Push starting a centrifugal clutch

[Wendell]

My basic understanding of centrifugal clutches (known as "semi-autos") said they couldn't be push started (also called bump or clutch starting) because I thought the clutch would only engage when driven from the engine side, rather than from the wheel side. I just found out by trying it, that my Atomik MotoX 70cc can be easily push started (no bumping necessary, and obviously no clutch required!). The same with the other bike with me at the time, a similar style of no-name bike.

Then I did an internet search and most of the posters said centrifugal clutched/semi-autos can't be push started, so I'm hoping someone here can help me understand what's going on.

General info: I only tried starting it once, but it seemed 3rd gear was the best, which is perhaps one higher than you'd choose with a clutch bike but the higher gear helps overcome the resistance of not having a clutch, and being a centrifugal it then didn't stall at the low speed. Or I suppose (mainly for the sake of theory) you could roll in neutral then select first gear which would probably require a bump. Or a trick I picked up from my net search: if you push the gear lever to the gear you want but keep pressure on it so the lever doesn't return to the "foot pressure off" position, there will be less resistance to pushing, more like if you were in neutral, which could aid in getting the bike up to speed, then you release the lever.

In hindsight I think I should have realized it could be push started because of the (high) resistance to rolling in gear with the engine off. Also, it has plenty of engine braking, I'm guessing as much as a clutched bike.
Thanks for any thoughts.

 

[Hillz]

yep,

for some reason after i pulled my CRF50 down to paint the old girl up, it would'nt start when i put it back together, so i push started it down the road & nearly looped out when it fired up

 

[Custom]

pitty/CRF engines arnt a true centrifical clutch but more of a slipper clutch with mechenical disengaugment on shifting by means of a cam plate and lever asembly atached to the back of the shiter shaft. So the fact that you can push start the bikes actually shows that they are working well

 

[Cactus Jack]

Yeah they work like a slider clutch as used by top fuel drag cars ... (crowerglide) ....

They obviously drag all the time and when engine revs rise they engage harder ...

The old "hold the lever down" trick works a lot like a manual clutch .... we used to drop burnouts and wheel stands on old step thru bikes using that method but if you over do the revs you could damage the tranny .....

 

[tormentandglory]

Those honda auto clutches are an engineering marvel. Theres a tapered cam device in the back of the clutch that makes it engage when you push it forward, so you get an engine braking effect when you back off the throttle. Most centrifugal clutches don't do that. You'll notice that you can wheel the bike backwards when in gear with no resistance. Only when you roll forwards does the clutch engage.

 

[Wendell]

Thanks guys, it makes more sense now. Does anyone have a link to a diagram or explanation of these clutches?

 

[tormentandglory]

And without the one-way mechanism, the kick starter wouldn't work either!

 

[orion125proseries06]

and honda being so awsome, that i have a trike (3 wheeler) from 1976 (thats 32 years!) we can do that on the trike, it is a semi-auto and we always used to push start it, they are indestructable (dad washed it down quit a few creeks) and it was shared between four brothers so its pretty good. and they are heaps easy to work on too, simplest engine you could find that actualy lasts!
they dont build em like they used to!

 

[numroe]

pitty/CRF engines aren't a true centrifugal clutch but more of a slipper clutch with mechanical disengagement on shifting by means of a cam plate and lever assembly attached to the back of the shifter shaft. So the fact that you can push start the bikes actually shows that they are working well

Interesting stuff. Would that explain why the shifter mechanism on my daughter's 70cc ducar is so difficult to move? I mean stiff spring load. Even difficult for an adult. As a result she leaves it in 3rd all day, but she's getting heavier now and sometimes needs 2nd.

Can I adjust this honda/ducar auto clutch in some way to soften the shift lever action for her, but still retain adequate clutch operation.

I'm also wondering about the idea of putting an auto clutch in another manual pit bike for my kids. Which leads me to ask:

1) how this XTPowerSports auto clutch assembly compares to the Honda/Ducar design: XT Power Sports Auto Clutch Assembly Does this USD99 kit also have a means of disengaging the clutch for shifting, or do the gears just get crunched? I'm assuming the manual clutch cable mechanism cannot be used as an over-ride with this kit.

2) Is this clutch the same as the stock Honda design: eBay Motors: NEW TRAIL BIKES HD AUTO CLUTCH XR50 CRF50 XR CRF 50 nr (item 200287852611 end time Dec-19-08 15:57:39 PST)

 

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CT-110 clutch

I have a CT-110 motor I am going to beef up and I was wondering if anyone knows what would be involved in converting it to a clutch like my 125cc lifan has since I do like having a lever.