Problem with kit chainring and grubee shaft bushing

Discussion in 'Frame Mounted Engines' started by Irish John, Apr 2, 2009.

  1. Irish John

    Irish John Guest

    I have two problems to report. I have built several Honda \Grubee frame mounts on the Schwinn Deluxe 7 spd with nexus hub. The 44T chain ring that is supplied with the kits is never quite round (see attached picture) and this means that on a geared or single speed hub the chain tightness will vary. I've known for a long time that the rear sprockets aren't quite round but this hasn't affected the running of the bike much. On a coaster hub the varying chain tension causes the chain to come off when pedalling hard up hills. When the chain comes off a derailleur it's a synch to fix but no so with a coaster hub. You have to undo the coaster brake and undo the wheel nuts and slide the wheel forwards enough to get the chain to ride back over the chainring. Has anyone else noticed this problem? Taking a lot of care aligning the chainring with the rear sprocket helps to get the pedal chain to stay on but the varying tension only causes the chain to come off when it is under a lot of load. The lack of roundness in the chainring makes a lot of difference to the chain tension even though it is only miniscule the chain needs to be good and tight on a coaster hub. The coaster brake fails of course when the chain comes off.
    The other problem is that the shaft bushing on a recent Grubee kit was not a good fit with the clutch bell housing and there was lateral play between the bell housing and the bushing. I'm not referring to the play inwards and outwards along the shaft that is fairly common on all kits and pretty easy to fix but a play at right angles from the shaft because the fit between the bell housing and the bushing isn't engineered well enough. This is the first timeI have encountered this problem and I will try to get a new bushing to see if it fixes the problem. If not the problem must lie with the bell housing itself.
     

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  2. fetor56

    fetor56 Guest

    Can't help much John but with your first problem i made absolutely certain(with digital calipers) that my wheel RIM was perfectly in-line with both chainstays.......possibly that helped.
     
  3. Irish John

    Irish John Guest

    I think what you are saying has a big bearing on it Fetor because the temptation to skew the wheel away from the drive chain is hard to resist if you don't want chain slap marks on your whitewall tyres. I do skew the wheel to the right precisely for this reason but I also take a lot of care to check that the pedal chain is running true onto the chainring. This usually works OK but the chain needs to be tight at all times to run true onto the chainring and a non-circular chainring prevents the chain from being tight enough. I make sure my wheels are really well balanced too and I even dish them to help with chain clearance on the left side but dishing only gets you an extra 2mm because the spokes are really short on a big hub.
    I think the only way to remedy the situation is to install a Schwinn 46T chainring which is more of an accurate circle so the pedal chain stays tight at all times (see picture).
    Another problem was that the rear 20T sprocket on the Shimano Nexus 7 spd hub sometimes was intalled the wrong way around. There is a convex and concave side and the Shimano exploded drawings don't show which side faces in or out (see attached exploded diagram). You just can't tell from the drawings but one is the right way and one is the wrong way. I forget which way is right but the wrong way will definitely cause the chain to come off - I think the correct way is concave facing outwards.
    It took me ages to work this problem out and I can tell you that the sprocket does sometimes come installed the wrong way around.
    You have the clamshell sprocket clamp on your bike so you probably don't need to skew the wheel to get clearance between the tyre & the chain but I do have to skew the wheel a little bit. However, since you drew my attention to it I think I could skew it less and that could help fix the problem. A 46T chainring is normally way too big for the Nexus 7spd but it enables you to get better pedal cadence at high speeds and that is more important on a motored bike than having proper gear ratios. There will always be plenty of low gear for hills and starting off anyway. The standard chainring for the Nexus 7 spd on the Schwinn D7 is actually 36T front and 20T rear but the 44T front chainring really suits it better for motoring - a 46T would probably be even slightly better still. Your point was definitely helpful and has got me thinking more holistically about the problem because a chain coming off a coaster hub is nearly as bad a situation as a big puncture.
     

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    Last edited by a moderator: Apr 3, 2009
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