Need gearing help with RB20 with NK Road Race exhaust

[ulyssescale]

I've been putting together a huge stash of parts to build a fast bike. I got a sweet deal on an NK Road Race exhaust and am trying to match some gears to it. I already have an Italian Li 150 set and now need to decide buy the front sprocket and clutch sprocket.

According to the AF RB Fact Sheet "Fitting Instructions for the RB20Cylinder Kit" the recommendation is that the Final Drive Ratio should be 4.8. However, I've always thought that engines getting power from higher RPMs (especially with the NK Road Race) should have a higher Final Drive Ratio. AF's Fact Sheet recommends a 17/48 sprocket combo.

Can anyone offer me some pointers on how to approach this? I didn't have much of a plan for this Lambretta other than to take it on the highway and drive like an idiot, but I'm afraid of having a too one dimensional engine that is a horrible experience at any other speed than bat s**t crazy.

[sydduckett]

I've been putting together a huge stash of parts to build a fast bike. I got a sweet deal on an NK Road Race exhaust and am trying to match some gears to it. I already have an Italian Li 150 set and now need to decide buy the front sprocket and clutch sprocket.

According to the AF RB Fact Sheet "Fitting Instructions for the RB20Cylinder Kit" the recommendation is that the Final Drive Ratio should be 4.8. However, I've always thought that engines getting power from higher RPMs (especially with the NK Road Race) should have a higher Final Drive Ratio. AF's Fact Sheet recommends a 17/48 sprocket combo.

Can anyone offer me some pointers on how to approach this? I didn't have much of a plan for this Lambretta other than to take it on the highway and drive like an idiot, but I'm afraid of having a too one dimensional engine that is a horrible experience at any other speed than bat s**t crazy.

after trying different set ups and diff gear boxes iv found that a indian GP 200 gear set up with 18 x 47 works really well. Pulls in 4th and also pulls uphill in 4th. In have a franspeed race as well which was similar to the NK mid i had on it before.

Hope this helps.

sef

[rbgaz]

hi, my rb22 with a nkrace would only pull in 4th with a 5.2 final drive with a 4.8 it went backwards uphill or into wind come to think of it it was s**t on flat road as well , sydbuckeets is 5.2

[ulyssescale]

Pulls in 4th and also pulls uphill in 4th. In have a franspeed race as well which was similar to the NK mid i had on it before.

sef

I'm assuming when you say "pulls in 4th" you mean you are able to stay in the powerband of the exhaust. Is this what you mean? Seeing as the NK high RPM exhaust produces power upto 9000 RPM I am not concerned about running out of pipe. In fact, I don't mind if I end up leaving a bit out there (by having gearing that doesn't manage much past 9000.



Rbgaz: When you say your RB22 kit went "backwards" you mean it sucked, right?

[twin peeks]

gp200 box italian 5.2 ratio let it rev ,makes it more rideable and the fly

[sydduckett]

gp200 box italian 5.2 ratio let it rev ,makes it more rideable and the fly

Thats the set up i had but i found i had a big gap between 3rd and 4th and unless i was giving it the beans it wouldnt rev on in 4th. Theres a bit of a gap between 2nd & 3rd now but its not as big and easier to get through. The indian gp box has one less tooth i believe. Now 4th is bang on even picks up from low revs...

I dont think you will be dissapointed with this set up as you have power band in every gear, it might be be a bit boggy round town and at low revs but as you seem intent of smashing the arse out of it i dont think you will be to bothered by this...

Id also reccomend not going lower than 3% with your oil, AF reccomend 4% but thats probably them covering themselves. Lots of people swear by 2% but i like to hedge my bets...

[rbgaz]

i think u find that af say 4% when using thier rb race crank so with other cranks it might not need this much, but i hav a the af crank and hav never used more than 2% and hav done over 6k

[ulyssescale]

Yeah, in regards to oil, you can bet if I am using my time to put together an engine like this I'll be spending more time above 6,000 than below.


What's the biggest flaw to using the AF recommended Li150 box with 18/47 sprockets?

[sydduckett]

Yeah, in regards to oil, you can bet if I am using my time to put together an engine like this I'll be spending more time above 6,000 than below.


What's the biggest flaw to using the AF recommended Li150 box with 18/47 sprockets?

I found that it wouldnt pull in 4th, as you couldnt stay on the power.

[ulyssescale]

I found that it wouldnt pull in 4th, as you couldnt stay on the power.

So the gap between 3rd and 4th is too great and I won't be able to get back into the pipe? So what kind of options do I have?

I'm guessing I'll need a higher gear ratio for 3rd, so it doesn't top out so soon. Right? And then go with a shallower 4th? Ugh. This is like f@@king school and s**t.