Chainline for a FLip/ flop hub

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martinn
Posts: 286
Joined: 1 Dec 2012, 8:20pm

Chainline for a FLip/ flop hub

Postby martinn » 6 Feb 2019, 9:20pm

Hi all,

I have a converted dawes running fixed. As this was my first foray in to this world, I have done this on the cheap so to speak.
I bought a cheap ready built fixed/ free wheel and had the adapter to space it to 130mm rear axle (The dawes had been previously cold set.)
The chain line for the fixed cog worked flawlessly. However when I tried to run the freewheel (Dicta if that makes any difference) this was clearly out by a long way.
So to get a good chain line if I ran it as a single speed, I put a spacer between the fixed sprocket and the hub, and the same size spacer between the BB and the chainring.
As I though the LH pedal would be displaced inward, i put a spacer between the pedal and the crank.

2 questions, is what I have done OK? or am I running a risk somewhere?
and 2, is this normal not to be able to run the fixed cog and the freewheel without some altering and additional spacing going on?

many thanks

Martin

Brucey
Posts: 32274
Joined: 4 Jan 2012, 6:25pm

Re: Chainline for a FLip/ flop hub

Postby Brucey » 6 Feb 2019, 10:37pm

most freewheels you can buy today owe their origins to BMX designs and don't have the same offset as fixed sprockets. Thus if you run a flip flop hub fixed one side freewheel the other then the chainline usually isn't quite right for both.

One solution is to run a slightly wider chainline and to respace/redish the wheel to suit. Suppose the fixed chainline is 42mm and the freewheel chainline is 44mm; a solution is to offset the hub by 1mm (redish the wheel to suit) and run 43mm chainline by respacing the BB or the chainring.

It is quite normal to have to make small adjustments of chainline on such machines. FWIW it is worth checking that the rear end is set symmetric; run a piece of thread from one dropout, round the head tube, back to the other dropout. If it is evenly spaced to the seat tube then the back end is reasonably square. It often isn't on cold-set frames because the RH chainstay moves more easily than the LH one.

hth

cheers
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martinn
Posts: 286
Joined: 1 Dec 2012, 8:20pm

Re: Chainline for a FLip/ flop hub

Postby martinn » 7 Feb 2019, 9:19pm

Thanks Brucey,

rogerzilla
Posts: 1304
Joined: 9 Jun 2008, 8:06pm

Re: Chainline for a FLip/ flop hub

Postby rogerzilla » 8 Feb 2019, 9:41pm

It depends how much you're going to use the freewheel, and what type of chain you use. if the freewheel is there as a "get you home" device when you're really tired, and you have a bushingless chain (which most are these days), just optimise for the fixed side and let the freewheel side worry about itself. Freewheels have a bit of wobble in them and the chain doesn't care about being run at a bit of an angle, so it will probably be silent. Otherwise what you've done is a reasonable solution but you have reduced the thread engagement of the fixed sprocket and therefore it's more likely to strip the hub. Putting spacers under the sprocket is more acceptable if you're running a fixed sprocket on a standard threaded hub with no lockring, since these nearly always have more thread to play with.

Assuming this is a fixed/free hub, the manufacturers should really fix this by making the "free" side without the lockring a bit deeper, since freewheels are nearly always 1-2mm off compared to a fixed sprocket, but they don't.

martinn
Posts: 286
Joined: 1 Dec 2012, 8:20pm

Re: Chainline for a FLip/ flop hub

Postby martinn » 9 Feb 2019, 3:28pm

The freewheel was 2mm out in my case. Seems a bit suboptimal that the wheels are not made to work in the fashion that most people would expect/ want them to.
Is there a sensible reason they are made this way?

I only really use the freewheel when on holiday, and plan a route that would be "challenging" on fixed. (1 in 5 descents)

Martin

Brucey
Posts: 32274
Joined: 4 Jan 2012, 6:25pm

Re: Chainline for a FLip/ flop hub

Postby Brucey » 9 Feb 2019, 4:37pm

yes it is because we are mostly using BMX freewheels.

BTW an easy way out of this predicament is to use a fixed/free hub (on a bike with two brakes obviously) with the freewheel mounted on the fixed gear mounting and the fixed cog with a 2mm spacer on the freewheel mounting. Just make sure that the fixed cog is properly tightened and/or is loctited to the hub, or can unscrew harmlessly (i.e. without jamming, easy enough if 126mm OLN or higher) if needs be.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~