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No there's no synchro. Car boxes are synchro mesh, as I understand it.

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[Viney]

A bike box is a proper, seqential straight cut box with no syncros afaik.

Originally Posted by jonboy

No there's no synchro. Car boxes are synchro mesh, as I understand it.

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Your quite right. My grey matter must be failing me, i'm the wrong side of 40. I learnt all this stuff at college when I was 17. I haven't been a mechanic for over 20 years now, so I must be beginning to forget some of it.

Doh!

[embee]

Originally Posted by I'm_a_Newbie

Helical cut gears are quieter, but the drawback is that the contact point is much smaller. This means the oil must withstand extreme pressure. This is what the EP stands for in EP80 or EP90 for car gearbox oil. My guess is that this is why bikes can use engine oil for the gearbox as well as the engine.

Kind regards
Tim

Not quite. Apologies for the techie stuff, but this is my field

You get small contact patches with spiral bevel or hypoid type gears, but it's not quite the same with spur and helical gears. They both have line contacts.

Helical gears are not fundamentally different to a spur gear ("straight cut" or in other words a helical gear with zero helix angle) in this respect.

The contact pressure between teeth varies over the contact path, but you still design the total contact length to give contact stress which the materials and the oil film will withstand. As you say, extreme pressure oils can operate at higher contact pressures. Oils behave in an interesting way such tht at very high contact pressure the viscosity increases and they become almost semi-solid, but the films are then very thin (often a few molecules).

The principle reason for using spur rather than helical for bike boxes is space. In order to have small diameter gears, the teeth need to be stronger for a given torque, and stronger means bigger "module" (= pitch circle divided by number of teeth), i.e. the teeth are big and stout rather than small and slender. This also applies to keeping the gears thin (i.e. axial length), and the gearbox short.

Helical gears are primarily designed for refinement (noise/vibration), and the basic principle is that as one tooth pair comes into contact at one end of the helix, another is just leaving contact at the other end, so the total contact length stays pretty much constant and the stiffness of the contact doesn't vary much. Because the contact starts just at one end, the effective stiffness of the new contact is relatively low, and then the contact length along the tooth grows so the stiffness changes gradually, so the noise is reduced.

Spur gears have the contact starting and stopping along the whole length of the tooth at one instant, which promotes noise. As one pair come into contact, the mesh stiffness sees a step increase, and a step decrease when they lose contact.

There are subtleties in the manufactured tooth form which can allieviate these issues, addendum modification etc.

To get the helix to work in your favour you usually try to get pretty close to (or a little over) one complete tooth pitch along the helix, and so the bigger the teeth (module) the longer the gear needs to be to get one pitch without having to have a very steep helix angle. The helix angle imparts axial force from the contact along the gear shaft, so you need thrust bearings, and also tries to bend the gear and shaft sideways. High helix angles are not desirable. In really big gears (marine propulsion etc) you see paired gears of opposite helix ("herringbone") which cancel out what would otherwise be huge axial forces.

Sorry for the nerdy stuff, I do get out sometimes..........honest

[quote="jonboy"]One of the big factors regarding oil and engine wear is the frequency of change. Even a cheap oil changed at 3k miles is likely to give you better wear protection than an expensive oil changed at 6k. Ideally (and if your pocket supports it) then an expensive oil changed very regularly would perhaps be slightly better, but only slightly I suggest.

I've seen other reports on the web that have shown that Mobil 1 car oil did in fact maintain its viscosity better (over a period of time) than some leading bike oils.


Car oil is ok for cars with there dry clutch systems ,but on bikes with a wet clutch, then bike oils have adatives to stop plate sticking and clutch slip.

Originally Posted by andy owen

Car oil is ok for cars with there dry clutch systems ,but on bikes with a wet clutch, then bike oils have adatives to stop plate sticking and clutch slip.

Yes but they won't necessarily affect a wet clutch providing the oil has a JASO MA rating.


.

Originally Posted by embee

Not quite. Apologies for the techie stuff, but this is my field

You get small contact patches with spiral bevel or hypoid type gears, but it's not quite the same with spur and helical gears. They both have line contacts.

Helical gears are not fundamentally different to a spur gear ("straight cut" or in other words a helical gear with zero helix angle) in this respect.

The contact pressure between teeth varies over the contact path, but you still design the total contact length to give contact stress which the materials and the oil film will withstand. As you say, extreme pressure oils can operate at higher contact pressures. Oils behave in an interesting way such tht at very high contact pressure the viscosity increases and they become almost semi-solid, but the films are then very thin (often a few molecules).

The principle reason for using spur rather than helical for bike boxes is space. In order to have small diameter gears, the teeth need to be stronger for a given torque, and stronger means bigger "module" (= pitch circle divided by number of teeth), i.e. the teeth are big and stout rather than small and slender. This also applies to keeping the gears thin (i.e. axial length), and the gearbox short.

Helical gears are primarily designed for refinement (noise/vibration), and the basic principle is that as one tooth pair comes into contact at one end of the helix, another is just leaving contact at the other end, so the total contact length stays pretty much constant and the stiffness of the contact doesn't vary much. Because the contact starts just at one end, the effective stiffness of the new contact is relatively low, and then the contact length along the tooth grows so the stiffness changes gradually, so the noise is reduced.

Spur gears have the contact starting and stopping along the whole length of the tooth at one instant, which promotes noise. As one pair come into contact, the mesh stiffness sees a step increase, and a step decrease when they lose contact.

There are subtleties in the manufactured tooth form which can allieviate these issues, addendum modification etc.

To get the helix to work in your favour you usually try to get pretty close to (or a little over) one complete tooth pitch along the helix, and so the bigger the teeth (module) the longer the gear needs to be to get one pitch without having to have a very steep helix angle. The helix angle imparts axial force from the contact along the gear shaft, so you need thrust bearings, and also tries to bend the gear and shaft sideways. High helix angles are not desirable. In really big gears (marine propulsion etc) you see paired gears of opposite helix ("herringbone") which cancel out what would otherwise be huge axial forces.

Sorry for the nerdy stuff, I do get out sometimes..........honest

Your quite right. I think I'll shut up now and go back to college. Well it was 23 years ago I learn't this stuff and I haven't worked in this field for 16 years.

Kind regards
Tim