You pretty much mirrored my own thinking there Dave!
I will have to decide by the time I start work tomorrow I suppose, so I guess sleeping on it is the best option...
Luke.
"120 tooth MINI-VELO"
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I will have to decide by the time I start work tomorrow I suppose, so I guess sleeping on it is the best option...
Luke.
Yeah I feel the same, the 120 is crazy, but the chain line on that short frame will make it tough to leave room for your derailleur. Besides, if you'd have never put that 120 up there we'd all be goin' on about how wild that 70 tooth is...
Carl.
Carl.
Luke I think the 70T sprocket would look allot nicer on your build and wouldn't look so way out there. But hey it's your bike so you know what you are going for. Just some thoughts to think about.
I would go with the 70 if it were my build... then I'd use that 120 on a build with 32 inch (or the 36 inch Coker) wheels 
Glen.
Glen.
The Renaissance Man
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My vote is for the 120! That's what we've come to expect from LukeTheJoker!
But I think you'll need a longer chain!
But I think you'll need a longer chain!
You should find a way to mount the bottom bracket dead center between the wheels. So it looks like 3 wheels. The center wheel being a touch higher so it doesn't hit the ground.
But wait a minute. What happens if you go over a speed bump or any sorta curb. The 120 will scrape
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But wait a minute. What happens if you go over a speed bump or any sorta curb. The 120 will scrape
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The 70T's more practical and won't require the mods to the frame, and yeah I'd be worried about that 120T bottoming out. Whichever you choose it'll look cool 'cause even 70 teeth is way out there.
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I want to see it with a chain guard!!??
I have to admit, I was expecting a different vote from most of you guys, I fully expected when I went to bed last night that the 120 tooth would take the day...
I just want to clarify a couple of things, mainly with the chainline/derailleur issues some are expecting with the 120 tooth. The chainstays are actually the exact same length as they were on the original bike, the bottom bracket/rear cluster relationship is switched height wise though, originally the bottom bracket sat 2 inches lower than the rear axle, now it sits 2.5 inches higher, that is mainly for the difference in tire size but was also slightly planned for the bigger sprocket... The original sprocket was 8.5" across, the 120 is 19.5", a difference of 11" or 5.5" each side, but the bottom bracket was raised 4.5", and since I will be using the "MegaRange" derailleur, which just happens to sit 1" lower, the chainline to the rear should be almost identical to standard. The slight worry I have is the chainline from the rear cluster to the 120, it is shorter due to the angles involved.
Here is an improved drawing, I hope it makes it clearer:
There would be 2" from the bottom of the 120 to the ground, the same amount that "BigJim" is off the ground, but "BigJim" is not used much for road riding, but also has a far longer wheelbase so it's ramp over angle is far less than this would be using the 120.
Luke.
I just want to clarify a couple of things, mainly with the chainline/derailleur issues some are expecting with the 120 tooth. The chainstays are actually the exact same length as they were on the original bike, the bottom bracket/rear cluster relationship is switched height wise though, originally the bottom bracket sat 2 inches lower than the rear axle, now it sits 2.5 inches higher, that is mainly for the difference in tire size but was also slightly planned for the bigger sprocket... The original sprocket was 8.5" across, the 120 is 19.5", a difference of 11" or 5.5" each side, but the bottom bracket was raised 4.5", and since I will be using the "MegaRange" derailleur, which just happens to sit 1" lower, the chainline to the rear should be almost identical to standard. The slight worry I have is the chainline from the rear cluster to the 120, it is shorter due to the angles involved.
Here is an improved drawing, I hope it makes it clearer:
There would be 2" from the bottom of the 120 to the ground, the same amount that "BigJim" is off the ground, but "BigJim" is not used much for road riding, but also has a far longer wheelbase so it's ramp over angle is far less than this would be using the 120.
Luke.
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That is a wild looking ride, would imagine you should get a quite high
top speed out of her? How about taking her over to the Lake Gairdner
salt flats in South Australia and strapping her out!
top speed out of her? How about taking her over to the Lake Gairdner
salt flats in South Australia and strapping her out!
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The Renaissance Man
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That was the one thing that I wondered about.
Even if it didn't run off, it would really wear on the chain and sprokets. Plus the chain ring teeth are not tapered which will further increase the wear.
You may be better off with the smaller ring.
Assuming a cadence of 100rpm in top gear, 90kph (55mph) should be achievable... The same set up with 700c tires would be maybe 120kph? (72mph?)
There must be something wrong with those figures though because the Donhue racer with it's 104/13 ratio and 700c tires has run 102mph (160kph) on rollers and 80+mph (130+kph) in the draft behind a Mark 3 Zephyr...
My 100rpm assumption must be a fair bit off...
Anyone know the rpm typically used for speed records?
Luke.
I will be tapering the teeth on whichever sprocket I use, I am going to do some measuring up on a couple of 20" 5 and 6 speed MTB's that I have, because they seem to work fine and I doubt there would be much more length than what this would end up being with the 120, I also may end up locking out the lowest (13t) ring on the cluster to help with the same issue.
Luke.
I say go with the big one simply because if you don't you will wish you had.
Thanks Kram, you are exactly right and that is just the push I needed!
Just measured the chain line on the 2 20" MTB's I have, one came in at 13.5" on the shortest ring, and the other 14".
A quick measure on the bike with the 120 mocked in place gives me 13.25"-15.75" depending on the gear used, I think that is close enough to run with, if necessary I will grind down the teeth on the smallest ring and use it only as a lock nut.
Bring on this evening, I have a need for cutting and grinding!
Already been outside doing some work, basically whatever I can out of the sun, got the forks straightened slightly to give it a more racer geometry, also widened the spacing and ground the dropouts to fit the wider 3/8 axle and hub on the front wheel, collected together the bikes I will relieve of their headtube's, turns out I will need 2 more as well as the original on the bike, that is one long headtube! Grabbed a donor fork to use for extending the fork stem, some other bits and pieces...
Luke.
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Go Big! But, the 70 looks killer too! I trust your judgement.
Another good productive evening on the bike! Started with the headtube extension, welded two more headtubes to the original one and then cut it to the right length, here is a pic from that point:
Then I lengthened the fork tube to match and moved on to lowering the front of the downtube, starting to look good! Also finished welding the rear stays and removed the old brake mounts, cleaned up the welds etc:
Starting to look like the drawing!
Standing back and looking at the bike, I decided to give the 70 tooth one last chance:
I think the 70 might be winning me over...
With the frame pretty much stitched up at this point, I decided to try it out for fit, it fits me at 6'4" perfectly! (You have to imagine the seat up at the right height.)
Really happy with the bike at this point, it is coming together really nicely and feels very solid but also very light and nimble, I guess using the 70 tooth is the best way to keep that feel with the bike. Going to have to make a decision on which sprocket soon, can't keep putting it off...
Luke.
Then I lengthened the fork tube to match and moved on to lowering the front of the downtube, starting to look good! Also finished welding the rear stays and removed the old brake mounts, cleaned up the welds etc:
Starting to look like the drawing!
Standing back and looking at the bike, I decided to give the 70 tooth one last chance:
I think the 70 might be winning me over...
With the frame pretty much stitched up at this point, I decided to try it out for fit, it fits me at 6'4" perfectly! (You have to imagine the seat up at the right height.)
Really happy with the bike at this point, it is coming together really nicely and feels very solid but also very light and nimble, I guess using the 70 tooth is the best way to keep that feel with the bike. Going to have to make a decision on which sprocket soon, can't keep putting it off...
Luke.
Really having trouble making up my mind on the sprocket, I think the 120 is too big (It looks bigger than the tires) But the sweetheart looks out of place, so I threw on the 55 tooth sprocket too and have 3 pics to decide from:
55 tooth:
70 tooth:
120 tooth:
Which do you guys and girls like?
Luke.
55 tooth:
70 tooth:
120 tooth:
Which do you guys and girls like?
Luke.
The 70T could work depending on the paint, seat and wraps used. If you went classic black, or faux patina, I think you could pull it off, especially with shellaced cork wraps on old timey mustache bars? It's just an idea. With the 55T you can get away with anything leaving you more options, modern, classic, squirelly hipster mish mash. It looks stock on a bike that's far from it.
With all that said, 70T would be where I would go.
With all that said, 70T would be where I would go.
