Monthly Archives: March 2013

Ape 50 top end and gearing comparison

Unfortunately, the images to this post were lost in a server crash.  i’m preserving the text in case it is of any help to anyone.

Most of the parts for our shop truck have arrived; I wanted to take this chance to illistrate the difference between the stock parts and what i’m going to.

First up: the cylinder and piston

As you can see, it’s bigger. much bigger. It’s also going to have a longer stroke. this takes displacement from 50cc to 136cc. In addition, it has a much improved set of transfer ports inside the barrel walls, improving the changeover from burnt gasses to fresh fuel and air. It is also set up for a cylinder inducted reed valve and larger carb, giving it much more power overall and a much wider spread of power.

Next, to take advantage of the power-

This is the old primary gear next to the new one. the old one is the one with the wee tiny separate gear and is a 14/69 tooth count, ratio 4.93.  the new set is a 29/68 tooth count, so 2.34 is the ratio.  If you’re coming from the Vespa smallframe world and are familiar with their gearboxes, you might think i’d lost my mind. when put in a smallframe scooter, this ratio gives you about 85mph @ just over 8,000 rpm. Fear not; there are other factors at play.  first off, the gearbox on this ape is different from the typical 4-speed scooter gearbox.  Actually, it’s more of a scooter 3-speed with a higher fourth gear added. (Remember the zirri short 4th gear kits? they use a modified ape 50 3rd gear as a 4th gear). it’s a 26/42 1.62 ratio 4th gear instead of a 22/46 2.09 like the scooter uses. But wait- that means if this combination was in the scooter, it’d get 105mph @ 8000 rpm. How does this work with the Ape 50? It’s the differential. there is a 34/52 1.53 ratio reduction in the differential. This drops the gearing considerably, to get us a ratio that gives 68mph @ 8000 rpm. in addition, this specific kit gives us some leeway to adjust the gearing easily later. there are 26,27,28, and 30 gears available separately that can be swapped without removing the engine from the frame. This gives us a range of steps from 62mph @ 8000 rpm to 71mph @ 8000 rpm. This should work well if we need to later fine tune the gearing once this is in service.

Stay tuned; i’ve already modified the cases for the new top end and expect to post that article next week. Feel free to ask any questions you might have in the comments; i’ve enabled using facebook to authenticate users so you shouldn’t even need to sign up for an account or anything,

UPDATE: This kit turned out to be a hair too high, even with the 26/42 combo. it might be right if used with the DRT short 3rd/4th christmass tree.