Chain Tensioner Replace roller with rear derailleur?

Discussion in 'Transmission / Drivetrain' started by deuskc, Jul 16, 2016.

  1. deuskc

    deuskc New Member


    Pretty new to the MB hobby as a whole so forgive me if this is a silly question. Most of the chain tensioners I've seen appear to use the standard rollers that come with all of the engine kits however some are shown using metal sprockets such as the one here (

    Is there a reason that metallic rear derailleurs such as this one ( can't be used? I've seen some mention them used as pedal side tensioners but never as drive-side tensioners. Is it that the teeth are too shallow for that purpose?


  2. crassius

    crassius Well-Known Member

    I don't believe teeth are needed, but may replace the usual sidewalls to prevent slipping off the roller - a bearing for it to run on is required due to forces and speeds involved.
  3. Steve Best

    Steve Best Active Member

    Deraileur or spring tensioner will not allow you to bump start your bike. Need a fixed tensioner or limited slack on the drive side.

    Teeth are not needed on an idler wheel. A smooth roller can run on the chain rollers (ideal) or even the side links.
  4. deuskc

    deuskc New Member

    Thanks for the replies.

    I believe bearings are built into rear derailleur wheels as a general standard, if you scroll down on the amazon product page I linked in the OP you can see a picture of one a guy has taken apart in the review section that shows the bearings inside the wheel. Which is why I thought it would be quick and easy to use as a metal alloy alternative to the plastic rollers.

    And I should've noted I'm building a 4-stroke bike so I wouldn't be bump starting anyway.

    I suppose I will use the plastic roller until it wears out since I already have it, I'm just thinking about what I'll inevitably have to replace it with.
  5. Randall

    Randall Member

    Steve, I have a spring tensioner on mine and am able to bump start my motor. Maybe it's just the configuration. I did put a higher tension spring on it to replace the stock spring that came with it.
    Steve Best likes this.
  6. Steve Best

    Steve Best Active Member

    The higher tension spring and I would guess there is not much more than an inch or so of slack?
    Any more would have teeth jumping and chains coming off.

  7. Randall

    Randall Member

    The higher tension spring and I would guess there is not much more than an inch or so of slack? --- Correct.
    Any more would have teeth jumping and chains coming off. --- Exactly.

    : )
    Steve Best likes this.
  8. Randall

    Randall Member

    FYI: My toothed tensioner just broke off a couple of days ago. I replaced it with the smooth roller. I think I have alignment issues. : )
  9. JunkyardDog

    JunkyardDog Active Member

    I like the idea of a toothed tensioner, but I can see where alignment issues could be a problem. The chain runs pretty much straight from the rear wheel sprocket to the engine sprocket, but the cheap crappy tensioner that comes with the kits clamps onto the chain stay, which puts it at an angle to the chain. The teensioners that come with the kits have a ton of problems. First, they clamp to the round chain stay, allowing them to rotate on the frame, unless you weld them in place. Second, they are made of super soft flimsy metal that you can bend your hands. Third, the cheap plastic roller is just that, cheap plastic that easily gets chewed up. It is also not nearly wide enough. The bearings? are pretty much worthless. The chain will beat one of these wheels to pieces in short order. Part of the reason for that is the chain, which is not uniform from one end to the other, and the other reasons are that the rear sprocket is simple stamped steel, not "hobbed" like a motorcycle sprocket, and then there is the rag joint, which is almost impossible to get perfectly straight. All of this together results in a chain with tight spots and loose spots. And when you have a non spring loaded tensioner that can't follow the chain as it constantly gets tight and loose as it turns, it causes the chain to "whip" This is what causes it to destroy the tensioner, or to come off the tensioner altogether. It can also pull the tensioner into the rear wheel, which is what happened to me.

    Derailleurs would make excellent tensioners if it were not for 3 things. One, they are too small for the motor drive chain. Two, they are not robust enough to handle the stress put on them by a motor driven chain. And depending on where they were mounted, they would have alignment problems just like the toothed rollers. They do make excellent tensioners for regular bicycle chain. I have converted multi speed bikes to single speed, and left the derailleur on it simply to tension the chain.

    A heavy duty tensioner, with a strong non bendable bracket and a roller made out of something tough and wide with good bearings, like a skateboard wheel, would be a 100 times better than what comes in the kits. But the problem of chain whip caused by the stationary roller would still exist. The best commercially available solution I have seen is a combination of a clamshell rear sprocket mount, which eliminates the rag joint, attaches the sprocket to the hub so it is centered, and does not put any stress on the spokes. Combine this with an engine mounted spring loaded tensioner, and that will solve 99% of the problems with the stock drive chain/sprocket/tensioner

    Engine mount tensioner

    Clamshell rear sprocket mount

    I don't know why parts like this don't come with the kits these days. The Chinese are still using the same old junk. My guess is price. Just remember, you get what you pay for.
    FurryOnTheInside likes this.
  10. O I use am arch idler arm its like a crescent its awesome ! Just needed to tighten it after chain stretched. That's all. I love it!