Centrifugal Centrifugal clutch engaging speed/RPM ?

Discussion in 'Transmission / Drivetrain' started by andyszyd, Oct 10, 2009.

  1. andyszyd

    andyszyd Member

    Hello everybody, I am a newbee on this forum, lurked for a while trying to learn from all experienced contributors.

    I am about to buy my first kit to put on my MTB.
    I want to use it on paved roads, but also off road, including step inclines, rough terrain (trails) intending to pedal assist when necessary.

    Mine primary concern is what is the lowest speed or RPM the clutch will engage and engine will effectiely pull?

    I know speed is relative to the gear ratio, I will need a low one.

    Do various centrifugal clutches have different or adjustable engagement speeds?

    I asked few vendors this question, never got clear answer, or they recommended NuVinci, or jack shaft drive.

    NuVinci is expensive and as heavy as half of weigh of some top quality bikes , jack shaft is difficult to install and I have a doubt if any deralilleur sprocket can survive the beating of combined engine and pedaling force for long.

    I want to keep it simple for starters.

    I will appreciate comments and analysis of experienced members, thanks all for your contributions to this wonderful site!

    Andrew S

  2. Mountainman

    Mountainman Active Member

    once engaged I would at least keep speed up to 3 or 4 mph
    so as not to be any slipping -- causing excessive wear

    adjustable -- basically no
    but one can buy different springs which will change THINGS up to plenty
    best done when we know what we are doing

    ride that thing
  3. augidog

    augidog Banned

    it seems that most utility engines start engaging at approx 3000rpm's stock...with the proper gearing (relevant to weight/horsepower) that's about the right range for a bicycle, imo. most (single-speed) kits using a utility engine & centrifugal clutch offer a decent range of gear choices.

    depending on your final config & how far you wanna take this, you may feel a need to raise the engagement speed, easily done with either a new clutch assembly or with luck a stronger spring for the existing clutch.

    my golden eagle tanaka 40cc (on a very heavy MB) currently engages at 3k but i've tweaked the heck outta the intake and exhaust (actually just bolt-ons from ADA Racing, good stuff) and it idles leaner & faster now, with smoother acceleration, plenty more low & mid torque, but no extra high-end...by the time i've got things rocked out it's possible i'll change to a slightly stronger spring, gear up, and pedal more during take-off.
    Last edited: Oct 10, 2009
  4. Mountainman

    Mountainman Active Member

    MBc Newbie

    since it's your first build there
    I am just thinking that your spring and engagement time will fit you just fine

    ride that THING and check it out that's what I am talking about
    may get so caught up in the good ride
    forgetting all about whats inside

    little spring don't mean a THING
    listen to that sweet THING sing
    I sure do like to ride this THING
  5. andyszyd

    andyszyd Member

    Thanks for advice guys, so you are saying by playing with springs tension I can change the clutch engagement RPM correct?

    Do different brands or kinds of clutches vary in this matter? (How they work and act?)

    At this point I am leaning toward GEBE drive with either Subaru 35 or Tanaka 40 for light weight and ease of installation.

    Also will check my local Harbor Freight store for 49/52cc water pump or auger engine.

    Ruled out friction drive, probably would not work well in off road environement dealing with dust, dirt and mud.
  6. Friction drive is not the choice for off road use. It requires a smooth treaded street tire and will just eat up an off road tread. The GEBE is a good choice for you; light, choice of easily changed drive gears from trail to highway, wide range of engines that will fit, etc. The best engine choice for you may be the Tanaka 40 for it's light weight and tunability for more low end and midrange torque as noted by augidog, or the honda 50 which is heavier but already has the low end and midrange torque inherent in 4 stroke engines. The Robin EHO35 is a fine engine but might be a little small for the heavy off road use you intend. You should leave the centrifugal clutch stock for a time to see how it works for you, then in time you may want to change the clutch springs in small increments to better suite your riding style. The springs are cheap and once you have some experience you will better know how changes in engagement speed will help. Good luck and have fun.
  7. augidog

    augidog Banned

    browse scooter and pocketbike sales sites for helpful pictures and info about replacement/hp clutches, you should catch on pretty quickly.

    common small-engine clutches use 2 or 3 shoes, & come in 52-54mm or 74-76mm sizes...both the EHO35 and PF-4000 use 76mm 2-shoe clutches.

    my costly personal & direct experience says: be wary of the 49cc scooter engines, i've been thru 3 of 'em, decent engines but QC is a weak point...and every time, i wondered why i bothered trying to replace my tanaka. truth be told, i wanted to try the starter/generator for lighting/safety...that din't work out so well.

    for anyone interested in my input: if (but not gonna) i were to try again (because i'd like a bit more displacement & can't find a tanaka 47cc anywhere) i'd get a "cag-47" and use the ADA Racing diaphragm-carby/billet-intake kit. QC is an issue with the cag, too...but it's been around for a while and has a fairly good reputation overall.

    dunno why i haven't tried a genuine mitsubishi, lord knows i've spent enuff on the clones, but from what i read they're nice offerings, too.

    again, direct experience...IF you choose GEBE and require low-gearing, you'll be using a 11-12T for the EHO35 or 12-13T for the PF-4000. the stock clutch config will perform just fine, and imo there's no beating the quality of either company's parts.

    like 'mike...all said, i recommend the PF-4000...it's built like a tank!

    just for giggles...my pf40, v-stack, hp-carby, juice-box, stock-crankcase, stalker-exhaust:

    and i hope i see you asking questions about wheels before you build...off-road motoredbiking will beat the heck outta most stock bicycle wheels...shop for suitable 12gauge wheels, which are available in a wide range of cost and quality.
    Last edited: Oct 11, 2009
  8. andyszyd

    andyszyd Member

    I just came back from my local Harbor Freight store, they will have 49/52cc engine with pump coming this week, can't beat it for the price I believe around 100 bucks and I have 15% disc cupon.
    I need to take the pump off, buy a clutch and put it in.

    They hade have very similar or identical engine with auger ($189) in stock, (this one has clutch installed but is almost twice in price, auger is useless for me.
    The engine looks solid and well build.

    I have 2 Chinese 160cc Honda clone engines, one powers my lawnmower, the other one pressure washer.

    They are quiet, start at first pull even after a few months with stale gas in the tank.

    Try that with Briggs & Stratton LOL!

    Even if this engine does not perform as well as Tanaka or Mitsu, or in case I kill it, that will be learning experience since I am new to motor biking.

    Better than experimenting with $300+ engine.

    By the way, do you think the Harbor Freight engine will mount with GEBE kit without major problems?


    better view of the engine:

  9. augidog

    augidog Banned

    i suspect that HF insists on better QC for their power equipment than most scooter-engine dealers.

    opinion only ---> 1.5hp/6500 rpm's isn't much for a 52cc, but folks seem happy with it. i wonder what size carby HF is offering on those. a stock tanaka 33cc easily compares and exceeds those specs.

    i cannot assure you (because i really don't know) that the pump-version will work...carefully check this topic for details:


    using the clone on the gebe-kit only required i file a bit off the flywheel cover where it contacts the mounting strap.
    Last edited: Oct 11, 2009
  10. andyszyd

    andyszyd Member


    Thanks for your thougthfulness, I checked the other thread before I bothered to go to the HF store, according to this thread you need shorter bolts to fit with BMP friction kit, nothing about GEBE.

    There is also an opinion on that thread, this engine is more like 2HP not 1.5HP.
    That is probably correct, Duropower.com has engine looking exactly like 52cc HF engine, the only difference to me is red color of fuel tank.

    The specs they provide claim 1.9HP at 6500 and 1.8 N.M torque at 5000.

    I like the light weight for 50cc engine 8.8 lb.

    Last edited: Oct 11, 2009
  11. augidog

    augidog Banned

    andy, i see you've done your homework...so the short answer is "just buy a stock clutch kit and get your feet wet."

    i'm glad to gab about what i directly know, and if nothing else i think i've expressed for you just how much a golden-eagle ring and belt can handle, see my site for "proof" :)

    i'd get all 3 drive-gears if you can afford the experience...the 12 will be boring as heck on the road but it will climb, the 13's in the middle duh, & the 14 should give you a hearty 33-35mph on asphalt with that engine. if your love of the motorsport outlasts the $100 engine, you'll feel confident getting something more high-end...if you get that far, you'll not regret it. when you have an engine for a while, you get attached, yaknow?

    have fun & keep us posted :cool2:
    Last edited: Oct 11, 2009
  12. andyszyd

    andyszyd Member


    Thanks for your help, professional advice and encouragement.

    I thing I swallowed the bug already, before I decide how many sprockets I get from GEBE would you tell me how hard it is to change one, how long it takes and what tools are required.

    Do they ever have their kits and parts on sale?

    On my future voyages and rides I will surely do some asfalt riding 10-20 miles to get to the mountains, and then back so highway gear would be helpful.
  13. augidog

    augidog Banned

    the kit w/o engine includes a clutch wrench & a 4-in-1 tool...that's all you need to change a gear & be back on the road in about 2 minutes.

    never seen the stuff on sale. i do know that extra belts & gears during a kit purchase will usually save you on extra shipping.

    btw: i'm a local-only gebe dealer but basically inactive, & i have no stake in any sales except my own. the only online sites i maintain is my ad-free journal, and the ad-free racing forums.
    Last edited: Oct 12, 2009