Hey guys, I'm no mathematics expert , and I don't know much about figuring these type things out. But maybe you can

My current gearing on my 06 fxd is 32t -70t belt drive.
I wanted to do a 30t front pulley for a while until I road one it didn't have enough top end for my liking but loved the bottom end pull.

so now I'm going with a chain conversion and was wondering how to get a little more pull than my stock gearing but not lose my highway speed (not much atleast)

My options are

24t -52t

24t- 55t

24t 60t

24t- 65t

 

The stock ratio 32 / 70 is going to be the best trade off.

Smaller front pulley (32 to 30) will result in better off the line but top speed suffers.

Bigger front pulley (32 to 34) will result in better top speed but off the line suffers.

If you want the best of both worlds - off the line and top speed - then you need to be looking into engine modifications. Perhaps a cam with more high end torque while retaining the same curve at the low to mid-range while using the 30t pulley.

 

Would a 24t - 52 be the closest to my stock gearing? I'm looking for a happy medium between my current gearing and a the 30t 70 combo

 

Try messing around with the Gearing Commander calculator website.

 

The stock ratio is 2.18 the 24-52 option would be 2.16 (less off the line torque) the 24-55 is a 2.29 ( more torque but higher highway RPM's) if you didn't like the 30 tooth you won't like that option. We have a 25 tooth front sprocket and if you run that with a 55 rear it will get you to a 2.20 ratio which may be more to your liking. Little more torque out of the hole but won't kill you on the highway.

 

Jeff, you can do the sums yourself, they are dead easy! Simply divide the larger number by the smaller one. So stock gearing is 32/70 which gives 2.1875. It is very unlikely your bike can pull that gearing to flat out at the red line, unless you have some pretty decent tuning mods. 30/70 gives 2.333, a change of just under 7%. If you want a little extra acceleration, without going wild, that is the set-up to go for IMHO.

I had a TC88 Dyna and changed the compensator from 25T to 21T, which really woke it up! Dropped rpms in top/fifth gear from 24mph per 1,000rpm to 20mph and gave 19% more torque to the back wheel. So I don't believe any of the ratios you mention are going to do any harm.

As Mark points out the closest to your current rear ratio is 24/52, which gives 2.167. That is just 1% different and frankly I don't believe you will notice any change. 24/55 gives a small improvement in acceleration of 6% and pretty close to a 30/70.

Frankly the numbers above are all fairly tame and a change of two teeth on the front pulley/sprocket is the minimum to give a noticeable change.

 

From what I have read the 30t 70 combo equates to 3.16 final drive ratio.
So I feel like i would be fine with a 2.29 would be tolerable. But I'm no expert.

I just know that riding the 30/70t combo bike was reving higher than I'd prefer ... while cruising at 80mph . 3-400rpm less I would have been ok with

 

Jeff, together we can do this! But first can we make clear what model year your bike is? Does it have a stock 6-speed or an aftermarket one? Is it a TC88 or TC96? It matters because gearing changed between those two and the TC96 was introduced late 2006.

 

I read your reply above and the equation is alot less confusing than I thought haha. My bike is a 2006 fxd 6spd tc88

 

The numbers we played with earlier are not quite right, your rear drive is actually 32/68, not 32/70 (which was stock for many years). So your rear ratio is actually 2.125:1. Your primary I believe is 25/36, which is 1.44:1, giving an overall ratio of 2.125 x 1.44 = 3.06:1.

An easy way to look at this is to note what your engine speed is at a chosen road speed, say 70mph. I reckon that will be 2,860rpm. If you are able to run your bike out to 5,000rpm in top gear that will give the exciting experience of doing around 122mph! I doubt if your bike can go that fast, unless it has been significantly tuned.

If you fit a 30T front pulley your gearing will reduce from 24.5mph per 1,000rpm to: 24.5 x 30/32 = 23mph, not much of a change. So 5,000rpm will be 115mph, still a bit of a stretch for a stock bike. Your rpm at 70 mph will increase only slightly to 2,860 x 32/30 = 3,050. Rear wheel torque will be increased by 32/30 = 107%, which should be noticeable.

Ignore that other bike you rode as it may not be the same spec as yours.

Any change of gearing needs to be a couple of teeth IMHO, to be worth the effort and expense, also to be noticeable on the road. Any more queries do ask!