Author Topic: New BB for Sport Raven Tour  (Read 1321 times)

onrbikes

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New BB for Sport Raven Tour
« on: August 10, 2015, 01:11:37 AM »
Am wanting to put a bash guard on the wife's Sport Raven tour and because the chain is mounted on the outside of the arms need to move to the inside arm.
I calculated that I need to move it 8mm, so that would mean I need a BB 16mm longer.

The BB that's in there is a BB UN54 71, at 110mm long.
I don't really understand what the 71 refers too, even after reading Sheldon Brown. Mine is a 68 and I think it refers to body length.

Would I be correct in thinking that I need a 68x127mm would give me the extra length?
Would the 73X112.5mm be virtually the same? By 1/2 mm.

She regularly gets grease on herself so am try o fix this, and have a guard that'll fit. I don't like the look of the Hebie.

Andre Jute

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Re: New BB for Sport Raven Tour
« Reply #1 on: August 11, 2015, 12:59:33 AM »
Am wanting to put a bash guard on the wife's Sport Raven tour and because the chain is mounted on the outside of the arms need to move to the inside arm.
I calculated that I need to move it 8mm, so that would mean I need a BB 16mm longer.

Unless you fit the BB axle asymmetrically, more sticking out on on side than on the other.

What you want to watch out for is the tread width, what the trendy cyclists call the Q factor. It is axiomatically assumed that cyclists, and lady cyclists more so, are narrow-treaded and pigeon-toed. It's nonsense (I for one like a good, solid, wide tread) but all the same you don't want to go too far.

The BB that's in there is a BB UN54 71, at 110mm long.
I don't really understand what the 71 refers too, even after reading Sheldon Brown. Mine is a 68 and I think it refers to body length.

Would I be correct in thinking that I need a 68x127mm would give me the extra length?
Would the 73X112.5mm be virtually the same? By 1/2 mm.

UN54 is the Shimano bottom bracket model number. 68mm or 73mm is the width of the BB; I have no idea what the 71 refers to. 110mm is the axle length, and that is what you want to change.

You need to move crank without altering the position of the chainring relative to the longitudinal centreline of the bike. This chainline must fall and remain at 54mm for a standard Rohloff installation.

However, to start with, what makes you think you need a new bottom bracket? Can't you just mount the bashguard over the chainring on longer studs with spacers. Rohloff sells a sets of spacers (indeed, gives them away with new Rohloff gear sets) to adapt various fastening positions on a compact crankset to use with and without a bash guard, and these might do if you could find the right studs.

If you can't fit the bashguard on standoffs, I think you can get away with a 118mm bottom bracket and a small amount of asymmetry.

She regularly gets grease on herself so am try o fix this, and have a guard that'll fit. I don't like the look of the Hebie.

I don't like bash guards much. I think one should be able to ride one's bicycle in street clothes, without any special prepaparion or pegging up your clothes. For this purpose the Chainglider is ideal. However, if you don't want it, a bash guard is simply dangerous unless your wife rides in skintight lycro. Recently, while I waited for a Chainglider-compatable ring and the suitable Chainglider to arrive for a new motor, I used a bash guard, and in two weeks got my trousers caught in it twice. If it weren't for my lightning reflexes (translation: I was going slowly enough to save myself a broken leg or serious bruises), I might have been hurt. If you don't like the Chainglider, I think an over-chain oil guard is safer than a bash guard, and works better than a bash guard which doesn't stop you getting bashed and doesn't stop you getting dirty either.

onrbikes

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Re: New BB for Sport Raven Tour
« Reply #2 on: August 11, 2015, 01:57:29 AM »
My first rule of life, "never argue with her".
She always wears shorts or tighter stuff, but she insists on putting her leg against the chain. Coordination issues.

It looks like the Crankarms are close to the frame as well. Because I want to put the chainring on the inside of the mounting bolts would need to lengthen the BB to keep the line correct.
I do like the idea of using spacers to mount the bash ring. I'll give that a go. Much easier solution.

My bike has the chainring mounted on the inside while the bash ring is mounted on the outside.

Thanks


mickeg

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Re: New BB for Sport Raven Tour
« Reply #3 on: August 11, 2015, 04:11:59 PM »
When I built up my Nomad, I did the calculations to figure out the spindle length I needed.  I then compared how far apart my pedals would be (Q Factor) and decided I wanted my pedals to be closer to the same width on my Nomad as on my other bikes.  Thus, I intentionally have my chain out of alignment by about 5mm.  It would be perfect if I ran the chain in the outer position on my double crank, but because I am running a bashguard I put the chainring on the inner position on the crank.

I find that having a chain out of alignment by 5mm is not any problem at all.  I can't comment on 8mm, but I guess my point is that chain alignment does not really have to be perfect, some misalignment is not going to do any real damage.

I did not like the high cost of bash guards.  I bought a 52t chainring on clearance for a fraction of the bashguard price, cut the teeth off with a saber saw, put the bashguard on the crank but before I put the chain back on I spent maybe 15 minutes with a file by turning the crank to file off all my saw cut marks.  Probably the only bash guard you will see that is stamped 52T.  After the photo was taken, I sprayed it black.