Chainset and BB for Scott Plasma 3 TT

Discuss light weight issues concerning road bikes & parts.
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portiabaines
Posts: 21
Joined: Mon Dec 31, 2012 8:31 pm

by portiabaines

Hi,

Just completed my first Duathlon of the year and came across an interesting issue. The course I had to ride was completely flat and I found myself totally under geared as I have my bike setup with a Shimano 7950 compact 50/34 chainset (use it as it's very hilly where I live and train).

So the big question is, what are my options as it looks like I can't easily change the 50/34 ratio to something bigger without the chainset looking odd. Should I just buy a non compact Shimano 7900 chainset or are there better/lighter options I could consider as I don't mind changing the pressfit bottom bracket at the same time? Guessing the bottom bracket will have to be a Shimano pressfit as I've tried looking around and ended up totally confused within 5 secs.
Regards

Portia Baines

audiojan
Posts: 778
Joined: Fri Jan 27, 2006 1:38 pm
Location: New Hampshire

by audiojan

What cassette were you using? 50-11 is slightly taller gear than 53-12, so that should give you all the top end you need... For really flat terrain, 11-21 would be ideal (straight block cassette, allowing equal spacing between gears), or 11-23 is a good option. Personally, I'm not a big fan of the 34, swap that for a 36 and you still have a useful compact, but much better shifting.
"Suddenly the thought struck me; my floor is someone elses ceiling" - Nils Ferlin

NGMN
Posts: 1536
Joined: Mon May 30, 2005 7:13 am

by NGMN

While I'd be a little surprised that a 50 was holding you back, as a 50/12 at 90rmp is 30mph, maybe just pick up some Praxis 7950 52/36 rings and get a 11 tooth cassette:
http://shop.praxis-works.com/Compact-DA ... oductId=32

audiojan
Posts: 778
Joined: Fri Jan 27, 2006 1:38 pm
Location: New Hampshire

by audiojan

NGMN wrote:While I'd be a little surprised that a 50 was holding you back, as a 50/12 at 90rmp is 30mph, maybe just pick up some Praxis 7950 52/36 rings and get a 11 tooth cassette:
http://shop.praxis-works.com/Compact-DA ... oductId=32" onclick="window.open(this.href);return false;



Sound advise, but if you're keeping 30MPH, then call Praxis as I'm certain they will sponsor you. :lol:
"Suddenly the thought struck me; my floor is someone elses ceiling" - Nils Ferlin

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portiabaines
Posts: 21
Joined: Mon Dec 31, 2012 8:31 pm

by portiabaines

Hi and thanks for the replies.

The cassette is currently a 11-28 so I'm guessing that could be changed also.

If I was to change the whole chainset what would be some good non-compact alternatives? What gear ratios are more normal for time trial bikes? Is there anything lighter than the 7950 which will easily fit?
Regards

Portia Baines

NGMN
Posts: 1536
Joined: Mon May 30, 2005 7:13 am

by NGMN

BB86 is probably the most "limiting" standard as you have to run a ~86mm length spindle and ideally a 24mm diameter spindle. There are some "hacks" to allow the use of 30mm spindles, but you'll use a smaller bearing with potentially shorter life span.

Dura Ace 7900 in a standard is a good bet, all things considered. Its on sale lots of places, good shifting, stiffness and decent weight.

Fairwheel bikes carries all usual more "exotic" options like Lightning, Clavicula...

sigismond0
Posts: 236
Joined: Tue Feb 05, 2013 8:29 pm

by sigismond0

I'd be interested to see some of the maths involved here. You say 50-34 with 11-28 is too low for you, but as noted 50/11 is 32mph at 90rpm. When you feel that the current chainset is under-geared, are you at the top gearing? What's your target cadence? What's a regular cruising speed for you?

Stuartclark1977
Posts: 88
Joined: Tue Sep 13, 2011 9:45 pm

by Stuartclark1977

I run 54/42 and 11-25 cassette and it works well. On a drag-strip course with a tail wind/slight downhill then 50*11 is not enough.

Having said that the last TT I did I was passed by Alex Dowsett like I was standing still...
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