Uber race crank

[drunkmunkey6969]

This is the mild 'flow assisted' version......a somewhat more extreme 'full race cut' version is in the pipeline for my RB20 build.....full write up to follow:

[paulmgreen]

That should be on your mantelpiece mate.... wasted in an engine!

[RinB]

Got alot more to go Sean to get that Comp Ratio down m8

[fishi]

Will this gain any kind of performance ?

Cheers
fishi

[byron]

tidy casing mouth
what rod are you using ?

[drunkmunkey6969]

OK, so.......what the f@@k are we doing machining down Lambretta cranks??

Well, its fair to say that this is a liitle experimental from our end, well at least on Lambos anyway, Sean has done a lot of these cranks on Vespas, and they prove to be a great success. Of course, the induction system on a Vespa is different to a Lambretta, but there's a lot more to this than just the inducton.

So, what are we hoping to achieve? Well a few things really, non of which are MAJOR big deals, but all add up, and along with the rest of thE engine mods, make for a more efficient and powerful engine. So we should see.....

1) A decrease in rotating mass.......ever seen BHP increase when a lightened flywheel is fitted? Same principle.

2) The crankcase volume does come into play on a Lambretta, slightly differently to a Vespa, but the gasses will flow in a smoother fashion (especially in conjunction with point 3).

3) The version we are doing for the racer will have the crank case metal removed to lower primary compresion, the reduction of material on the crank will further assist lowering the PC also (see pic below).....especially as the 'full race' version will be more of a 'bell' shape than a circle.

4) Increased lubrication to the crank big end.....which on an RB kit (which is what we'll be using it on) is a useful thing!

So.....decrease in rotating mass, improved gas flow, lower primary compresion, increased lubrication. Add these items to all the other minute changes we make (a finger boost port direct from the inlet tract to the casings, etc etc), and the race engine should be a sweet little number.

Casings with metal:


Casings without metal:

[RobD]

Hmm interesting.

I agree that you'll lower primary compression if that's what you're aiming for.

There's a limit to the amount of rotating mass you want to remove otherwise there won't be enough inertia to keep the engine going between power strokes and it won't tick over as I'm sure you'll know. In my mind as much of the weight removed as possible should be removed from the flywheel therefore reducing the stress on the crank pin / web interface. Not sure if I explained myself clearly there I suppose what I'm trying to say is that if I know I can take a kilo off the rotating mass before the engine becomes erratic at tickover than I'd rather take off all the kilo from the flywheel than say 750g from the flywheel and 250g from the crank webs. Minimise the stress on the crank.

You should get an improvement if you reduce the reciprocating mass i.e. that of the piston and rod assembly. It's accelerating and decelerating at such tremendous rates that any reduction in inertia will have a similar effect to lightening the rotating mass. Again making the cranks life easier. Modifying (lightening / removing stress raisers) the piston would be beneficial (or finding a lighter piston ) and possibly polishing the rod if it's not already been done but you'd only remove a couple of grammes there.

I still can't see that you're going to get appreciably better gas flow but will be interested in your findings. A better big end bearing is apparently sufficient for the RB225/250.

It does look nice but I'm too lazy to go to that length on the crank just to lower primary compression a bit. Good luck though and this is only my opinion, it's good to see you're prepared try stuff out and publish details.
Will be interested in your progress as I'm putting together my RB250 top end over the coming few weeks.

[drunkmunkey6969]

I agree that you'll lower primary compression if that's what you're aiming for.

It is.

There's a limit to the amount of rotating mass you want to remove otherwise there won't be enough inertia to keep the engine going between power strokes and it won't tick over as I'm sure you'll know.

Yes, we intend to do the race version as a 'bell' shape, following the contours of the weighted end of the crank circle. So the remaining mass is opposite the rod/pin ....like a couterbalance?

In my mind as much of the weight removed as possible should be removed from the flywheel therefore reducing the stress on the crank pin / web interface.

Of course.

I suppose what I'm trying to say is that if I know I can take a kilo off the rotating mass before the engine becomes erratic at tickover than I'd rather take off all the kilo from the flywheel than say 750g from the flywheel and 250g from the crank webs. Minimise the stress on the crank.

I always make this mistake (not emphasising the end use)......the full race version is letteraly that....full race, so tickover is of lesser concern, its revs and power that we are interested in.

I still can't see that you're going to get appreciably better gas flow but will be interested in your findings. A better big end bearing is apparently sufficient for the RB225/250.

Lots of things are 'apparently' sufficient.....but it depends how far you're taking the rest of the tuning before it becomes a weak link and needs further consideration.

Hopefully the whole set up will be in balance......bore/stroke ratio, rod length/stroke ratio, port time area, mass of crank/flywheel/piston/rod combo, port timings, carburation, exhaust, gearing etc.......to give us a fruity little number!

[stockcar]

what sort of cost is involved in this sort of mod??

[RobD]

There's a limit to the amount of rotating mass you want to remove otherwise there won't be enough inertia to keep the engine going between power strokes and it won't tick over as I'm sure you'll know.

Yes, we intend to do the race version as a 'bell' shape, following the contours of the weighted end of the crank circle. So the remaining mass is opposite the rod/pin ....like a couterbalance?

[/quote]

I look forward to seeing it, I've a picture in my mind of what you mean (doesn't mean that's it's anything like what you're going to make ). Being a bell shape opposing the crank pin won't you trap a large volume of the charge behind the crank at BDC and release it as the transfers are closing?

I suppose what I'm trying to say is that if I know I can take a kilo off the rotating mass before the engine becomes erratic at tickover than I'd rather take off all the kilo from the flywheel than say 750g from the flywheel and 250g from the crank webs. Minimise the stress on the crank.

I always make this mistake (not emphasising the end use)......the full race version is letteraly that....full race, so tickover is of lesser concern, its revs and power that we are interested in.

Can make it a bit more difficult to ride though, don't forget driveability (tractability?) might make you quicker round the track than outright power.

Bit off topic but just out of interest does anyone know at what point you start to lose power by reducing the rotating mass? There must be a point at which the energy required on the compression stroke is more than the inertia/kinetic energy supplied by the flywheel? Bit of a sad sack maths question I know