yet another stupid question (transmission/clutch)

[KnifeEdge_2K1]

I heard from somewhere that a double/twin plate clutch would be harmful to transmissions with syncros ... I'm not sure how credible that source may be but i cant find a logical connection between them. Can anyone help clarify this? I would think a twin plate clutch would be better than a single plate clutch because it can handle more power/torque compared to the single.

[Matra et Alpine]

I heard from somewhere that a double/twin plate clutch would be harmful to transmissions with syncros ... I'm not sure how credible that source may be but i cant find a logical connection between them. Can anyone help clarify this? I would think a twin plate clutch would be better than a single plate clutch because it can handle more power/torque compared to the single.

If the clutch is capable of transmitting more torque without slipping then it's down to the gears to cope with the strain.

That's the only reason.Anythin else will be myth.

name me a transmission designed in the last 30 years that doesn't have syncros ? THAT comment confused me Again, bad gear changes will tear up syncro dogs if the clutch has strong bite. Down to driver there tho


I've heard some real BS about what a gearbox can and cannot do based on "dragsters use autos so they're what everyone needs"

However, get it wrong and you're left with an expensive coffe grinder.
Rally Mini before JK gear replacements shred 2 sets of helical cut gear.
Makes a REAL and expensive mess in a Mini cos the oil is shared with the engine and all the shards end up EVERYWHERE

[KnifeEdge_2K1]

If the clutch is capable of transmitting more torque without slipping then it's down to the gears to cope with the strain.

so is there a way of "reinforcing" the transmission? im assuming this isnt a run of the mill procedure and would require lotsa of time, effort, specialists .... and money

[Matra et Alpine]

so is there a way of "reinforcing" the transmission? im assuming this isnt a run of the mill procedure and would require lotsa of time, effort, specialists .... and money

Yep

First you need to make sure the casing and shaft mountings will be strong enough. THAT's the most expensive bit

2 options for strengthening the gears.
First the gears can be made from stronger materials.
Second the gears can be cut in a different pattern.
and of course you could do both

Gears are cut in a helical pattern - so if you look at a gear the teeth are angled. This makes the gear engagement 'easier' and a lot quieter ! BUT it weakens the componoent dut to the angle of the forces. So a 'siimple' upgrade is to go for straight-cut gears.

For some 'boxes it can be quite cheap. eg Jack Knight produced straight-cut gears for Minis' in the 60s and they are still available today and cost about $1500 for a replacement 4-speed set. For ~$5000 you can get a whole new 6-speed straight cut strenghtened 'box and final drive.

But to do one-off would be VERY expensive.

[M Doe]

Hate to disagree with you again. I have always believed that helical cut gears can withstand more torque as there are more teeth in contact with each other at any one time. This is because they are of course mounted diagonally and so the duration they are mated is extended. There is also the added advantage that there is more material resisting the shear force applied to them, and arranged in a better fashion. One could say that helical gears have a higher second moment of area, as more of their material is away from the neutral axis (running through the tooth perpendicular to the direction of force). Technically the helix angle should allow for a constant teeth area to be in contact at any time and so more torque can be transmitted.

You say that they make gear selection easier? I do not I agree with this. The gears are constantly meshing with one another all the time. It is the dog/sincros that do the selecting.

So if I'm correct why use straight cut gears? This is purely a move to reduce resistance and rolling losses (I have seen a car loose 20hp on the rollers due to its gear box). Remember this resistance rises as the speed increases! But my god are straight cut boxes noisy! I don’t think I could put up with it!

[KnifeEdge_2K1]

One more thing to add, in almost all of today's manual transmissions the forward gears (1 through 5 or 6) are all helical while the reverse gear is strait, the reason for this is because when you select reverse you need an extra gear between the input and output shafts to reverse the direction of the spin (the idler gear). Sorry if i do not provide a very good explanation of this but its the best I can do.

actually helical gears aren't as strong as spur (strait cut) gears depending on your interpretation

helical gears create thrust forces on the bearings inside the transmission which is basically a force trying to move the gear up and down which is virtually non existant in a spur gear setup

the helical gear setup's advantage over spur gears are that they are generally alot smoother and quieter.

If you're talking about adding more horsepower or torque it would be generally a good idea to move to spur gears because they reduce the thrust load on the bearings of the transmission. Since as you've already mentioned all the gears inside a transmission are permanently engaged the "noise" difference will be not be as significant as you may think

just to clarify
the noise created is due to the engagement of each tooth on the gear, not the gear changing itself
the reason why helical gears are quieter is because each tooth makes minimal contact with each other as they engage and gradually increase contact while spur gear tooths engage right away

[Matra et Alpine]

Hate to disagree with you again.

It's OK, everyone on UCP has the right to be wrong

[M Doe]

Yes thrust is a problem with helical gears, there’s no denying that. Does anyone have experience of ways of reducing the effect? Gear pears with apposing helix angles perhaps?

As for torque carrying capability my money is still on helical gears, for the reasons explained before. If any one can explain to me why a strait cut gear could transmit more torque, I am waiting to be persuaded.

If strait cut gears are no noisier than helical gears, can carry more torque and are undoubtedly easier (and cheaper) to machine. Why don’t all standard road cars have them? Please explain!

[KnifeEdge_2K1]

Yes thrust is a problem with helical gears, there’s no denying that. Does anyone have experience of ways of reducing the effect? Gear pears with apposing helix angles perhaps?

As for torque carrying capability my money is still on helical gears, for the reasons explained before. If any one can explain to me why a strait cut gear could transmit more torque, I am waiting to be persuaded.

If strait cut gears are no noisier than helical gears, can carry more torque and are undoubtedly easier (and cheaper) to machine. Why don’t all standard road cars have them? Please explain!

they ARE noisier, but in a performance car noise isnt much of a problem but while a passenger "civilized" car it would be annoying

i dont think helical gears can transmit MORE torque than the strait cut gears, even though there is more surface area on the helical gears they are angled (hence helical) and the force would be transmitted diagonally, most of this force would be transmitted to the output gear but some as i've said before would be turned into a thrust load onto the bearings. this is not used to drive the vehicle and is just absorbed by the mechanics of the transmission itself.

as for why standard road cars have helical gears instead of strait cut ones, since most standard road cars have relatively low amount of horsepower and torque there is no disadvantage to having helical gear setup. however when you raise the horsepower and torque enough the helical setup will show signs of its shortcomings

[M Doe]

Your not loosing that thrust force; there is a reaction force that must be equal apposing it! The gear is not flying through the gearbox wall is it? There will obviously be resistance added here but you’re not loosing any force sideways!

Did you understand what I was saying about the helical gear tooth having a higher second moment of area than that of the spur tooth?
If you did I think you would agree with me.

Your right about everything else though.

[Matra et Alpine]

As for torque carrying capability my money is still on helical gears, for the reasons explained before. If any one can explain to me why a strait cut gear could transmit more torque, I am waiting to be persuaded.

For a given cog size there is more steel in the gear tooth for a straight cut gear than there is for a helical. DAMN, wish we had 3d cad on UCP )
Whilst a helical has more contact area, the tooth shape is lightly narrower.
So in a given space you get stronger gears with straights.
It's a FACT.
This does NTO need any more depabte other than clarification of the why.
It *IS* fact. I've rallied for nearly 30 years now and IT IS FACT.
OK now ???

If strait cut gears are no noisier than helical gears, can carry more torque and are undoubtedly easier (and cheaper) to machine. Why don’t all standard road cars have them? Please explain!

They are a LOT noiser than helical. Helical are there for quietness lone.

[Matra et Alpine]

Your not loosing that thrust force; there is a reaction force that must be equal apposing it! The gear is not flying through the gearbox wall is it? There will obviously be resistance added here but you’re not loosing any force sideways!.

It's a lot of force and especially in competition it produces large 'impact' damage on the mainshaft thrust bearings as the forces during gearchange occur ie releasing clutch after up.down change and lots of grip!!
Mini's and MG's will destroy the 'box castings with the force

[johnnynumfiv]

are all helical while the reverse gear is strait

on most transmissons, 1st and reverse are the same gear.

[KnifeEdge_2K1]

on most transmissons, 1st and reverse are the same gear.

really 1st and reverse are the same? ive never heard of that b4

as for the previous question about how you're not "loosing" the thrust force, you actually are, the structure of the bearing itself absorbs this force

its like you pushing against a really big building, the force is not gone, its just disipated throughout the entire building. same concept here
since the vertical thrust load isnt doing any work to drive the car it's essentially "lost"

[Egg Nog]

1st and reverse are not usually the same gear, but they both often lack syncros. Moreover, reverse is usually a straight-cut gear (which is why you get that noise when you back up).