(image credit: http://aroundtheedges.wordpress.com/)
Nope i'm not going to tell you how Sir Chris Hoy rampaged in the keirin tonight. 10th world championship! Awesome power - watching him kick then kick again to come round for the lead and then power on the gas for the win got me pretty damn goose pimply, i can tell you...
Anyway: 6 speeds...i've been reading around what folk have tried and it all seems pretty system dependent. We dont know how the 17-34 cassette is going to go on the back of the Vertigo just yet. the spacing has some leeway i would imagine because the spokes are so highly angled, the modded cassette can be moved in towards the middle of the wheel as well. The further away from the derailleur it is, the teeth it can cope with. This seems to be the stumbling block for some set ups. It also means you have to use lots of b-tension (which changes the arc of the top pulley compared to the cassette). Unfortunately, the more b-tension, the less smooth the shifts. There is also a minor matter of the limit screws being longer so the range of throw can be kept in check.
I want to use a saint mech, because they are tough. They are also shadow style mechs, which means they are tucked under the cassette further out of harms way but critically, closer to the cassette, decreasing their ability to cope with a 6 speed/large range set up. It is possible it aint going to like the 34 tooth low gear. Options: spacing the deraileur out from the drop out, sometimes by as little as 2mm can be useful apparently. Im not sure what i think about that...yet!
I could also drop the 17-34 modified cassette. The reason i am using that spread is primarily because it comes on an alloy carrier. The singlespeed cassette body of DT hubs is aluminium and individual thin steel cogs will chew it up for breakfast.
Other options. Use 18-32 xt cassette part. i drop a gear ratio, and get less useable range, but the 32 low gear may work. The xtr cassettes come on carriers and could give a similar range, but no real benefit over xt there.
I could also use a road cassette: in looking at the older style 9 speed ultegra 6700 i can use the two low gear parts - 21-24-27 on an alloy carrier and 17-19 pinned together. Then, because the pinned together bits dont have a spacer attached, (see the tech docs) a chris king cog should nudge up along side very nicely, maybe with a judicious 1mm spacer, to give me 15-27 6 speed. With the large range widget in place on the saint mech i reckon if the 17-34 doesnt work, this will. Only issue - the teeth on the 15 tooth king are likely to be a little taller, so the shifting onto and off may not be hugely crisp, we'll see if it is needed.
In that scenario, the gear development would change only very slightly if i use a 36 tooth ring. It is possible that this would allow me to put the ring in the middle position also - which MAY be better for chainline...we'll see.
With 36 tooth c'ring and 15-27 i get 121.5 to 218.7 development versus
with 40 tooth c'ring and 17-34 i get 107.2 to 214.4.
I could also use 35 or 34 if i needed more climbing gear.
Given the gear i run on the singlespeed (33-19) gives a development of 158.2 im pretty happy with this range.
Damn. My brain might have just cramped.
So once again, here is Hoy, on the BBC winning his 10th (!) world title...
4 comments:
Jon, don't go for a Shadow derailleur if you want smooth shifting. Shadow derailleurs are more like SRAM ones. Not as brutal, but a lot less smooth than regular Shimano stuff. A lot more tolerant to abuse and lack of care, though.
I rode an XT Shadow derailleur and a 6 speed cassette (13-15-18-21-24-27T) on my Chumba last season (search my Flickr gallery for some pictures). I used a block out of Ultegra CS-6500 cassette and mixed Miche and Shimano for the remaining three cogs. It worked really good and I think I won't ever go back to a full cassette, given the benefits of a strong rear wheel.
Miche has a huge choice for cogs, covers some dimensions that Shimano doesn't (18T separate cog, for example) and has some "start" cogs up to 16T, if I remember well. That might be cheaper than King and won't require any spacing or modification. The finish isn't as good as Shimano, but it worked pretty good on my Chumba.
As for the gap between cogs, I tried an even value between cogs on the whole cassette (12-15-18-21-24-27T). I'm not really powerful and going to the 12T from the 15T was like going to overdrive. I went for the 13T instead and I think I should have gone for an even smaller range (14-16-18-21-24-27T).
If you go the road block way, have a look at the smallest SLX cassette (11-28T), the three bigger cogs are on an aluminium carrier and the following two are pinned together. Basically, it's an old 9 speed Ultegra brought back to life for a cheap price.
If you have any question, just let me know!
THANKS! awesome advice :-)~
Jon,
Just to mix it up a bit...I ran a 12/25 9sp block on an MTB for a while and in the end, the close ratio shifting got me. I found that I needed to pop the shifter twice to be in something usable due to the abrupt up/down of the trails I was riding. I'm going to fiddle around with your cassette this week to work out the chain line and any shifting issues. I suspect that I'll need to get a longer low limit stop screw to prevent overshifting the derailleur into the spokes.
Sean is right, some road cassettes just don't feel right for MTB riding. But it's personal matter. I've heard people complain about the close ratio or the complete opposite. There's no rule in that field, just test and see what suits you.
It's hard to avoid the biggest range when you set up a single ring drivetrain, though. The biggest road cassette (12-27T) are good for me and have the same gear range of the 7 or 8 speed cassette we used to ride 15 years ago
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