Having a think about timing chains & gears.

[Ian Smith]

Hello Folks, I hope you're all doing well.

A bit of an idea going on here. We're all familiar with chain rattle on a lot of BMC-based vehicles, my resto included. However, casting my mind back, I remember the well-loved original Mini, with its transfer case & gears. This made me think, how about a gear-driven valve train, instead of the chain? 

Going back to the Mini, I approached MED Engineering to see if the Gates style can be upgraded for a Triumph block. Fair play to MED, their response was very polite, but no, sadly.

Still, if you don't ask, you don't get....

Moving the idea about, would it be possible to run a set of reduction/ up-speed gears to replace the timing chain?

Too complex? have a look at the gear-case on a Napier Deltic.

A bit too big for a 948 Herald, but you never know....

Cheers,  Ian.  

[JohnD]

Complex?    You would need an odd number of gears (Not even, rather than peculiar)  between the crank and the camshaft, so that they both span in the same direction, as provided by the chain.   I won't try to calculate how big they should be to drive the camshaft at half crank speed, but one won't do, too big so three gears.  Does get complex, doesn't it?

Chain rattle probably means a worn tensioning spring.  Easily cured, although modern repro springs are said to have a short life!

John

[RedRooster]

I've got a Med mini belt drive set up gathering dust, I went back to a duplex chain. 

[rogerguzzi]

Hello 

           You would need a gear for the camshaft and one for the crank with half the amount of teeth and one in the middle to make the cam turn in the right direction number of teeth would not matter.

But better still you have set up like the old Velocette Motorcycles with odd number of teeth so you finish up with a hunting tooth and on the Velocette it is 91 revs before the same teeth mesh again and also Helix gear teeth for less noise.

But probably all a bit complex for a ordinary Triumph engine? 

Roger

ps plus the middle gear has to be adjustable for position to get the mesh of the gears just right?

[zetecspit]

If you get to see/hear a very low mileage Triumph engine running, there is very little noise, if any, from the timing gear.

If yours is particularly noisy, replace the 2 sprockets with the best (NOS if possible) and also the timing chain and tensioner.

Somebody had properly hardened tensioners made, I think Cghris Witor has some, and Tony Lindsey-Dean. Not massively expensive.

 

As a curveball, back in the early 90s, there was an article where somebody fitted a timing belt to a Triumph. That would be rather easier, especially if you could find the article (TSSC mag from memory) I think he used pretty std bits from cars of the era, the hardest bit was making a plate replace the timing cover, fitted behind the new belt.

Edited September 22 by zetecspit

[Nick Jones]

Was going to say belt drive…… Quite tricky though sealing it all and providing thrust plate arrangement for the cam.

Good quality chain parts is the sanest way forward IMO.

[Ian Smith]

The cambelt conversion sounds interesting!

[JohnD]

Cambelts are designed to be durable, but should be replaced in a manufacturer's design from 40K miles.    In a bespoke design might not last that long.   And will need a tensioner, but usually a idler roller, so possibly quieter.

But if chain rattle is bothering you on a Triumph engine it must be really loud!

John

[Escadrille Ecosse]

Engineering department response. 

BMC A and B series engines, unlike the Triumph don't have a chain tensioner which makes them inherently a bit noisier especially as they wear.

The Napier Deltic is an engineering marvel but took quite a bit of development, especially in respect of the gearcase. It is also renowned as being one of the most powerful and loud diesels ever built for non-marine use.

Working out gear ratios is simple.

The third intermediate gear actually makes things easier as it fills the 'gap' between the crank gear and cam gear. Otherwise they would have to be quite huge

Arranging a sufficiently rigid bearing arrangement for that third gear would be rather more involved. This needs precision to ensure correct gear mesh and cope with loads. Lubricating the bearing and gears would require some engineering. The splash and hope of the chain drive would not be sufficient.

Also the camshaft bearing may not be adequate for the radial load generated by the gear on the cam. This would also be in the opposite direction, ie 'up' compared to the chain drive which is 'down'. The cam bearing lubrication is designed around the 'down' pull.

Biggest issue though is getting the gears properly designed, hobbed and then hardened adequately. Otherwise they will wear prematurely, fail catastrophically or just be noisy as all hell.

Fitting all this into the limited space at the front of the engine without completely redesigning the crankshaft nose, pulley, water pump drive, alternator, radiator, etc and of course the timing cover might well be challenging too.

Even well executed the gear drive will almost certainly be noisier than the chain drive. That's one of the main reasons car designers use chain drive for the cam. Chain drives are also about the most efficient transmission transfer systems available, for smaller drives anyway. Significantly more so than gears. Which is of course why they are so much quieter.

And then there's the cost. Unless you have the facilities and expertise yourself then just getting the three gears made will probaly cost more than the entire car. And finding someone to do the design and then make the gears would be challenging in itself.

Mr Renold invented the roller chain about 140 years ago. It just works. Which is why it has been and continues to be used in the internal combustion engines pretty much ever since that was invented over 120 years ago.

And with decent components still works very well in the small Triumph engine.

 

Edited September 22 by Escadrille Ecosse

[Escadrille Ecosse]

The other thing of course is why MED make the belt drive. 

It's not for noise, or any other reason associated with the basic engine arrangement.

It's there because it's the least complicated engineering solution in the Mini to fitting a twin cam head.

Edited September 22 by Escadrille Ecosse

[JohnD]

Thank you, Colin!  Everyday's a schoolday, especially when we have the class of teacher we have on Sideways!

I thought a gear train would be complex, but had no idea how much so!

Now, how about a cross flow head for a Triumph?   Or is that why such engines have overhead camshafts?  I've tried drawing one and can't get around the push rods.

John

[Nick Jones]

There are plenty of cross-flow engines with pushrods in existence - the famous Ford "crossflow" for one and many American V8s.  There are even pushrod engines with "hemi" type combustion chambers - also American (and UK) V8s plus various French I4s that spring to mind.  It's spark plug placement that is the bigger challenge usually.

For a cam-in-block engine with short distance between crank and cam, a chain is ideal.  Nice short chain that minimises the amount of stretch.  Hence why certain BMC engine designs thought they could get away without and real chain tensioning device and Triumph made do the with rather rudimentary sprung blade.  To be fair, it works pretty well when the chains are decent quality with ground outside edge and decent quality tensioner.  These last two have been somewhat problematic in recent years with unground chains chewing through tensioners made of bean tins rather rapidly.  Chris Witor (and probably others) is selling hard-chrome plated tensioners now and it is possible to get decent chains, though care is needed.

It might well be possible to co-opt a less primitive tensioner design, but questionable whether it's really worth the effort.

[rogerguzzi]

1 hour ago, Nick Jones said:

There are plenty of cross-flow engines with pushrods in existence - the famous Ford "crossflow" for one and many American V8s.  There are even pushrod engines with "hemi" type combustion chambers - also American (and UK) V8s plus various French I4s that spring to mind.  It's spark plug placement that is the bigger challenge usually.

Hello All

               Most Motorcycle engines are crossflow and push rod operated !

Yes a gear drive needs to be well lubricated and helical teeth to be quiet!

In my Velocette KSS OHC the cam is vertical shaft drive so bevel gears bottom and top but what they did is the whole lot is filled with oil when running so very little gear noise if set properly(that's why they stopped making then takes to much time to set up right and lots of shimming?)

Roger 

[andymcp]

Many motorcycle engines use manual cam chain adjusters, with a screw and locknut, or coil spring tension.