76 replies to this topic

[stretch tech]

 

(well technically there is no such thing as centrifugal forces but that's a different story)

I'm so glad somebody else said that.


The primary gear floats because the clutch drives it and it sits on the crank, if it were fixed there would be no clutch. With the side load the tensioned chain would impose you'd probably have to engineer a bearing for it. The Princess had a conventional clutch and flywheel with the input drive on the extreme end of the whole transmission case and after the clutch which eliminates a lot of issues.

 

 

It makes sense when you think about it...

 

okay i see how that works now...do you reckon you could solve this floating issue by boring and machining out the sprocket with spilnes and then producing a splined sleeve (longer than sprocket face width) that fits to the crank. The spline sleeve would then move as the original gear would have but allowing the sprocket to stay put whilst still transmitting drive ?

 

Carlos: there are chains robust enough to take the forces easy, it's finding one small enough to fit within the transfer case that is the issue. 

 

Joshua: hmm in the event of a chain failure, i guess there really isn't a lot of option as a break is out of the question really.

 

Gremlin: i'm not 100% sure what you mean. could you explain more ? a sketch perhaps ?

 

Cooperman: it's not the power loss reduction that i'm focused on as the gains in reality wouldn't be massive. The small power increase will just be a bonus but i can imagine the advantages would be seen in a race situation. For road use, the main advantage will be the elimination of side loads on the bearings caused by the the nature helical gears. The chain could then be a viable substitute for SC drops without the noise issue.

 

SledgeHammer: i've never seen tensioners like this...interesting idea...

[tiger99]

I know there is no such thing as centrifugal force. I am a physicist by education. However, a chain manufacturers web site called it such, and most of the people on this forum probably use the term, so it seemed appropriate, and the chain will ride up on the sprocket teeth, regardless of whether there is the mythical centrifugal force or the actual centripetal acceleration.

 

But the important thing is that we are getting good ideas here, and people are thinking deeply about the problems.

 

As for primary gear bearings, when the clutch is engaged, 99.9% of the time when the car is moving, there is no rotational movement between crankshaft and primary gear. The existing gears, like any others, have a tooth pressure angle which generates a force, trying to push the primary gear upwards, so it will effectively try to run off centre on the crankshaft, taking up any clearance in the bushes, and the splines will chatter in the clutch disc, which will not move laterally between the flywheel and clamping plate. Now, we swap to a chain, and instead of a force pushing the gears apart, dependent on the pressure angle, the full chain tension is pulling the primary gear down, and it will again wobble with respect to the crankshaft, to the full extent of the clearance in the bushes. The only difference is that the force will be 3 or 4 times larger, the bush clearance will be the same.

 

With the clutch disengaged, in either case the drop gears are spinning freely, back-driven from the road wheels via the gearbox, or by their own inertia during gear change, but as the clutch is free, there is no torque, except residual drag, and so no torque, and pressure driving gears apart, or chain tension pulling them together.

 

The real difficulty is when the clutch is slipping, and good drivers minimise the duration of that. But in a hill start, for instance, it can not always be avoided, and in that condition there is high chain tension and high rotational speed difference between primary gear and crankshaft simultaneously.

 

It could be concluded that the vertical loads with chain drive will be larger than with gears, and so the primary gear bushes and gearbox input shaft bearings will require investigation and analysis, possibly upgrade. It would initially seem that bushes reamed to the minimum acceptable clearance would be best, but there is scope for other detail changes too.

 

While we are messing about in the clutch area, might we not also want to fit one of the modern hydraulic conversions where a cylinder with built-in thrust bearing acts directly on the clutch, thus getting rid of the operating lever? These are usually used on RWD cars, and so the cylinder, piston and thrust bearing are hollow for the gearbox input shaft to pass through, but on a Mini that detail is unimportant.

[Cooperman]

That is very well-explained, Tiger.

Of course, it might be better to use roller bearings in the primary gear, but lubrication is not possible and running them dry would be an issue. Anyone remember working on the original oil-fed primary gear set-up. Blooming nightmare!

Chain drive would really only be feasible on a race car with very regular 'strip-&-rebuild' cycles and where a slight power increase to knock a fraction of a second off a lap time is important despite the cost.

It is interesting though, isn't it. A proper engineering design project.

[tiger99]

I wonder about needle rollers in the primary gear. Because they only run when the clutch is disengaged, maybe grease lubrication, with an oil seal at each end, would suffice? The crankshaft would be skimmed and an inner sleeve fitted to give a suitably smooth and hard surface for the rollers.

 

I do remember the oil-fed primary gears, but fortunately my old van had been modified. Others around had not. I knew just about enough to avoid buying one of those, although I saw a few....

 

Issigonis did make a few mistakes, largely due to the amazingly short time he was given to develop the Mini, and I think that was one of the worst. Possibly not him personally, but one of his very small team of highly competent people. Nowadays they would need 100 times as many people and 5 times as much time, despite computer simulation of everything.....

[Mr Joshua]

(well technically there is no such thing as centrifugal forces but that's a different story)

I'm so glad somebody else said that.

 
I was tempted!
 
Seriously I'm following this topic, and it's got some fantastically knowledgeable people commenting!
 
If you can put the power of a hayabusa turbo through a chain, I'm sure you can put 80BHP from a mini through it

If only life were that simple.

[sledgehammer]

was it Issogonis's idea to have the engine in backwards to avoid the idler gear in the first place ? (seen pic somewhere)

 

he could have had the engine run backwards , but fit it in the current position (different cam , starter , oil pump etc)

 

that would also eliminate the idler gear

 

anyway back to ...

[tiger99]

Well said Mr Joshua! It is because it is not simple that such a conversion is not readily available. Actually I am glad that it is difficult, because it discourages some of the well-known aftermarket suppliers from amateurishly cobbling together mod kits that will only give trouble....

 

I will throw in another "difficult" idea, get rid of the idler gear by reversing the engine rotation, as per original prototypes. Well, actually the engine sat on the box backwards, and rotated the same way, but realistically we would need to keep it the normal way round and reverse the rotation.

 

Several problems, distributor shaft would want to climb out of block, and cams and valves would all be wrong, so replace timing chain with pair of gears, to reverse camshaft, and all is well again, including oil pump.

 

Pistons must be reversed, so thrust during power stroke is on correct side.

 

Cranksaht drillings need to put oil in correct place to achieve proper hydrodynamic lubrication of the bearings, so would need to be plugged and re-drilled, or of course new forging drilled appropriately.

 

If crankshaft centre line is offset from bore centres (don't know if it is, but on some engine designs it is, to help with piston side loads) there are difficult problems, unless it is within the limits of boring and sleeving to get it right again. I know that overbored 1275 engines have two cylinders offset for another reason.

 

The clutch centre plate damping springs, and/or pressure plate drive straps would be wrong.

 

Alternator "may" work correctly in reverse, depending on design, but water pump will not. Electric pump is a possibility. Fan too would be electric.

 

Now the big problem, which may not matter with later gearboxes. The reason the engine was turned round had nothing to do with carburettor icing, as commonly supposed, but was due to the inertia of two large diameter drop gears being far higher than three smaller gears, which caused havoc with the weak syncromesh in the A35 gear cluster. We have far better syncromesh nowadays, so I don't know if this would still be a problem.

 

Any thoughts on this?

 

Edit: sledgehammer just beat me to it with that idea.

 

Another edit: I forgot that the starter would need to be reversed.

Edited by tiger99, 08 February 2014 - 05:12 PM.

[stretch tech]

tiger: my apologies, i did not mean to cause you any offense. i was merely stating the fact. the top bush on the crank would have to be modified to prevent the floating of the sprocket so it would be a good time to make something more robust to withstand the greater forces as you describe. perhaps a split bush which the sprocket could be pressed onto ? 

 

Cooperman: the more we delve into this topic the more apparent it becomes that a chain drive is going to take more work than i first thought. the stripping and rebuilding hopefully could be avoided as iwis chains are pre stretched and are by far the market leader in terms of life and wear resistance.

[tiger99]

Don't worry, I don't get offended by healthy technical debate, although sadly some people here (not so far in this thread) may do. Actually, I think they will stay away from this discussion, as it is a bit too technical.....

 

I have just been to www.iwis.de for a look. Interesting stuff...

[sledgehammer]

I wonder when we will have the materials to do a reliable strong enough 'wet' toothed belt drive - would save a lot of problems (and create a few)

 

on new cars , the cam drives are going back to chain at the moment , not always with reliable results - some causing spectacular failures

 

the only other drive I could think of is a Bevel Gear which isn't good when in small area & would be a mare to set up esp in a weak alloy housing

 

my money is on Double helical gears - expensive to make - but easier to fit

[Carlos W]

on new cars , the cam drives are going back to chain at the moment , not always with reliable results - some causing spectacular failures

 

Mazda have had problems with their diesel engine cam chains stretching.

 

Honda had gear driven cams in some of their motorbikes in the 80s

[stretch tech]

yeah i'm surprised this thread has lasted so long with out any non constructive replies from the minority on here.

 

that's the one, work has had a few training days with them and the range of chain and the money spent on development is outstanding - so much so that you can't believe it's a viable business...

 

sledge hammer: i think double helical gears could be a better alternative to chain for the reduction of wear and reliability. we are starting to introduce them at work so both options are just as available to me. I don't think bevel gears are a viable option in this case as you'd still be stuck with a greater number of gears and have to somehow mount an axis that runs perpendicular to the crank down to the first motion gear. That coupled with the space issue ...

[Cooperman]

Can I buy an Iwis pre-stretched duplex timing chain for a Mini? Please let us all know if they are available.

 

I like Tiger's idea of pre-lubricated needle rollers for primary gear.  Maybe a case-hardened sleeve with a wall thickness of 1 mm fitted over the crankshaft end with oil seals recessed in at each end to allow for the thrust washers. After all, wheel bearings are pre-lubricated and the grease doesn't normally get onto the brake discs. I'm sure the crankshaft tail diameter could go down a bit to allow for needle bearing diameter. There could be a bearing at each end and these could be quite long. Anyone fancy trying this.

[Tamworthbay]

on new cars , the cam drives are going back to chain at the moment , not always with reliable results - some causing spectacular failures

 
Mazda have had problems with their diesel engine cam chains stretching.
 
Honda had gear driven cams in some of their motorbikes in the 80s

They did indeed. The VFR750F used them very successfully (although head slims were a nightmare). It was all due to the spectacular failure of the cam chain system on the Vf750F. As in the old joke, knock knock who's there? Honda cam chain tensioner.

[Cooperman]

On current BMW's the timing chain is at the back of the engine, but the chains are said to last the life of the vehicle.
But if you have, say, a 100,000 mile 7-year old BMW and the chain snaps, the cost of repairs is so high that the vehicle is scrap so, one might accept, the chain has lasted the life of the vehicle. Cynical, moi?!