Gearing issue

[Scooterdude]

Hi guys, just done a 200+ round trip today on my series 3 with 175 iron conversion, when I did the rebuild I was advised to fit a 16 tooth front sprocket which I did ( can't remember how many teeth the clutch sprocket has) but it started life as an LI 125 but with a pacemaker gearbox, well it zips through the gears like a moto cross bike and pulls like the space shuttle in top, non of this thrashing the nuts off it in 3rd and changing in to 4th and it dies here! Anyway was thinking of upping the front sprocket to a 18 or even a 19 tooth... Any good?

[Doom Patrol]

Surely that would make it even harder to pull?

[holty]

if it pulls a 16 easy then it would be worth trying a 17 front sprocket which will give you longer gears and a little more top end speed, with slightly less accelleration, will save you revving the nuts off the engine all the time, it might pull an 18 just depends on how much torque the engine has and how much you weigh etc, sometimes its best to just try it and see if you like it, my rb250 is pulling well on a 21 front sprocket but would be useless on yours.

[Scooterdude]

Not sure of the torque figure as I've never had it on a dyno but it seems pretty good, I got caught out on it once in traffic and ended up pulling off in third! No problem. 14.5 stone on a good day haha.

[holty]

ive had a look in the gearing calculator, its on the main page with an 18 front sprocket it will do 66 mph at 6000 revs if it has the power,

[Doom Patrol]

Sorry if I'm missing the point here, but if it won't pull 4th gear how's it going to take a larger front sprocket? You'd just be making the gearing taller.

[rosscla]

OP says it pulls fine in top and doesn't have to thrash third to get it there.

[Doom Patrol]

Okay, obviously I misread it. I thought he said it died. Carry on....

[Adam_Winstone]

^... that's how I read it too! TBH I'm still not 100% sure whether it pulls well in 4th but revs out and screams or whether it zings through 1-3 and then "dies'" in 4th, which I would take to mean doesn't pull correctly.

I'd always look to gear iron 175s with 5.2:1 4th gear, which is IMO perfect for all round use. Let's not forget that 5.2 is the same as an Innocenti GP200 and/or SIL Stg IV (tuned) GP200 so please don't mistakenly think that an iron 175 will pull the original 4.8 without any issue. 4.8 is fine with a 20mm carb and a standard exhaust on an original 175 barrel + head but most iron 175s will find it easier to pull 5.2 than 4.8, unless you weigh 7 stone. Over gear it and you'll regret it, spending most of your time using the first 3 gears.

Adam

[holty]

to quote the original post (it pulls like the space shuttle in 4th) i think he means it pulls well, in other words it might be under geared, i agree with you adam, over gear it and you would spoil the ride. and spend a lot of time in 3rd, revving the nadds off it before it will pull in 4th. i would try a 17 front sprocket and see if you like it.