Gearing insight....

J

jspbtown

Active Member
#1
#1
Would love some feedback on my current gearing. Not really interested in top speed or out of the whole acceleration per se but rather is it too extreme?

So:
  1. Tecumseh HS50 motor. Runs well. No real mods other than a minibike style exhaust.
  2. Hilliard Extreme clutch with 2300 engagement springs, 12 teeth, and a #35 chain
  3. Clutch goes to a jackshaft with an 18 tooth sprocket
  4. Jackshaft to rear rim has a 13 tooth sprocket
  5. Rear sprocket has 60 teeth
I get a final ratio of 6.923 to 1.

Is this so far out of whack that the clutch won't even move the bike? Hilliard is suggesting its a gearing issue but asked for more details. And yes...I have ordered a Max Torque clutch but its not in yet.

Just thought I would seek out some more insight.
clutch3.jpg clutch4.jpg
 
ole4

ole4

#2
#2
That ratio should give you great acceleration. sounds like a clutch issue. When you give it throttle does it just rev up or does the clutch hold revs down?
 
J

jspbtown

Active Member
#3
#3
Clutch does not hold revs down. While up on my bench (with obviously no rider) it will have a slight pull at WOT but I can rock the bike back and forth with ease. No way it would move the bike with someone on it.

Its weird....6 bikes under my belt and none had this problem. Even used the same jackshaft set up (but with a Predator motor) which I know is more powerful but that bike would pop a wheelie out of the hole.
 
Charles S

Charles S

#4
#4
That might be a good overall ratio for a small wheel bike but you may need a higher ratio (AKA: lower gearing) for that large OD tire. For a given amount of power, the larger OD the tire, the higher ratio (lower gearing) needed (search "effective gear ratio"). If you browse the media section you will notice most minibikes have a rear sprocket relatively close to the diameter of the wheel rim, including large wheel bikes (with the exception of bikes with motorcycle-type engines/transmissions which have a primary reduction, crankshaft to clutch, plus the multiple reductions of the transmission). I'm guessing that your sprockets would be fine if combined with a torque converter but you may need a higher ratio (lower gearing) with a clutch, maybe something around 9:1(?)... @ole4 has much more experience than I do, though (he posted while I was typing and searching photos). FYI: Your master link clip is backwards, the closed end should always lead.

https://oldminibikes.com/forum/index.php?media/pxl_20210511_192857888-jpg.152489/

https://oldminibikes.com/forum/index.php?media/93c0b218-cba8-4a7b-9716-637811f95a3d-jpeg.151346/
 
Last edited:
J

jspbtown

Active Member
#7
#7
I have not tried to remove the rear chain. I do know the rear wheel and jackshaft spun very freely the several times I have had the clutch removed. It also rolls very easily when I did have it off the table.
 
D

desert rat

Well-Known Member
#9
#9
Is this clutch # 2? I would get a tac I have five different ones. The mechanical one goes in the bolt hole in the end of the shaft, it's old. One is a leave on digital. An optical, uses a reflective tape. A digital muti meter, you can clip it on the kill switch wire to get a signal from the coil. What I'm getting at is before condemning the clutch again get a rpm reading that you know for sure what it is. Second thing to look at is , is the throttle plate actually going to full open? You can "tweak" the linkage to get this and that does not affect the governor max rpm but will get you to max rpm which you may not be achieving right now. One thing you could try as just a quick look is to just grab the linkage and yank it open and see if the clutch grabs, ya that's over riding the governor for a couple of seconds but it will tell you real quick if it is rpm related. just be ready for it to hook up all at once. Just my two and a half cents.
 
J

jspbtown

Active Member
#10
#10
I am appreciating the insights....

Yes...Hilliard clutch #2. After I wrote to Hilliard they confirmed that the black springs I had on clutch #1 (3000 rpm engagement) and since that one got a little warm I decided to go with a new clutch with the white springs (2300 engagement). I figured if it was just the springs I could get some white springs for the other clutch for my next project.

So second Hilliard clutch has white springs (2300 rpm engagement). No difference in making it move.

Regarding the throttle.....I have hand pushed the carb to its max setting and held it there. I can easily hold the bike in place on my bench when doing that. It will creep but I can hold it bac with just one hand. It seems to rev up nice.

I have a multimeter. So I connect the positive lead to the kill switch wire and then the negative lead to ground? And then what do I look for for a reading at WOT?

Lastly...just for fun I swapped out the coil with a new one I had laying around and it runs the same...meaning it idles well and seems to rev nicely.
 
D

desert rat

Well-Known Member
#11
#11
Mine is a muti meter cause I got it in the mid '80s , don't know where i put it right now. These are a dime a dozen at garage sales. Try to find one that has 2/3/8.
 
minibikin'

minibikin'

Sho-nuff
#12
#12
Charles S said:
That might be a good overall ratio for a small wheel bike but you may need a higher ratio (AKA: lower gearing) for that large OD tire. For a given amount of power, the larger OD the tire, the higher ratio (lower gearing) needed (search "effective gear ratio"). If you browse the media section you will notice most minibikes have a rear sprocket relatively close to the diameter of the wheel rim, including large wheel bikes (with the exception of bikes with motorcycle-type engines/transmissions which have a primary reduction, crankshaft to clutch, plus the multiple reductions of the transmission). I'm guessing that your sprockets would be fine if combined with a torque converter but you may need a higher ratio (lower gearing) with a clutch, maybe something around 9:1(?)... @ole4 has much more experience than I do, though (he posted while I was typing and searching photos). FYI: Your master link clip is backwards, the closed end should always lead.

https://oldminibikes.com/forum/index.php?media/pxl_20210511_192857888-jpg.152489/

https://oldminibikes.com/forum/index.php?media/93c0b218-cba8-4a7b-9716-637811f95a3d-jpeg.151346/
Click to expand...
^^^ This. You're calculating static gear ratio. You need to take the circumference of the rear tire into account. You r static gear ratio on a 6" wheel bike verses a 12" wheel bike is off the spectrum....
 
J

jspbtown

Active Member
#15
#15
I have a tach/dwell meter from my old days restoring kits cars. So I guess a 4 cylinder reading divided by 4?

Oh...and by the way.....I installed a Max Torque clutch just now and it seems much better. Gotta finish work and get the seat installed and try it again. Avenger.jpg Deserter.png manx.jpg Sterlingfrontright-1.jpg
 
J

jspbtown

Active Member
#17
#17
So I installed the Max Torque clutch and got more movement. I am a big guy...280lbs...so its starts off a bit slow but as it warms up it goes pretty good and I can feel good engagement of the clutch when I back off the throttle and then stab it again. I also tweaked the carb adjustment, played with the governor, and the linkage.

So now I am torn.....is it not responsive cause I am a "big boned" adult? Am I expecting too much out of the little 5 hp? Or is the engine power down for some reason? I am finishing up another 5hp so I might try swapping them out.
 
mustangfrank

mustangfrank

Well-Known Member
#18
#18
jspbtown said:
So I installed the Max Torque clutch and got more movement. I am a big guy...280lbs...so its starts off a bit slow but as it warms up it goes pretty good and I can feel good engagement of the clutch when I back off the throttle and then stab it again. I also tweaked the carb adjustment, played with the governor, and the linkage.

So now I am torn.....is it not responsive cause I am a "big boned" adult? Am I expecting too much out of the little 5 hp? Or is the engine power down for some reason? I am finishing up another 5hp so I might try swapping them out.
Click to expand...
You're asking too much from a basic centrifugal clutch, you need a torque converter setup or a much larger rear sprocket and a high end centrifugal clutch. It does sound like your engine might be down on power, a compression test might clue you in.
 
J

jspbtown

Active Member
#19
#19
Thanks for the insight. Compression test yielded just over 90psi.

Let me pose this question...Since finding a rear sprocket was a bit of a challenge how about a larger sprocket (on the engine side) on the jackshaft? Currently I have 12 teeth on the clutch and 18 teeth on the jackshaft.
 
massacre

massacre

#20
#20
jspbtown said:
Thanks for the insight. Compression test yielded just over 90psi.

Let me pose this question...Since finding a rear sprocket was a bit of a challenge how about a larger sprocket (on the engine side) on the jackshaft? Currently I have 12 teeth on the clutch and 18 teeth on the jackshaft.
Click to expand...
using a bigger sprocket at the jackshaft from the PTO would help some. Using a smaller sprocket from jackshaft to rear wheel would help even more.
 
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