Slower with shift kit

[BrandonG]

So a couple weeks ago I put on the SBP shift kit on my 66cc. Love the low rpm cruising but before the shift kit I could hit 35 (maybe 36 on a good day) with a 44 tooth. The motor screams. So I figured with the shift kit Id be hitting around 40 at least. Nope, cant get above 32. I have an 8 speed bike, and Ive tried top speed in 4-8th. 5th is the best at 32, but 4th will pull good right up to 30. I noticed I had a worsening head gasket leak, so spent a couple days perfecting the fix (glass and sandpaper to flatten my anglefire head). That got me from 30 to 32 top speed. Also I found an intake leak where the carb connects and fixed that (sprayed starter fluid and now its perfect). The shift kit chains are not too tight. Cant think of what is slowing me down.

I have the NT carb on the stock air/fuel ratio and thats worked great. stock jet but I have a 68 n 66 if I should try those. At full speed I tried slightly choking the motor to make sure it isnt running lean, and it just bogged and slowed. so maybe its rich at full speed? Also It feels like its hotter than it used to be. Lean? Maybe thats the raised compression with the head sanding or it could just be me.

Also Im at sea level if that helps for tuning tips.

My spark plug is brown, but maybe 3 months old. Does the color reflect the tune of the last ride or average tun of the life of the plug?

 

[crassius]

I'd first be looking for a bad bearing in the chain run, or maybe something rubbing.

 

[Sidewinder Jerry]

I use a Staton Inc shift kit system with my autoshift bike. I've done lots of experiments with different setup searching for what works best. I did lots gear and speed calculations; then tried it out. What I discovered is with these small engines you're only going to get so much speed and that's all there is to it.

So then it becomes about designing a setup which gives you the best overall performance. I currently have the only bike in the world that shifts gears automatically through a shiftkit. What I've found is less gears is better. It allows you to widen the range between gears.

I'm currently running a custom built 5 speed freewheel (34, 28, 24, 20, 16). You can go to my profile to see pictures of how I built it. I use a Surly 34 stainless steal chainring. My top level ground speed is 29 mph. The engine is a Robin Subaru EHO 35. The gear box is 18.75:1. The GB-SK 18-44.

This is a really great setup for me since Tennessee has very steep hills. I weigh 250 lbs and can still go up any hill here at least 8 mph. My state has an automatic transmission requirement. So what I've built is a 5 speed automatic transmission system making my shift kit legal here.

To conclude try reducing the number of gears you have and concentrate on overall performance.

 

[wheelbender6]

Sounds like gear ratio to me. The SBP kits usually come with plenty of gear ratio though.

 

[BrandonG]

Ill check my chains again.

I can get it to any rpm at 30mpg because I have so many gears, and Ive tried running high low and all rpms inbewtween. what sounds to be around 5000 is what I run in 5th at full speed

 

[Sidewinder Jerry]

The primary thing about a shift kit is versatility. Without knowing what kind of reduction system you've got coming off your engine to the jack shaft. Then what is being used to the shift kit. It's hard to make recommendations. However you do mention having a 8 speed bike. You don't say if its a freewheel or a cassette you're using. In an 8 speed you can get an 11-34 in either freewheel or cassette. Shimano has them in cassettes and DNP has them in freewheels.

Use 1st and 2nd for going up hill. Use 3rd, 4th, 5th and 6th for level ground. Use 7th and 8th for down hill.

 

[crassius]

if you're sure you measured 35mph with the 44t, then get out your notepad & calculator and track thru all your gears to find one as close as possible to the ratio of the 44t and try that gear - if it is very close in ratio to the 44t, but still much slower, then you're losing power to friction somewhere

 

[The_Aleman]

The SBP system does increase drivetrain losses because of it's complexity. My brother's GXH50 + Q-Matic used to always beat me in top end by 2-3MPH, although because of my shift kit, my HS will leave him way behind accelerating below 30 - especially off the line. He has 2 reductions, I have 4 not counting my Sturmey's internals.
Of course, he has a Honda and I have a Huasheng, but a well-tuned fully broke-in Huasheng is pretty close if not a little faster than a stock GXH50.

I know you have a 2-stroke, but I'm using the different drivetrain losses as an example.
A chinagirl on a bike typically has 2 reductions (internal 4.1:1 and output to wheel). You now have 4 because of the jackshaft.
Your top speed will suffer slightly - unless you're accelerating downhill

 

[BrandonG]

Before shift kit: 35.4 mph @ 8250 rpm
After shift kit: 32 mph is my max. Heres what rpms would be:
3rd- 8100 rpm
4th- 7100 rpm
5th- 6000 rpm
6th- 5300 rpm
7th- 4600 rpm
8th- 3900 rpm

Peak power should be around 5500, not at 8000+. So I should be going faster than the previous 35 because I can run rpms closer to peak power. The problem is Im having excessive mechaincal losses.

Put on a new drive chain because the old one was trash. Didnt help much.

 

[Goat Herder]

For the ease of gearing and cruising etc. J Shafts are great! That is a treat. They will always rob some power tho from the extra moving mechanic drag and parts put into motion. Chains rattling etc. Straight from the source to the ground for maximum output in my own experience had the top speeds.

 

[Sidewinder Jerry]

It would help to know exactly what gears are on the rear wheel, on the shift kit and so on.

 

[Sidewinder Jerry]

If a drag bike is all you're wanting two chain rings on the shiftkit would be a great setup. A single sprocket is all that would be needed on the rear wheel. One chainring would be low for take off. The other high for top out. A derailleur in the rear could still be used for a chain tensioner. You could still keep the multi 8 speed on the rear wheel just use one sprocket when drag racing. And shift the front chainrings on the shiftkit. On my Staton Inc shiftkit I can put up to three chainrings on it.

 

[BrandonG]

10t engine output to 17t jackshaft
10 t jackshaft to 44t chainring
36t chain ring to rear gears.
Rear gears are 11, 13, 15, 17, 20, 23, 26, and 30 teeth.

I have 2 chainrings, from the jackshaft and one to the rear gears. How would having 2 drive chainrings and one rear gear be better than using 2 gears at the rear wheel and 1 drive chainring?

 

[Sidewinder Jerry]

Example: 22-11 take off; 44-11 top out. You just went from a 2:1 to a 4:1 ratio. With the chain only moving one gear.

 

[Goat Herder]

Example: 22-11 take off; 44-11 top out. You just went from a 2:1 to a 4:1 ratio. With the chain only moving one gear.

On this Jshaft he speaks of the chain starts with the engine sprocket , left J Shaft sprocket, Right Js Shaft sprocket , Crank sprocket , Adjacent crank sprocket, finally to the rear wheel.

I think I counted three chains and a sum total of 7 sprockets there. With only thinking about one gear in the rear. That is just to transfer the power there.

 

[Sidewinder Jerry]

Tomorrow I'll calculate up what he has with the information given. The Staton shiftkit has the outside drive coming from the gearbox and you can put up to 3 chainrings on it to drive the rear wheel.

 

[crassius]

saw a post in another thread by jaguar (who knows a lot about these bikes) in which he says the shift kit loses 8% of your power in the friction of all the gearing

 

[BrandonG]

So the new losses in friction are greater than the new gains from better gear ratio? Is everyone losing top speed with the Shift kit? I was under the impression I'd gain at least a few.

 

[Sidewinder Jerry]

My bike is an autoshift system. I've tried many setups what I've found is you're only going to go so fast on level ground and that's it. Going down hill is a whole different story. Some people even build down hill racers.

 

[BarelyAWake]

So the new losses in friction are greater than the new gains from better gear ratio? Is everyone losing top speed with the Shift kit? I was under the impression I'd gain at least a few.

It's a commonplace impression, but if ya ponder it for a mo it makes sense why you'd not gain any top speed - if the final (top) gear ratio is the same as a single, where would the "extra" power come from? If it isn't either the single speed is over/under geared or the shifter bike is*... so being able to shift or not to your top speed/gear makes no difference at all except one critical detail...

...that ol' underdog, acceleration.

Top speed is nice and all, but if it takes you all day to get there & you lose it the second there's even the slightest hill, it's not all that useful... unless ofc you live in a perfectly flat world w/o any winds lol

It's all about application, what you want from your machine - if you live where there's lots of twisties & hills and/or don't want to sacrifice top speed - but also don't want to be screaming near the redline just cruising around, then some sort of jackshafted shifting setup may be something to consider... whereas if you just want simplicity & efficiency & are willing to make a compromise between acceleration & max speed, then a single speed setup is for you.

While there are parasitic frictional losses with a jackshaft setup, they're comparatively minor - I've both a shiftkit and a DIY jackshaft setup & ridden for thousands of miles, I would say the concern isn't the added friction which at most would be about the difference between two different Tshirts & flutter drag, or a few PSI in your tires or even just their tread pattern - the number one concern should be reliability, the maintenance the additional chains & bearings require, which while not a dealbreaker (I wouldn't build another w/o a jackshaft) is most definitely far more an issue than a few arbitrary percentage points


*obv it's OK for a shifter bike to have it's final "over geared" to help keep cruise RPM down as you've a couple other gears to work with before ya get there - I skipped that to help simplify the example.