Shift Kit - Best Gear Ratio?

[moronic_kaos]

What is the lowest possible gear ratio that these motors will handle with a shift kit? Kind of a two-part question, like...

1. What is the highest gear (lowest tooth count) on a rear hub that these motors can push (without loosing enough RPM to defeat the purpose of itself) on flat ground if they've already got going from previous gears
2. How does that compare with the standard rag-joint sprocket sizes in total gear ratio (in terms of tooth count)

I guess what I'm saying is what is the calculation to convert the equivalent rear hub teeth with the standard rag-joint teeth (to get an idea in comparison with the different rag-joint sprockets I've already ridden), and at what point would the motor peak at a top speed and not even be able to maintain it's own speed on flat ground due to too low a gear ratio.

The best example would be for a 26" wheel with an 66cc (80 or whatever you want to call it) motor that's 100% stock.

 

[BarelyAWake]

Well I suppose it would all depend on what you're using for gears... not exactly the number of teeth - but what bike are you putting the shiftkit on? The reason I ask is that if it's a 21 speed bike, you'll be going through the 7 speed freewheel on the rear hub and as you'll be able to use all 7 speeds it may be a moot point as I've heard that it covers the range quite well, even perhaps too many gears with the first gear being even a touch too low and the seventh being a very tall "cruise" gear.

Any fine tuning of the gear ratios beyond that is so specific to the build in question, it's performance, the terrain, the build's weight, rider weight, your riding style, etc. that I'm not sure that there's an easy answer - other than the shiftkit itself was designed with the "average" 21sp bike in mind so it's pretty close... the kit even comes with two choices of secondary gears for the jackshaft, a quick and easy swap.

I've a very heavy vintage bike w/a shiftkit going through just a three speed hub, and even then it's very close. I'm running a 10t for the secondary shiftkit gear and it's just a touch too tall for maximum acceleration performance (not quite matching my engine's powerband), but it's about perfect for quiet & efficient cruising. I was going to swap it out for the 9t... but Pablo suggested I swap the 44t chainring on the pedal crank for the 48t instead, but TBH it's so close and I'm a slacker so I've not done it yet lol

I'd suggest starting out with the "average" - what comes with the kit and fine-tuning the ratios to suit your bike and riding style, there's such a wide selection of gear options available that it'll be a piece of cake to dial it in to exactly what you want be it a screaming demon or a comfy cruiser - or even a bit of both

 

[jauguston]

I can speak to that issue as I just went through getting the gearing right for my setup. It could be a little bit different for other lighter riders but this will be pretty close. I started out with the stock single speed sprocket on the left side and quickly decided I hated it. It is on a 21 speed mountain bike.

I then purchased and installed a Sick Bike Parts shift kit. The kit quality is first class. I opted for the HD version with the better freewheel bearing. It was a big improvement over the single speed but for me the gear ratios were way to high. I then purchased their optional 30t front chain ring in place of the 36t that came in the kit. That was a big help but low gear was still too fast. I next purchased their seven speed rear chain ring set with the big first gear ring. That is where I am now and I think I will leave it there. I have been testing with a GPS in a handlebar mount to monitor speed and a Tiny Tach to monitor engine RPM.

What I have ended up with is a nice slow first gear that will allow starting out from a stop without having to pedal and a 6th gear that will go 30 mph at 5100 rpm. For me at 250 pounds 7th gear is 4300 rpm at 30 mph and my stock Silver Grubee Sky Hawk 66 with the slant plug head will not pull it.

SBP has a 24t front chain ring available that I "may" try some day but for now it is pretty darn good.

If you think like me that the gear ratios are too high I would warn you NOT to change gearing between the engine and the freewheeler sprocket. Make the changes where I did. If you change the jackshaft ratio you will make it much more difficult to get the motor to turn over with the pedals to start.

 

[moronic_kaos]

Well I've got 3 different setups in my mind, and I'm trying to determine what I should do. I'm currently running a cranbrook, and I like the bike itself. The way it's built is very smooth and I trust it at 30+ mph as it shakes very little.

My first thought is that there's a women's 24" sitting in my friend's backyard with a 7 speed hub on it. Thought about pulling both wheels and just swapping them, plus the derailleur and shifters, but I'm not sure how I'd get the brakes to work without investing a ton of money into disc brakes.

The second is buying a 3-speed hub for my 26" wheel and a left-side trigger-shifter, but I'm not sure as I don't want to have to buy all new spokes to fit the wheel to the hub (I just re-spoked the wheel 2 weeks ago). Then try and use the derailer on the bike mentioned above and if it didn't work, buy a newer one.

The third idea is that there's a $70 mountain bike at Target (they don't show it online, but it's been at the store for the past 2 years). It's got 7 gears in the rear. Was thinking about pulling the whole system, rear wheel, shifter, brakes (front and rear), etc. Then pop a motor on what's left of the mountain bike (perfect frame with plenty of clearance, wouldn't even need a u-bolt or any weird adapters for the front mount as the bottom tube is of standard thickness), using my cruiser's coaster-brake wheel and left-side sprocket (my friend has an almost complete kit and wants another bike. He don't care about shift-ing ability). Perhaps I could get a return on my money, like $30 selling the bike to him.