Have I got this right? Chain line.

[Bobtoo]

I've been doing some measurements in an attempt to work out what BB I need to buy for the Royal.

2017-09-03_04-08-56 by RichardB5, on Flickr

In the above picture I have the chainring on the inside of the spider on what used to be a triple, and a small chainring on the outside to act as a spacer. The bottom bracket is the original, which was about 117mm I think.

Working on the assumption that it seemed ok like that I bought short chainring bolts to let me run it as a single, and a 110.5mm UN55 BB to compensate for the chainring's new position.

It looks a bit better but the chainring bolts are very close to the chainstays.

It was at this point I decided to do a proper job and take some measurements.

The OLN measures at 122.5mm and it's 16.5mm from the inside of the drop out to the centre of the sprocket. I make that 44.75mm from the centre line of the frame to the centre of the sprocket.

The centre of the chainring to the other side of the seat tube is 56.0mm and the OD of the tube is 28.8mm. I make that 41.6mm from the centre of the frame to the centre of the chainring.

I think that means I need a BB that protrudes 3.15mm further than the one I have, which is 6.3mm overall length. As the existing BB is 110.5mm I think I'm looking for as close to 116.8mm as I can get, which would be 118mm if I get a UN55 again.

Does that sound right or have I got it all completely wrong?

 

[S-Express]

Can we see a pic of the actual chain line?

 

[Bobtoo]

Are these any good?

 

[S-Express]

The main issue with a chain line is that it's straight, or straight as possible. That looks ok to me.

 

[Tim Hall]

The photo with the ruler held on the chain ring does suggest the BB is a bit short, assuming you're holding the ruler straight. I'd disagree with @S-Express slightly - surely a line between two points (chain ring and sprocket) is going to be straight. What you want is for it to be straight over the whole of the chainring, IYSWIM. Does it run quietly? How close are the chain ring bolts to the chainstay? I put a 1mm spacer on the chainring side of my UN55 to nudge the chainring bolts out a touch.

 

[Deleted member 1258]

Are these any good?

View attachment 373755

View attachment 373756

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In the photo's it looks OK

 

[Cycleops]

Looks pretty good to me and it's as near as you're ever going to get it. Sure it runs out a fraction but so do most geared bikes a lot of the time.

 

[3narf]

Personally, I wouldn't worry unless it starts unshipping the chain...

 

[3narf]

Really? You would be happy with it chattering away and telling you that you are wearing stuff unnecessarily?

Clearly, based on the picture showing how close to straight it is, it's highly unlikely to be noisy or cause undue wear.

 

[Bobtoo]

Thanks for all the replies. It doesn't feel quite right as it is, so I do have to do something.

The crank has to move out anyway because it's too close to the chainstay, so I've ordered a 115mm BB. If it still seems a bit out of line I'll add a spacer to the BB. I'll report back once it's been done.

While I'm here I have another question. The hub is designed to be fixed on one side and five speed on the other. I can see that you could flip a fixed/SS wheel on a whim, but flipping this kind is a bit more complicated. What is the thinking behind the design? Is it for winter/summer, work/weekend or to let you use the same bike for different kinds of events?

 

[3narf]

The idea is you can ride fixed or freewheel depending on the the ride, but this way madness lies. Ironically fixed and freewheel singlespeed are perhaps the most diametrically opposed disciplines in cycling, and to find yourself on the exact same bike, suddenly not being able to freewheel, is a recipe for disaster of epic proportions.

Best to have a completely different bike with a fixed wheel if you must indulge in that activity; personally, I think they belong to the first half of the 20th century...

 

[Bobtoo]

This hub isn't just fixed or free though, it's fixed or derailleur.

 

[Tim Hall]

This hub isn't just fixed or free though, it's fixed or derailleur.

The thread that takes a five speed freewheel (your derailleur option) can also (and should) take a screw on single speed freewheel. It's the same thread. Or you can fit a different fixed sprocket, using a BB lockring as a jam nut.

 

[Cycleops]

Give me a jam doughnut every time.

 

[Sharky]

While I'm here I have another question. The hub is designed to be fixed on one side and five speed on the other. I can see that you could flip a fixed/SS wheel on a whim, but flipping this kind is a bit more complicated. What is the thinking behind the design? Is it for winter/summer, work/weekend or to let you use the same bike for different kinds of events?

I think you have to back to the 60's or earlier. In those days, fixed gears were still extremely popular and many riders were "on the fence" as to which is best. So when building up bikes, the option to have either was plus point when buying components.