DIY Shift Kit Project--'The Shifty Beast'

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lowracer

lowracer

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Oct 17, 2008
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Furry,
I owned a Mazda RX-7 turbo many years ago with a rotary engine & I hated it.
Zippin, I probably hit about 40 mph & I didnt get low and aero w/ the Mitsu engine which isn't a high revving powerplant. I geared this puppy for the Tanaka 47r which I still have in a box. I may either look into the lay back seatpost idea or remounting the pocket bike transmission over the top tube of the bike closer to where the triple clamp forks would contact the engine if turned completely to the left. This would allow some more clearance and allow either an expansion chamber pipe or china girl std pipe wrapped in header wrap.
-Lowracer-
 
unknownbmxa88

unknownbmxa88

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Mar 15, 2013
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montana
thats sweet. you did you diy shift kit simular to my gx35 full suspension ...mine has eaten 2 derailers so i think im going to a internally geared hub
 
lowracer

lowracer

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I did some brainstorming today about where to re-mount the engine for more legroom. I was either going to a rear-rack mount but bolted solidly to the bikes frame (not moving with the rear wheel), or just higher and more forward than previously mounted. I decided on an "Above-Frame Mount".
You always hear about "In-Frame", but what is "Above-Frame" you might ask yourself?
Here are a few pics of where the engine is going. It should allow me to monitor all engine functions by site....who needs guages?
I've had a few few front rack front wheel drive mounted engines in the past (see avatar pic) & enjoy having the engine and drivetrain visible, provided they aren't oil spitters (like the Cags). I'm gonna mount the Tanaka 47r this time around and its tight.
Also, I needed to remove my rear derailleur engine chain tensioner (located on the non-drive side of the chain), since under pedalling loads it puts the derailleur onto the drive side of things causing chain problems.
By mounting the rear portion of the angle aluminum with one bolt through the seat tube gusset, the front portion of the angle aluminum can be raised a bit and clamped down on top of pieces of skateboard riser pads, creating an adjustable chain tensioner. I'll also wrap and tape the section of top tube with some thick inner tube for protection and vibration dampening. I'll get rid of the rear derailleur tensioner, and just get the engine chain perfectly tensioned (like on my fixie bike). I still have a few half links of #410 chain I bought for my fixie to further fine tune the length of the chain. This should allow pedaling, engine power or both without throwing a chain...We'll see.
-Lowracer-
 

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unknownbmxa88

unknownbmxa88

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i would almost do a rear rack mount. having that engine there seems a bit iffy to me, lol but i dont have much room to talk cuz my modified in frame gx35 has the rod pointed right at my nuts and i guess they can blow a rod through the valve cover if the redline too long.
 
lowracer

lowracer

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We'll see how the 'over top tube mount' works out since I've committed myself to trying it out.
Reasons it could be a dud:
1- noise
2- heat
3- muffler interference
4- oil spit
-Low-
 
lowracer

lowracer

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I got some more work done on it last night and its coming together.
Using 1 half link for the chain was necessary to get it to the correct length.
The adjustable transmission mount allowed for take-up of the chain slack.
I added some additional supporting aluminum & the one in the 1st pic also serves as a chain protector (leg protector). I think I'll add a short piece of silicone tubing to the end of the muffler with a hose clamp to route the exhaust further below my left foot.
The muffler clears my leg and pedal stroke much better than the previous mounting of the transmission in-frame. Lastly, I'll need to mount the fuel tank.
-Lowracer-
 

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lowracer

lowracer

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Worked on the bike most of the day today after riding my regular pedal bike for exercise this morning.
I got the fuel tank mounted and took it for a spin. Conclusion, The Tanaka 47r with a shift kit really wont be doing much pedaling. I did tool around my block under pedal power mode. When you pedal, you are also turning the 1:5 transmission which is not hard to pedal, but very inefficient. I think I'll add back the rear derailleur chain tensioner to the engine non-drive side and not pedal the 'Shifty Beast'. I may need run the legs around every now & then for bystanders & police, but the front freewheel wont be catching.
Here is the latest pic, but I took it before adding the derailleur & some silicone hose to the muffler tip.
-Low-
 

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lowracer

lowracer

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I've been itching to take it out for a rip now thats its done.
I just got back from about a 20 miler to really put her through her paces.
This thing hauls butt!
Only problem is: my ears feel like I just got back from a rock concert.
Its really fun to click through the gears accelerating.
Downshifting 1 or 2 gears for turns and getting back on the gas, also fun.
I'm very glad I wrapped my pipe with header wrap or I'd have a few burns.
I'm very happy I finally did a Shift Kit.
I may give it a go one more time to mount the engine behind me to reduce the hellacious noise to my poor old ears...
-Lowracer-
 

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Dan

Dan

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May 25, 2008
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Really looks cool LowRacer. Has a funky cross look of a madmax/mars rover/military ATV thing happening.

Looking forward to seeing your rear mounted version.
 
lowracer

lowracer

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Thanks Dan.
I'm still contemplating the rear mount.
I rode it a bit more yesterday & it wasn't as obnoxious (maybe cause I didn't flog it as much...lol)
I let a buddy of mine ride it and the look on his face as he passed by was priceless...like a kid in a candystore.
After a few laps around the block he started to get the hang of the shift.
I'll need to go out & buy some more high octane no-ethanol gas today (7 miles away) for the next bunch of fun filled motorized bicycle miles.
:)
-Lowracer-
 
5

5-7HEAVEN

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Aug 2, 2008
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Gear ratios & speeds are as follows:
12T-44T
24T-9 speed cluster
8625 rpm's-26" wheel diameter-5:1 PB transmission
--------------------------------------------------
34T=25 mph = 25.97:1
30T=29 mph = 22.92:1
26T=33 mph = 19.86:1
23T=38 mph = 17.57:1
20T=43 mph = 15.28:1
17T=51 mph = 12.99:1
15T = 11.46:1
13T = 9.93:1
11T = 8.40:1

As I see it, your 1st gear is ok, but hard to climb from the bottom of steep hills and ramps from a stop. Second and third pulls well in midrange.

Your engine feels best on the flats in 4th gear. Sixth thru 9th gear is too high a gear ratio for this engine.

What you need is a smaller trans sprocket AND a larger chainring. Instead of 12t/44t, your optimum is 10t/65t. With that change, here's what you'll have:

34t = 46.04:1
30t = 40.63:1
26t = 35.21:1
23t = 31.15:1
20t = 27.08:1
17t = 23.02:1
15t = 20.31:1
13t = 17.6:1
11t = 14.9:1

In 1st gear, you will be able to climb from bottoms of extremely steep hills and ramps from a stop. On flat ground, you can start off in 3rd gear. Fifth gear will be lower than what your present First gear is. Presently, your last four gears are not useful. With this gear change, all 9 gears are useful. You will prolly like using just 3rd, 5th, 7th and 8th gear. Ninth gear is your highway gear.

Once you change sprockets, the 47R engine will come alive!

Regarding the noisy engine. It looks like you're using the Happy Time basic muffler. If you change exhaust to Sick Bike Parts' high performance pipe, engine noise will reduce dramatically. When used on my loud CY460 engine, noise levels dropped so much, I swear I could hear the piston ring scraping the cylinder @ idle!

Your best exhaust is the 3-piece Tanaka-specific pipe, which allows the stock muffler to bolt onto the tip. This pipe makes the most hp and quietens down the 47R greatly.

Hope this helps.xct2
 
5

5-7HEAVEN

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Aug 2, 2008
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If you can't find 10t/65t, then use an 11t/72t in T8F chain drive.

It'll be close enough.
 
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lowracer

lowracer

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Oct 17, 2008
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5-7,
I have a 10T sprocket on a different project that I'm going to 'borrow' to experiment with. Adding just that to my existing drivetrain will get my 1st gear ratio from 25.95:1 to 31.15:1 & give me a more useable 6-7th gear. Its flat as a pancake here where I live so I have no hills to speak of, but I do feel 1st gear isn't being optimized with my 12T right now.
Seriously though, I kinda wish I had a 3 speed internal hub since I feel all the shifting is a bit much everytime I take off & then stop (I pedal a fixed gear road track bike for reference). It is great fun though 'going thru the gears' and reminds me of my old riding dirtbike days. The ratios are close and keeping the engine 'on the boil' adds to the fun factor.
5-7, your insight has always helped me with my motorized contraptions over the years and I thank you.
-Lowracer-
 
unknownbmxa88

unknownbmxa88

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Mar 15, 2013
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i have been looking at a three speed hub for mine too. my bike has eaten 2 derailers in the last 50 miles lol. im contemplating buying a 5 speed sturmey archer with a drum but im shourt on cash at the moment, and i dont know if they hold up.. i know the three speeds are supost to be ideal, i am just worried the gears would be too spread out.
 
lowracer

lowracer

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Oct 17, 2008
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unknownbmx,
I'm wondering why your eating derailleurs?
I know that if a rear derailleur isn't perfectly aligned & adjusted, it can ghost shift between gears. Under load with a high rpm engine that could be very bad.
If you stick with a rear derailleur setup, order new derailleur inner & outer cable. Lube the cable & get it set up making perfect shifts in pedal only mode.
I still think an internal hub would be best due to the straight chain line.
-Low-
 
16v4nrbrgr

16v4nrbrgr

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Mar 17, 2012
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North Bay
I hope my SRAM X7 plays nice with the Wippermann Connex chain for my shifter. I'm glad you're getting good acceleration and its working out well, as your gearing is very similar to mine. I think it would be neat if there was a 9 speed cassette that went from a 50t downward to maybe a 15t, but that doesn't exist and I'm kinda stuck on narrow motor sprocket and trans reduction gearing so I hope at least my bike performs well on gentle inclines, this thing I'm making is really a toy dreamt up when I thought I'd have the cash to go to SoCal races, so uphill performance doesn't concern me too much.

It would be interesting to have a "low range transfer case" of sorts by using the sbp shift kit with a 3 speed derailleur going to the rear, then a MTB could have a range of gears for the steepest of trials stuff to fast smooth straights at 50+. Of course with the complexity of it, the reliability goes down, so its difficult to have your cake and eat it too.

Keep it up lowracer, your proofs of concept push drivetrain configuration boundaries!
 
lowracer

lowracer

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16v,
Thanks.
You can buy a larger 36T, 38T, or 41T rear cassette sprocket on eBay that a normal MTB rear derailleur can handle. They run from $37-$43 & are made of stainless steel. You just have to remove your smallest rear cog. The ebay seller is mtbtools & you can do an advanced search to find all his items.
Personally, If I had to do it all over again, I'd probably just buy the SBP 130 bcd adaptor for the front freewheel & run a 53T & 24T on the front freewheel and std rear cassette 11-34 with a 10T or 12T trans sprocket. I prefer to run std #410 bmx/fixie 1/8" chain from engine to 53T since that's what I use on my 2 road bikes (pedal only).
Either tomorrow or on the weekend, I'm going to switch my trans sprocket from 12T to 10T to see how that performs. I think it will work out better off the line & allow another gear or two on the rear cassette to be utilized.
-lowracer-
 
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