NuVinci rear sprocket skipping

Discussion in 'Rack Mounted Engines' started by duivendyk, May 3, 2008.

  1. duivendyk

    duivendyk Guest

    When I started building my NV based system I decided that it would be nice to retain the front derailleur on my Raleigh comfort bike.This would allow me to keep my pedaling speed within comfortable limits, also the rear derailleur could easily be adapted to function as a chain tensioner.
    A chain tensioner is worth having because with vertical dropouts there is no way to adjust the chain (except by adding or removing links), which is hit or miss, more
    likely the latter, obviously something to be avoided.
    After I had finally got everything together,I tried things out with the bike suspended from the joists in my basement,the 43 cc. Mitsu started right up and everything seemed to work just fine,including the front derailleur.As an extra precaution I had added a chain guard so that the chains could not possibly interfere with one another.
    When I started to road test the bike, things went fine in the power assist mode,but the chain started skipping teeth when in the pedal only mode esp. when going up an incline.At first I thought that the derailleur idler was too far from the rear sprocket and reworked things so that it was quite close to it,this did not seem to help at all .Finally I increased the chain tension substantially by installing a spring.Surprise,that did the trick !.It turns out that the cause of the problem is the rear 16 T freewheel sprocket that comes with the hub,it has a tooth profile that is not compatible with a variable length chain,or certainly not optimal with any standard chain, as far as I am concerned.
    A properly designed chain sprocket provides a semicircular "pocket", between the teeth (with some sideways clearance) for the chainrollers to nest into and the tooth flanks are fairly steep to keep them in place.The NV sprocket has a narrower pocket,it looks like less than 120 degrees, the sides are relatively long and at a more shallow angle,this makes the chain more prone to ride up that slope when under heavy tension and skip to the next tooth, and so on.What keeps this from getting out of hand is, either the chain length is fixed and without much slack ( "riding up" increases the effective diameter of the sprocket), so a longer chain is needed,or enough tension is put on the back chain to keep the rollers in their pockets and not ride up the side of the teeth.It could be that the european chains have smaller rollers.My chain was brand new one for derailleur bikes made by Sachs, over there.I don't know yet if increasing the chain tension will make it difficult to operate the front derailleur,may be I'll have to do without after all.
    My first impression is that the NV hub is not a particularly efficient transmission device compared with other internal gear hubs I have had experience with.Comparison is difficult since it has a very wide range and the motorized bike is a lot heavier,JJ
     

  2. I sometimes have the same problem with my shimano 7 speed cassette, but I can usually whip it by adjusting the cable tension on my shifter. Just one or two clicks one way or the other will usually do it. I find myself having to do this quite often.
     
  3. sparky

    sparky Active Member

    This is why I gave up on derailers. BMXs and internal gears for life!
     
  4. duivendyk

    duivendyk Guest

    Apparently there was more to derailleurs than I had thought in my innocence,never had any problems before with the Shimano variety,just clean them once in a while.I can still use the front one but it's not too happy shifting,I don't have to do it all that often anyway with my power assist,JJ
     
  5. SirJakesus

    SirJakesus Guest

    With the wide range on the NV I'm surprised you need the front shifter at all. I haven't tried any steep hill climbing but in low gear my bike is very easy to pedal around even while sitting down. Maybe you just have to experiment with which size front sprocket best suits your needs for motored and un-motored riding and choose the compromise if it keeps giving you trouble. I geared mine straight with the large chainring (44t maybe?) and used the heavy duty single speed chain found at wal-mart. It seems like it gears the pedals a tad bit high which I kinda like while pedaling motored because I dont have to spin the pedals like a madman, theres always a good amount of resistance.
     
  6. sparky

    sparky Active Member

    I can't understand the need for the front derailer either. Isn't that the whole point of getting the NuVinci?
     
  7. Yeah...I never had any problem with the chain jumping until I replaced my back rim/hub, (shimano hub), and with that came this 7 speed cassette, which should be perfect for my Schwinn Alloy 7 cruiser. The stock shimano freewheel didn't give me any problems at all. The only time I use 6th or 7th gear anyways, is when I'm negotiating a long hill. 7th gear is pretty comfortable at 20-25 mph I suppose. Just have to tweak the cable tension once in a while.
     
  8. duivendyk

    duivendyk Guest

    I would agree that at first blush, it looks a bit odd to want to extend the wide 3.5 range of the NV hub even further with a front deraileur.But there are solid arguments for doing this:
    1) Keeping the pedal speed within a comfortable range, 60/85 rpm has it's appeal,at least for a geezer like me. With a fixed ratio to the rear wheel,the pedal speed is proportional to the motor rpm,which can vary by something like 80%' that is 4000/7200 rpm, or 55/100 at the pedals,which is pushing things.With a front derailleur (38/48T). this can be reduced to 42% , or 60/84, that's is more to my liking.
    2)Limp back capability,while transmission range is important,how it is used,the actual overall gearing is what really matters.On my Raleigh bike with front&rear derailleurs,which I converted, I had a minimum ratio of 28/34 or 0.82 and a max ratio of 48/13 or 3.7 from the pedals to the rear wheel. The overall ratio range of 4,6 exceeds that of the NV hub by 30% .You would not do too well with just the NV in the hills around here!
    My lowest NV + front derailleur ratio is 1.28 (38T front,16 T rear, NV ratio .54). which is 56% higher than what it was on my bike prior to the conversion.So riding the much heavier bike up any incline is going to be pretty tough slogging.Moral behind this: don't run out of gas,carry tools,spare plug.On second thoughts I could have been better off with a larger rear freewheel sprocket (18 or20 T might have been better (those are specials at Staton I think),then the minimum ratio would have been 1.02,still 25% above my pre-conversion ratio.This raises the pedaling rate in the assist mode too.Take a look at my table below that lists pedaling rates for various engine speeds and sprocket sizes.
    You might say, why worry about what can go wrong,I try to think things through, which has allways served me well in my engineering work.
    3) Practical considerations,the NV hub is designed for fixed length chains and for bikes with horizontal dropouts,which permit for/aft movement of the rear axle to adjust the chain.
    In American (Chinese) derailleur bikes with fixed vertical dropouts no such adjustment is possible,nor is any required since rear derailleur takes up the slack.Taking out 2 links at the time with a fixed length chain with chaintensioner leads to exessive slack,unless youhappen to luck out .You might as well keep the rear derailleur and use it as a chain tensioner in a fixed mode,if so why then not also keep the front derailleur,it still can be useful !(The lowest speed 28 T cannot be used it gets too dicey as far as clearance between the chains is concerned)not to mention the skipping problem.
    Below is a table of pedaling rates (cadence, in bikers lingo) for different engine speeds and various front &rear sprocket sizes.The freewheel sprocket on the Staton gearbox has 16T and the NV input has 27T, for an overall reduction of 31.64. from engine to NV input
    The front derailleur sprocket sizes are 38 and 48 T, the rear one 16,18 or 20,the combinations are shown as 38/16,48/16 38/18 etc
    PEDALING RATE
    Rpm-----38/16---48/16----38/18----48/18---38/20---48/20
    4000------55------43--------62-------48-------69-------54.
    5000------69------54--------76-------61-------86-------68
    6000------82.5----64.5------93-------72.5--- 103-------81
    7000------96------72--------108.5----84------121------94.5
    It is obvious that either the 16 or the 18 T rear sprocket are reasonable choices.The 18T looks advantageous in light of better performance in the emergency mode.
    Since I was crunching numbers anyway I have added a final table of min. and max.road speeds, listing engine rpm,rear wheel input rpm. min/max speeds based in a NV ratios of 0.53 and 1.86 with a wheel circumference of 200 cm or 79 " (26 " wheel).
    Eng.rpm----rearwheel inputcorrection---- min. speed-----max. speed
    ------------------------------------(mph)------------(mph)
    4000-----------126-----------------5.0--------------17.5
    5000-----------158-----------------6.25------------ 21.9
    6000-----------190-----------------7.5--------------26.2
    7000-----------221-----------------8.75------------ 30.7
    In flat terrain,a larger gearbox output sprocket could be used, depending of course also on engine size.With a 20T sprocket,the max speed would become over 38mph. assuming that the engine does not run out of torque.I don't know what the rev. limit on my Mitsu TLE 43 is,I guess around 7500 rpm.Info is hard to come by,JJ
     
    Last edited by a moderator: May 10, 2008
  9. duivendyk

    duivendyk Guest

    Deraileurs may have their weaknesses,for sure, they are rather vulnerable mechnically and I can appreciate that hard-core mountain bikers would rather not to mess with them,the weak part of geared hubs is the control cable etc,the SA was pretty vulnerable with that silly chain,the old Shimanos were much superior in that regard,and I'm pretty the sure the modern Nexus hubs are quite good too_One nice thing about deraileurs is that they fail gracefully and you can figure out what's wrong and you may be able to jury-rig up something. Also the thing usually will not totally die on you,in contrast to an SA hub that once completely froze up on me,in some god forsaken part of Scotland.I had to hitchhike my way, bike&all to a bike shop,it rained too.I had busted the clutch cross which was in 2 pieces,not cheap to fix either.Most internal gear hubs don't get adequate lubrication,keep them oiled if you can find a nipple!.The problem with the geared hubs from our perspective is that there is no room axially to stick another sprocket on the drive side,as Staton managed to do with the NV hub,JJ
     
  10. SirJakesus

    SirJakesus Guest

    I agree about carrying tools and making sure to never run out of gas. I dont have a spare spark plug for the TLE43 yet but I've yet to encounter a sparkplug so fried that I couldn't give it a little love and bring it back to life.
    It's still a neat idea, if you get it to stop skipping totally and make it solid I may be interested in the mod in the future. Thanks for posting :)
     
  11. duivendyk

    duivendyk Guest

    I allways carry spare plugs esp.with 2 stroke engines.Ignition used to be a weak point,in the old days of leaded gas.You can change one in a jiffy,it might work and you're back in business.It takes a 3/4" deep socket,an odd size,the common ones are 13/16"
     
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