MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - How gearboxes work - youtube clip

http://www.youtube.com/watch?v=ftdgB93QOD8&feature=related
Daniel Thirteen-Twelve

Brilliant. Great post.
Lawrence Slater

Gearboxes are black magic! Even after dismantling mine into its individual components and rebuilding I couldn't for certain tell you how it worked! ha ha

Will watch clip later, see if it helps.

Malcolm
Malcolm Le Chevalier

That is most illuminating. But why does the rib case 'box whine in all the indirect gears? I can understand first gear which has noisy straight cut teeth, but why is there the noise (albeit reduced) in 2nd and 3rd ratios?
Mike Howlett

Mike, as a mechanical engineer who studied Mechanical Transmission systems as his masters subject I should be able to answer that question. However its actually quite complicated and there are a number of reasons. Its usually though to do with the actual design of the gears and what module is used - designing a gearbox is actually quite an empirical process as there are lots of objectives/requirements to meet which are independent of each other (eg, speed, torque, - ratios, tooth stress both static and fatigue - tooth end profile, gear material and hardness processing and other wear properties, gear sizes, thicknesses, box size, oil used - not to mention bearing selection, given the fact the forces all change when you change a gear - choosing bearings is very hard) All these things are usually combined with what engine is being used, what diff ratios are being used, what size wheels are being used, what the weight of the car is (so swapping to other gearboxes can sometimes cause more problems than they solve). Anyway, with all these considerations naturally you end up with some compromise, and there are gearboxes which are known to whine when they leave the factory. But after a number of years use there is inevitably some tooth wear which will change the profile of the tooth at the tip, which changes the stress on the teeth, which creates your whine. Second gear is well know for this problem as it takes a lot of acceleration load.

You will find it might whine more/or less when loading the other face of the teeth (eg when engine braking) - this is due to wear, it can either wear itself into a whine or out of. Mine currently whines more on the overun. Its nothing to worry about, just annoying.

C L Carter

All the gears are in constant mesh including first gear which is straight cut.
Daniel Thirteen-Twelve

Thanks CL and Daniel. Very interesting. Of course, 1st gear is always in mesh - I should have realised that from the video. But the MGB 3-synchro box has a straight cut first gear and that doesn't whine like an A-series unit. I remember getting my first midget, a 1964 model, in 1971. Because of the noisy gearbox, and not knowing any better, I went to University Motors in Epsom and asked about a Gold Seal exchange gearbox. The guy behind the counter said not to bother as the new one would sound just the same as the one in the car! The noise is apparently built-in.
Mike Howlett

the most likely cause of whine in 2nd and 3rd gear is noisy lay gear bearings

the noise will be much less in 4th gear because there is no longer any load going through the lay gear



Norm
Norm Kerr

Yes - in the Midget box, the first gear is shifted sideways to engage with the straight-cut on the layshaft; this requires more movement than merely selecting through the dog-gears, which is why you move the lever further forward than 3rd.

I rebuilt my Midget box in ~1980 to replace layshaft (hardening had worn away under the needles)... managed it without a manual, and with no 'spare' parts...

A
Anthony Cutler

<<All the gears are in constant mesh including first gear which is straight cut.>>

Not in an A-series engined Midget 'box, it ain't.

First gear is straight cut, but is not constant mesh - it is a 'crash' gear.

And not all 1st gears are noisy.

I have had several Spridgets with quiet gearboxes. Noise is a symptom of wear/damage.
Dave O'Neill2

First gear is in constant mesh.

What makes first gear a 'crash gear' is that it doesn't have synchromesh which isn't the same thing as being in constant mesh. Gears are taken to be in constant mesh when each pair of gears turn at all times.
Daniel Thirteen-Twelve

When was the last time you looked inside a Spridget gearbox?

First gear is NOT constant mesh.
Dave O'Neill2

Not recently and I've only type 9s in the garage!

Tell us how it engages if it's not in constant mesh.
Daniel Thirteen-Twelve

'First speed gear assembly can be controlled so that the gear can be meshed with the small spur gear at the rear end of the laygear to give first speed'

I never knew that! I thought gearboxes were either fully constant mesh or not. However, it seems the Spridget box is a mixture of both.

Sounds like another reason to fit a T9.
Daniel Thirteen-Twelve

Dave,

On the Spridget box is the reverse idler in constant mesh with anything?

Daniel Thirteen-Twelve

OK, here's one I prepared earlier...

The sharp-eyed among you may spot that they are Morris Minor gears, but the principle is the same.

The first speed gear is also the second speed synchro hub. As seen in the photo, the gears are in neutral. If 1st speed gear slides to the left, it engages with the laygear, thereby selecting first gear. If it slides to the right, it engages second gear.

The reverse idler is only meshed with either first speed gear when first is selected, or first speed gear and lay gear when reverse is selected. If second third or fourth are selected, the idler is not meshed with anything.


Dave O'Neill2

is this correct?

(great image, Dave!)




Norm Kerr

Yep, that's it.

I hadn't seen Daniel's question about the reverse idler when I took the photo, otherwise I'd have included it.

Nice labelling Norm. How do you do that?
Dave O'Neill2

To make that, I grabbed your image, and put it on an XL spreadsheet, and then making the labels is simple. When done, I save a copy as a .jpg file for uploading here.


Norm
Norm Kerr

I figured it was excel from the background, but I couldn't see anything relevant in excel 'help'.
Dave O'Neill2

Does anyone know how to download these vid's for storage on a local computer?

What software do you need? I've tried before, but not been able to do it.
Lawrence Slater

to download your own copies of videos from the internet, it is super easy if you've got a Mac: use Grappler

http://thelittleappfactory.com/grappler/

if you only have a PC, though, I've no idea but I expect that similar exists for them too

grappler works so well, and is so simple to use that even I was able to figure it out!

Norm

Norm Kerr

For a PC have a look at this http://www.makeuseof.com/tag/18-free-ways-to-download-any-video-off-the-internet/
Mike Howlett

Thanks Mike, I'll check those out. Mines a PC, couldn't afford a Mac. :)
Lawrence Slater

This thread was discussed between 26/07/2012 and 28/07/2012

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.