewland says that changing gear as fast as possible is kindest to the gearbox.

[Holst]

RobM77 said:

Holst said:

RobM77 said:

Sorry, yes, I meant a dog box. I know very little about gearboxes :-)

So essentially thus is just down to speed? If that's the case, why didn't Fangio just change gear quick enough to avoid DDC then?

I guess that reliability had a part to play as well.
DDC will be kinder on the box and clutch. In fangios day gearboxes would have been more fragile than modern examples.

Thanks. So I guess the answer to my question is purely down to fragility? Does that explain why rally drivers in the 80s were DDC but single seater driver's weren't? Was it the length of the stages and the power of the engines that meant longevity was of more concern in rallying than in single seater racing (max 1hr30min between gearbox changes)?

The Hewland link in my original post is worth reading - Mr Hewland says that changing gear as fast as possible is kindest to the gearbox.

Im not sure.

I think the reason is that if you DDC properly then both shafts should be spinning at the same speed, allowing a smooth shift.
I dont see how a propper DDC change could be worse than Hewlands method.

I think Hewlands point is that if you are not going to DDC and want a fast change the faster you can move the gears the better.
This fast method will still cause some wear, so maybe in some formula (like a rallying) it was safer to still DDC.

We really need somebody who has experience of these sorts of gearboxes to comment as im just guessing.

[RobM77]

Holst said:

RobM77 said:

Holst said:

RobM77 said:

Sorry, yes, I meant a dog box. I know very little about gearboxes :-)

So essentially thus is just down to speed? If that's the case, why didn't Fangio just change gear quick enough to avoid DDC then?

I guess that reliability had a part to play as well.
DDC will be kinder on the box and clutch. In fangios day gearboxes would have been more fragile than modern examples.

Thanks. So I guess the answer to my question is purely down to fragility? Does that explain why rally drivers in the 80s were DDC but single seater driver's weren't? Was it the length of the stages and the power of the engines that meant longevity was of more concern in rallying than in single seater racing (max 1hr30min between gearbox changes)?

The Hewland link in my original post is worth reading - Mr Hewland says that changing gear as fast as possible is kindest to the gearbox.

Im not sure.

I think the reason is that if you DDC properly then both shafts should be spinning at the same speed, allowing a smooth shift.
I dont see how a propper DDC change could be worse than Hewlands method.

I think Hewlands point is that if you are not going to DDC and want a fast change the faster you can move the gears the better.
This fast method will still cause some wear, so maybe in some formula (like a rallying) it was safer to still DDC.

We really need somebody who has experience of these sorts of gearboxes to comment as im just guessing.

That seems like a good summary to me. If you're not going to DDC then just get it over with as fast as possible. I'm not sure I've ever tried a slow gearchange with such a box, but I should imagine it would crunch, and DDC would avoid it.

I have experience of using Hewland straight cut dog boxes in racing cars, but I've always driven as described in the Hewland article, with a SDC H&T on downchanges.

Edited by RobM77 on Wednesday 31st March 15:37

[davepoth]

Dog boxes hate slow gearchanges, you are supposed to slam the gears through as quickly as possible to stop the revs dying off when shifting up. Double clutching is the way to drive one when you aren't at 10/10, or are trying to avoid shock loads in the driveline (although it will wear the box quicker)

[RobM77]

davepoth said:

Dog boxes hate slow gearchanges, you are supposed to slam the gears through as quickly as possible to stop the revs dying off when shifting up. Double clutching is the way to drive one when you aren't at 10/10, or are trying to avoid shock loads in the driveline (although it will wear the box quicker)

Thank you, I think that explains it nicely

Thanks to everyone who responded.

It leaves the question though as to why rally drivers in the 80s double declutched, and apparently racing drivers in times past did too. I've even found this current rally driver double declutching:

http://www.youtube.com/watch?v=muxVLOaw2ik

Is this because of 4WD or something?

[Holst]

He may be trying to control the engine braking more accurately.
I guess in a rally car this might be more important than on a track car (especially if your sideways in a forest)

It might also be down to how the diffs work. In an old quatro I can imagine it being easy to lock a wheel when down shifting. Where as a modern car with trick electronic diffs should be able to control harsher engine braking.

[Super Slo Mo]

RobM77 said:

Thank you, I think that explains it nicely

Thanks to everyone who responded.

It leaves the question though as to why rally drivers in the 80s double declutched, and apparently racing drivers in times past did too. I've even found this current rally driver double declutching:

http://www.youtube.com/watch?v=muxVLOaw2ik

Is this because of 4WD or something?

No, he's effectively trying to match the speeds inside the gearbox to get as smooth as possible a downshift. It's a dogbox, so doesn't have synchromesh.

As you change down a gear (normally), to make a smooth shift generally you need to increase the engine revs so that the revs are at the appropriate level for the lower gear. This lessens the work the clutch needs to do, and is generally referred to as 'rev matching'. I think you already know about this bit right?

By lifting the clutch mid gear shift and revving the engine (a bit like normal rev matching), you increase the speed of the gearbox between the clutch and the disengaged gears (don't know if this makes sense). Because in a dog box there's no synchromesh, which is effectively a type of clutch that matches the speeds within the gearbox, ideally you need to manually match the different gear speeds to prevent graunching, crunching, and possibly damage to the box itself.

I'm struggling a bit to explain this simply, I might need to think about it a bit more.

Basically, when you change gear, there's a mismatch within the gearbox between the bits attached to the clutch, and the bits attached to the output (wheels if you will). On an upshift, the engine and gearbox will naturally slow, so changes are relatively easy with a dog box. On a downshift, you need to increase the speed of the engine, clutch and front end of the gearbox. So:
1: you disengage the clutch while you move the shift to neutral
2: re-engage the clutch
3: increase the engine revs
4: disengage the clutch by which point the gearbox will be spinning nice and fast but not connected to the road
5: disengage the clutch
6: select the lower gear (hopefully you'll have got it right)
7: and finally re-engage the clutch.

All as quickly as possible!

Does that help at all?

[RobM77]

Holst said:

He may be trying to control the engine braking more accurately.
I guess in a rally car this might be more important than on a track car (especially if your sideways in a forest)

It might also be down to how the diffs work. In an old quatro I can imagine it being easy to lock a wheel when down shifting. Where as a modern car with trick electronic diffs should be able to control harsher engine braking.

During threshold braking on a track of course any perturbation in braking force on the driven wheels will lock a wheel. I can only assume it is something to do with the 4WD system and backlash. Bear in mind also that trick electronic diffs are only in F1 and the top level of motorsport; the vast majority of racing cars just use a standard diff.

[Super Slo Mo]

A couple of links I found which might explain things a little better than I did (above).

http://en.wikipedia.org/wiki/Double_clutch

http://www.torquecars.com/articles/double-de-clutc...

Plus, as ever, PH has, the more understandable explanation, including a discussion on clutchless changes.

http://www.pistonheads.com/gassing/topic.asp?h=0&a...

The 4WD system has nothing to do with it.

Edited by Super Slo Mo on Saturday 3rd April 10:42

[RobM77]

Super Slo Mo: thanks for you posts :-) I do actually understand H&T and DDCing quite well, and can do both (I've raced for 10 years, and grew up around 1930s cars), I just wondered why DDC is standard on classic cars, but not on racing cars, when both have similar gearboxes. As described above, it's obviously down to speed, but that doesn't explain rally drivers DDCing.

[Super Slo Mo]

I wasn't trying to be patronising or anything, honest, but I was just attempting to explain (badly I think) how the gearbox internals work. I knew you raced, and thus assumed you're proficient in the art of H&T, and quite possibly DDC too.

I don't know why rally drivers DDC, and racing car drivers generally don't. Is this true across the board, do all rally drivers do this? It might just be that some prefer to do it to help gearbox longevity (I'd imagine if you'd paid for it with your own money, this would be important).

It might also be the case that the gearbox on a rally car has heavier/stronger internals (I have no idea), which carry more inertia, and thus need more of a hand when changing gear.

[RobM77]

That's ok, sorry I wasn't meaning to sound dismissive Good points re: rallying.

[VYT]

Not sure if this has been said previously. Often single seaters had a sequential shifting dog box akin to that used in a bike or a kart, it is not possible to DDC on those because there is no neutral between gears. You still need to blip the throttle on a down shift so as not to shock the transmission or more importantly throw yourself into a spin as the clutch comes out. I drive a Formula Ford which has a standard H pattern 4 speed Hewland box. Up changes only need a lift of the throttle I use a modified DDC routine on the down change. I put the car into neutral as I lift off the throttle, no declutch. Then blip the throttle, declutch , select the gear, let the clutch out. Little bit less busy than the full DDC process. As has been said before properly executed DDC will be kinder to the dog rings in the box. I imagine a week long rally would be enough to destroy a box whereas an hour or so on a track probably wouldn't. As it happens I have just finished repairing the Gearbox in my FF due to a failed circlip. The dog rings are looking fine so i'll be continuing to DDC.