Changing to compacts
1- I've seen a shim 105 5750 crank. Will this fit my existing set up?
2- do I need to change the bottom bracket?
Thanks for any feedback in advance!
Replies to This Discussion
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Permalink Reply by Pete Roberts on -
Thanks Forest :)
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Permalink Reply by Jim Cooper on -
+1
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Permalink Reply by Frank on
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Stick with compacts cause they make mechanical sense!
A tighter cassette as in a 11/23 or 12/25 will give much better spread of gears as a 12/28 where suddenly you be left wanting an in between gear!
Also, turning a smaller chainring on a higher cadence is easier on the knees and unless you are racing or riding lots on the flats, a compact is the way to go!
The pros even use compacts on very hilly climbs but they are pros for a reason, their leg strength is amazing!
Over to you!
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Permalink Reply by Terry C on
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The problem with compacts is that in fact, they are not more efficient than standard cranks. Using compacts means more time in an 11T or 12T rear sprocket. 11 teeth is border line on chordal action ( for anyone not an engineer, Google that ), and therefore greatly reduces efficiency over say 52-13T or 14T. 11T sprockets do get used with conventional chain rings of course, but only in sprints and down hill.
For a rundown on number of teeth v efficiency this may help . . http://www.g-cog.com/VBMX/spicer.pdf
Something that should be taken into account for each individual rider, is of course crank length. To be able to spin a compact, you would probably go down a size, say from 172.5 to 170. This is also related to the rider's leg length.
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I hear you but surely most of us set up our bikes to spend most of the time in a gear where the chain is almost a straight line front to back!
Chordal action is rarely spoken about in cycling from my few readings and not sure how the vibrations etc would affect the ride but certainly, throw that in and see.
Its each to their own, but for me, I find a compact way way easier to live with and more importantly how it translates to real world riding.
Not sure where you got more efficient from but Yes, there is an advantage for standards but only if your riding and legs suit that.
In SA, with the hills we have, a compact makes sense and only on downhills will I drop down to an 11T, the rest is spend in the middle of the cassette if I can. Hence the Titanium where it counts.
Thats just me and seems to work and since my power(watts) is nothing like the 2.5KW's the pros do, I am happy and by all accounts the person in question. perhaps a 12/27 or 26 be perfect than? Just be vary of big gaps from 24-27 or 28!
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Permalink Reply by Trent on -
thanks Frank and that is my concern the gap from 24-27 or 28. the 25 seems to do me pretty well for most of the climbs i have done (Lofty, Willunga, Cox's hill, S.Exy etc) but just wonder if i could go a little bit faster without grinding. I am leaning more to the 12-27
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Trent
do some pilates or spin classes, that will get you over most of what adelaide has to offer, seriously, a 34/25 gets a pretty good cadence but a 12/26 could do the trick, I know it sounds like a small difference but I bet its all you need.
Or, ride more and it will get easier and soon will want a 11/21, my mate runs that with a 52/38 and does the hills I do no worries but does strain the BB more but he is only 59KG.
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Permalink Reply by Trent on
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Thanks once again Frank. 12-26 or 27 i think will do the trick for me. 25 does the job most often so maybe i just need to MTFU a little bit =-)
I would LOVE to get out and ride more but with work, family and study i am lucky to get out twice a week.
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Chain line is something that most riders should consider, and it is why I use a 52T. I live in a very hilly area - - north west Tasmania, but also do a lot of ks on relatively undulating roads, and use 52-15 most of that time, which gives me a straight chain line on my bikes.
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Permalink Reply by Dahondude on -
Terry, not many people ride around in a 50/11 or 50/12 for very long. According to Steve "the God" Hogg, @100 rpm a 50/12 combo gives you 52.9 km/h so even faster for a 50/11. There are only a few pros in the world that could hold that sort of speed for more than a few minutes and certainly no-one on AC could do that for more than a few hundred metres, even with a howling tailwind!. For the rest of us the 12/13 tooth cogs are something you might use one a week in a crit bunch sprint, or pedalling down a long hill.
You also say "therefore greatly reduces efficiency over say 52-13T or 14T." Thats ratios of 4 and 3.71 which is the same as 50/12.5 and 50/13.5 (clearly you can't have half teeth so round up or down to 12 or 13 and 13 or 14).
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Although I live in a hilly area, north west Tasmania is subject to some quite strong winds. As the coast line faces north, a lot of riding here on the undulating roads is east / west or vice versa. If I have a tail wind on Bass Highway, I will be in 52-13 for 20kms at a time.
Tomorrow ( Wednesday ) I will actually be in 4WD, as I will be on snow covered roads again !
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