DB30 97cc Jackshaft?

#1
Hi All,

I've been lurking around for a while and it appears that I waited too long to buy two PMR jackshaft kits from HotRodMiniBike.com. Is there another place to buy them? If not what are my options?

I have two stock DB30s and two young kids riding them. I'm not quite ready to swap out the engines (until they are a little older) but we would love the increased low end torque and higher top speed. Additionally, I loved the idea of the kit that comes with everything I need, and bolts on quickly.

Thanks in advance!
 
#3
Yea, I chatted with Brad & Shawn a few times.... ''Good Guys''.
Brad custom machined me a velocity stack once for a little Lifan on the Lathe, & it turned out awesome.

Anyway,,
What I did with my MM-80 ''pretty much same engine'' as the DB's,, was I got a Jackshaft plate & parts from BMI carts & parts.
Nice quality aluminum Plate was like 38 bucks & aluminum housing with bearings & bolts was around 13.

You have to get a ''short'' 5/8 shaft, shaft collar,, thrust washers,,, key stock,,,, & #35 chain 5/8 bore B sprockets also.
Also need hardware to bolt the plate up to the side case.

If you get to wanting those, I will chat again with you on sprocket sizes & ratios. ;)

I 100_5525.JPG
 
Last edited:
#4
Added,,
In re reading your initial post Michael,,

with the jack shaft,, you are ''not'' going to get both more bottom end torque & more top speed,, it's going to be one or the other.
Your stock DB 30 rear sprockets are probably around 75 t., & am guessing the stock clutches are probably 11t.
That gear ratio is actually pretty high already.
PMR's jackshaft kits had B sprockets in them to make the gear ratios even higher, so as to help get them rolling better from a stop ect. especially for larger persons riding.
That in turn with the governor still in, & limiting the top rpm's to around 3,800,, top speed would be ''less'', but would get out of the hole ''a lot'' better.
Going with Max torque 5/8 bore SS clutch's, with Black springs,, would help some just in that, as the engagement of the clutches is @ higher rpm's & will allow the engines to spool up rpm's more before the clutch shoes grab,
which translates to less stress on the crank & rod ect. ;)
From my experiences with the 100cc & under engines, the black springs are a must,, even with jackshafts.
I
 
#5
Thanks I74,


I have inadvertently revealed another reason that I like the idea of a jackshaft "kit"... my lack of knowledge about jackshafts. :)

I looked around on the BMI page and it seems like it won't be too difficult to piece together everything that I need, but I don't know what I don't know so I anticipate there might be problems when I start to put everything together. Then multiply that by two since I’ve got two DB-30s.

A couple of initial questions:

1. Shaft. What length of 5/8" keyed shaft should I buy? Can I just buy it extra long and cut it to length with an angle grinder? (probably a dumb question) How long of a shaft is typical for this type of set up?

2. Gear ratios. I have a bunch of kids, but the youngest two (8 and 10 years old) will be riding the minibikes the most. We mostly ride them in the mountains of Colorado (about 8000 ft elevation) and are interested in low end torque. (This seems to be difficult at high elevation.) From what I’ve read, it seems like we should shoot for a 7:1 gear ratio. Does that sound like a good jumping off point?

3. Chain. Should I just buy a bunch of #35 chain “by the foot” and master links to make the two chains for the jackshaft?

Thanks again for the help!
 
#6
Hi Michael,
I lived all over Colorado for years....
Firstly,,
The 5/8 shafts you need are the 4.5 in. ones,, for 12.95 ea.
They are ''way to hard'' to cut off with a grinder,, so I would just buy 2.
Looks like the Jackshaft, hub,, & hub bolts come all together for 38.95,, which is a really good deal.

If you have a Theisen's,, I would buy a 10ft. roll of the #35 ''DID'' chain, & a 4 pack of DID master links.
That quality chain is made in Japan & is a great deal @ - 20 bucks for a 10 ft roll.

Stock gear ratio with an 11t clutch & 75t rear sprocket is 6.8 - 1.

I would count both sprockets first to verify.

If that's what it is,, getting to an 8 - 1 + ratio should be pretty good for hills, but you will definitely need the black springs to also help keep the front ends down. ;)

Let me know what you have, & we will go from there …..

I
 

DaddyJohn

Well-Known Member
#8
Added,,
In re reading your initial post Michael,,

with the jack shaft,, you are ''not'' going to get both more bottom end torque & more top speed,, it's going to be one or the other.
Your stock DB 30 rear sprockets are probably around 75 t., & am guessing the stock clutches are probably 11t.
That gear ratio is actually pretty high already.
I

I was thinking, another way of putting it is the jackshaft's main purpose (in this application) is to allow you to lower your gearing without resulting in a rear sprocket that's as big as the tire.
 
#9
Thanks for all the help!

I'll try to put together a list of parts (and links to parts) that I order from BMI. Hopefully this will save someone else some time putting together this jackshaft project in the future.

Thanks again!
 
#12
About .005", actually, the 16mm being the larger (16mm = 0.629921, round it to 0.630", 5/8" = 0.625"). Chinese engine, metric crank. A 5/8" clutch is going to have a bore slightly larger than 0.625", it has to have some clearance to slide easily onto a 5/8" crank. A 5/8" clutch should fit a 16mm crank. If it doesn't slide on easily, polish the crank with a strip of 400 grit, or finer, sandpaper.

If you gear the stock engine to where you have ample torque, you're going to have a rather low top speed. What @I74 hasn't told you is that he soon got rid of the small engine and jackshaft and put a larger engine on the bike. ;) The jackshaft mod is more complicated than an engine swap, you will have two chains to align and adjust slack. IMO, an engine swap is a much better option. You can limit the throttle opening if you feel a need. With a stock 70T DB sprocket and a 12T clutch sprocket, you would have a 5.833:1 ratio which should be fine with a 196-212; you could replace the rear sprocket with a smaller one later for a lower ratio (less torque, higher top speed), if desired.

https://www.oldminibikes.com/forum/...-rare-chrome-mbx-11-pipe.168165/#post-1229864

https://www.oldminibikes.com/forum/...pping-in-a-gx-200-in-the-modded-mm-80.167374/

https://www.oldminibikes.com/forum/index.php?threads/baja-blitz-motor-upgrade.169736/
 
#13
About .005", actually, the 16mm being the larger (16mm = 0.629921, round it to 0.630", 5/8" = 0.625"). Chinese engine, metric crank. A 5/8" clutch is going to have a bore slightly larger than 0.625", it has to have some clearance to slide easily onto a 5/8" crank. A 5/8" clutch should fit a 16mm crank. If it doesn't slide on easily, polish the crank with a strip of 400 grit, or finer, sandpaper.

If you gear the stock engine to where you have ample torque, you're going to have a rather low top speed. What @I74 hasn't told you is that he soon got rid of the small engine and jackshaft and put a larger engine on the bike. ;) The jackshaft mod is more complicated than an engine swap, you will have two chains to align and adjust slack. IMO, an engine swap is a much better option. You can limit the throttle opening if you feel a need. With a stock 70T DB sprocket and a 12T clutch sprocket, you would have a 5.833:1 ratio which should be fine with a 196-212; you could replace the rear sprocket with a smaller one later for a lower ratio (less torque, higher top speed), if desired.

https://www.oldminibikes.com/forum/...-rare-chrome-mbx-11-pipe.168165/#post-1229864

https://www.oldminibikes.com/forum/...pping-in-a-gx-200-in-the-modded-mm-80.167374/

https://www.oldminibikes.com/forum/index.php?threads/baja-blitz-motor-upgrade.169736/
If he didn't have smaller kids riding these bike Charles, I wouldn't of put info about the Jackshafts.
The set I did though with my little Lifan engine pulled my 200 + lbs with out much trouble.
Basically did it because of being a challenge
The Stage 1 GX200 that is currently in it though is like a piece of heaven, & I basically can cruise @ an idle, & it is quick as a Harley for about a block.

Can not recommend an engine like that for 8 & 10 year old kids though ..

The 5/8 bore Max torque clutch that I put on my little lifan engine fit fine with just a wrap of aluminum heater duct tape around the crank to take up the slight gap.
The ID of the clutch on mine was actually a frog hair larger than the OD on the crank
 
Last edited:
#14
Can not recommend an engine like that for 8 & 10 year old kids though ..
Leave the engine stock and limit throttle opening. If he keeps the bikes he will likely end up doing the engine swap after a while anyway and the jackshafts will be in the scrap pile. JMO. He could do the jackshaft on one bike and engine swap on the other, see which is better...
 
#16
Added,,

Michael,
Would still like to know what the teeth count on your sprockets are.

If you do have 70t rear sprockets on them, going with 75 tooth ones with Max torque 11t SS clutches & green or black springs,, would noticeably help things out a bit right there .;)

Even better would be split sprocket hubs, & 79t split sprockets.
That would give a ratio of 7.182 - 1
 
Last edited:
#17
The 5/8 bore Max torque clutch that I put on my little lifan engine fit fine with just a wrap of aluminum heater duct tape around the crank to take up the slight gap.
The ID of the clutch on mine was actually a frog hair larger than the OD on the crank
Consensus is that the clutch should "float" on the crankshaft, allowing a bit of lateral movement.
 

Top