Uber race crank

[fishi]

perhaps a dyno run, WITH and WITHOUT mods to the crank will give us a serious information about the benefits of this kind of mass reduction

[drunkmunkey6969]

perhaps a dyno run, WITH and WITHOUT mods to the crank will give us a serious information about the benefits of this kind of mass reduction

I agree, and i often think that about a lot of tuning mods that we do.......but scooters is my hobby, not my living....so who wants to pay for the back-to-back engine rebuilds and dyno runs??

You are correct with your idea, but as with many things......time and money prohibits.

[Powolotti]

the idea behind those crank shape (i refer to the bell or mushroom shape) is lowering crankcase volume and therefore pressure. I think to that point everybody follows, but what are the benefits? More room in the crankcase means more place for gasses to enter. But now, things are getting complicated. There are great pumping losses inside the crankcase, so you need an exhaust that controls the pumping (said in an easy way). For example, if you would fit a Clubman you will notice a lack of power against a full circle or HPC crank. On some of the exhausts around there is no noticeable difference in power output and on some it works, altough it must be said the torque and power curves are shifted a bit up the revrange as well (compared to a HPC crank on same exhaust)

[Powolotti]

oh, and not forgetting better balancing

[fishi]

oh, and not forgetting better balancing

but balancing can also be done to a full circle crank like it does MB with there ALPHA or Tino Sacchi,

or is this not the same balacing.

[Powolotti]

it is, you need to either take away weight on the crankpin side or add weight on the opposit side (tungsten inserts) - most have a mixture of both. You can come to the same weight factor with both ways - you´ll end with either a HPC or LPC crank.

[Avantone]

oh, and not forgetting better balancing

How are cranks balanced in practice in the context of the rod/gudgeon pin/piston/rings etc?

[Powolotti]

first step in balancing a crank is to find the weight for the masses that go up and down: piston, rings, gudgeon pin, bearing, circlips and you´ll also add the weight from the small end of the con rod. For this the conrod can be in a crank for instance and the small end is put on your scale - it is important that the rod is horizontally aligned.

Now you add all the above and get a weight of let´s say 600grams. Write this down and also write down the weight from the smallend, we will need this later.

Now you need two prisms where you rest your crank, you´ll notice that when you turn it it will come always to a halt in the same position. Now put a piece of wire through the smallend (like a hook) and put nuts on it. Add nuts until the crank stops after beeing turned or pushed without having a heavier side.

When you´ve found this put all the nuts and the wire on your scale and add to this (again) the weight from the smallend you have measured before. Let´s say this weight is 200 grams.

You see now that the moving masses (up and down, 600grams) are opposed to a weight of 200 grams from the webs. This is roughly said a 33% balancing factor. To better this a lighter piston can help - this also shows why a 66mm piston usually runs smoother then a 70mm piston! But, in our case, a lighter piston is not enough (or we have alredy fitted the piston we want to use). We must remove weight on the crankpin side or add weight on the opposit side as mentioned before to get our balance factor right. For a horizontally cylinder around 50% balancing factor seem to work quite well, this means in our case we want to add/remove 100 grams - please note the weight examples are for easy calculation, usually it is more material we have to modify!!

[fishi]

it is, you need to either take away weight on the crankpin side or add weight on the opposit side (tungsten inserts) - most have a mixture of both. You can come to the same weight factor with both ways - you´ll end with either a HPC or LPC crank.

let me say this in other words:


standard Lambretta cranks like, AF´s, MEC.Eur´s and Mazzucchellis work best with common exhaust like JL-Road/Race, MB Dev-Tour, old PM´s, AF-NK, KPR1-2-3-, Taffspeed MK-x and all others on the market, some good, some better,

HPC cranks like Tino´s or MB´s ALPHA can gain extra/more power with the exhausts mentioned above but with the advantage being balanced, so less vibrations

and

LPC cranks with the bell or mushroom shape have the advantage to be balanced,

BUT/AND

even gain more power from a todays Lambretta engines when used with a hard sucking exhaust design.

Is a pipe like PM´s Fat Mamba a hard sucking exhaust or have we still to wait for a new pipe, done specially for a LPC crank ?

[Powolotti]

you are absolutely right!

The Mamba goes in the right direction, altough I say it is too short for a proper road/rally pipe. Imho still the best available at the moment to work with LPC cranks. New pipes are in the pipe line and I think these will be beneficial for LPC cranks (as they are adapted from vespa pipes that work well with low primary compression, so it´s just a guess)