520 is a size of chain, 525 is the standard size but is slightly heavier and thicker that a 520 chain so some fit it to clear wider wheels etc.
in my experience ;
14-40 made for a pretty quick feeling bike, off traffic lights etc it pulled away easier ( or felt it )
14-42 was a fast feeling bike, again it pulled from lights really well and climbed the revs really fast, I've never 'topped out' a 400 in 6th on the road so can't comment on speeds/revs but it was a great back road bike. I'd hate it on a motorway but.
15-40 is what I'm running now. JT steal sprockets from rick and a 525 JT chain too.

A very helpful set of replies.

1st gear is very tall, so any gear reduction will improve the starting and low speed performance.
I'm not going to do any high speed running, so I don't mind lowering the ratio a bit. I've got to fix a few things first, then will calculate the rpm at about 140km/h with a 14 / 42 and 15 / 40 and see which I think is practical.

I'm curious as to why 140kmh?

You could just sell it and get an nc35 and youll have much nicer road gearing.

just to know what have track riders.
Cause i have original set up and i have some troubles with the 1st gear....
cause it too long, i have to engage it on slow turn, but i've too much motor brake...

any suggestion for track use ?

Goulime wrote:just to know what have track riders.
Cause i have original set up and i have some troubles with the 1st gear....
cause it too long, i have to engage it on slow turn, but i've too much motor brake...

any suggestion for track use ?

I used to run 14 - 42 with a 520 (NON O ring) chain and would go up to a 45 rear depending on the track.
It will also depend on the tune of your bike.
This was on an NC35

thanks skinnydog !
i will go to le mans by the end of the month. i had some issue last time on shorter track (Carole) so i don't know what i've to do.

Goulime wrote:thanks skinnydog !
i will go to le mans by the end of the month. i had some issue last time on shorter track (Carole) so i don't know what i've to do.

I have never been to Le mans, but if you are on the GP circuit, i would guess that you need near standard gearing. probably 14f 38r or 14f 39r
The start straight is very long and anything less you will be topping out half way to the first corner.

Many thanks skinnydog0_0 !
Hope i will can fight with tom4!

Hi, I use 14/42 for my track bike based on the advise I got from here. It's seems pretty good for most uk tracks, haven't tried silverstone though lol! Pulls really well out of the corners. Bit of a mix match AFAM front Renthal rear and Regina chain, 520. Unless they have changed now do not buy a Regina chain, it's only brand to use a solid link instead of a soft link. My tool wouldn't do it and had to peen it over with two hammers.