Compact to double for TT?

uphillstruggle · May 2013

Started TT's on my road bike with clip on bars.

Its currently a compact with a 12-28 cassette from cycling in the hills.

Going off power results i am struggling to keep the power high on the easier parts of the course presumably in part due to me either having not enough gears or too many gaps between them.

Would it be worthwhile changing to say a 11-25 cassette alone or would i be better also changing the bike to a 53/39 chainset?

Grill · May 2013

At 100rpm in a 50x12 you'd be going 32.6mph, and 35.9mph at 110rpm. I doubt you're spinning out and if your power results aren't coming from a PM you'd best ignore them.

That said gearing choice should be based upon preference. From a cost/benefit perspective you'd be best going with an 11-23 cassette to keep your ratios close.

giropaul · May 2013

I've also been looking at this issue (but for riding kermesses).

"Back in the day" a 12 was unusual, a 13 more usual. When I time trialled I used a 55 ring (55/50) with a 12 up block. Top with that is 123". A 50x11 is 122.7" - virtually the same gear. (I was doing 21 minute 10s and 56/57 minute 25s - without tri-bars in those days - and seldom used the 12)

I'd definitely go with Grill then - 11-23 - you'll get decent chain-lines on the gears you will be most in.

On the track (outdoors in those days) we regularly did 23 minute 10 mile events - usually on about 50x15 (90"), so "spinning out" on anything over 100" shouldn't really be any issue.

maryka · May 2013

I used to use a compact and 11-25 for TTing back when my TT bike was doing double duty for (hilly) Ironmans. When I switched to TTing only, I went with a double and now use several cassettes (11-23, 11-25, 12-17) for various courses. I just never need the 34 ring in TTing whereas in Ironman I'd use it for every hill.

Dual carriageway courses will be very fast, I've done well over 35mph on downhill/tailwind sections of those and I'm little and slow compared to the fast guys! I'd wager that most people don't pedal 100rpm in a tucked TT position very comfortably going over 30mph. But some of the hilly sporting courses will need a decent climbing gear (39x27 for me), especially the winter/spring races on poor roads, wearing heavier clothing, riding through denser air, etc.

Cheapest for now is to get an 11-23 cassette with your compact but if you're keen to do more typical TTing on faster courses you will want a double.

Edit: have just seen you're on a road bike with clip-ons, so I would keep the crank a compact if that's what you find yourself needing for road riding (I have a compact on my road training bike). But if you get hooked enough into TTing to get a TT-specific bike, then you'll want a double on that.

SBezza · May 2013

Grill wrote:

At 100rpm in a 50x12 you'd be going 32.6mph, and 35.9mph at 110rpm. I doubt you're spinning out and if your power results aren't coming from a PM you'd best ignore them.

That said gearing choice should be based upon preference. From a cost/benefit perspective you'd be best going with an 11-23 cassette to keep your ratios close.

Not as simple as that with a TT, to the OP. I wouldn't even think about TTing on a compact to be honest (that is me personally), there are many many times I struggle to put out the power in a 53/11 on faster parts of the courses. Once I get over 100rpm on the road the power seems a struggle to get out, though I generally TT at around the 90rpm mark.

I now use a 54 and a 23/11 cassette, admittedly I might put out more power than yourself, but if your comfortable rpm is around the 90 rpm mark then a compact is likely to be limiting on alot of courses. I can still hit speeds where the rpm gets into a range where putting the power out suffers, power isn't just about cadence, if you can't comfortably put enough torque through the chainset at the higher cadences power will suffer, though to be honest by that time I am normally travelling very fast so it doesn't bother me too much. I would never go as fast using a compact chainset though.

Start with a 11/23 cassette, though be aware that the 11 sprocket is not that efficient, and it would be better for efficiency to get a bigger chainring and be further up the block.

markos1963 · May 2013

I would agree with getting a 11-23 cassette to start with and see how you go. You could then get some 110 BCD chain rings in 53/54/55 with a 38/39/42 whatever you prefer instead of getting a new chain set. I run a 55/39 with a 12-23 just to get a straight chain line in 55-16 which is my favourite gear

Mettan · May 2013

markos1963 wrote:

I would agree with getting a 11-23 cassette to start with and see how you go.

+ 2

@ OP - on a DC you'll get plenty of usage out of the 12,13,14,15 - well worth having the close ratio.

uphillstruggle · May 2013

Thanks for all the advice.

The club course is relatively fast and flat and i feel that a tailwind on any of the slight downhill sections will have me struggling to take maximum advantage. My cadence averages at 90 and if it goes to nearer 100 i am struggling it seems to keep the power up.

Ill give the 11/23 a go. After my first go at a TT i can see it getting addictive and having shocked myself with a sub 25 am keen to improve as quickly as possible. Dont want to loose out on the fast bits having done all the hard work!

SteppenHerring · May 2013

50x11 is a higher gear than 53x12

Going by Sheldon Brown, on a 700cx23 tyre, 100rpm on 50-11 is 35.5mph, on 53-12 it's 34.5mph. If you were serious about TTs then a 55/42 chainset might be worth it. I'm in terrible condition at the moment and only push 53-11 on the downhills and I'm not spinning out until about 45mph.

Cassette is the cheapest option.

Johnpsanderson · May 2013

Interesting debate - I suspect it'll come down more to your personal preference for riding than anything else. Once you've progressed beyond 'I need the lowest gears I can just to get up any given climb' a compact setup does lend itself to someone who is happy/able to put out their best performance with higher cadence. Whether you want to work towards being able to do that (doubtless it is trainable) or not is another question.

sub55 · May 2013

I TT on 56---11/21
For those that permently bang on about cadence and spinning out on a compact , i`ll explain why.
On the flat it puts me in the middle of the cassette ,which means if you hit a down hill section or a strong tailwind ,you have higher gears if you need them and vice a verser, If you end up flogging your guts out into a headwind. It also gives you a straight chain line most of the time and being bigger circles is mechanically more efficient.

NITR8s · June 2013

This is a question I am currently thinking of. I am riding the same set up and feel that I am disadvantaged by everyone else riding standard chainsets.

I know I would need to be going like 30mph+ before I spin out, but I cant help think if riding a standard chainset would make much of a difference.

Johnpsanderson · June 2013

NITR8s wrote:

This is a question I am currently thinking of. I am riding the same set up and feel that I am disadvantaged by everyone else riding standard chainsets.

I know I would need to be going like 30mph+ before I spin out, but I cant help think if riding a standard chainset would make much of a difference.

I think you've provided your own answer - which is "not unless you changed your cadence" when you fitted the new chainset. Otherwise you'd continue to use the same ratios - you'd just have the chain on a different sprocket on the cassette to achieve the same gearing!!!!

dru · June 2013

you have to find the gear range and cadence you are happy riding at.

When I started I thought it was all about 53 x 11 and mashing it out.
Looking at my average cadence I was 80 ish.
I discovered that I could ride an easier gear and turn the pedals quicker, be comfortable and quicker.

MY pb last year was a low 22, and I didn't ride anything lower than 53 x 15 (also that was giving me a straight chainline as well)

So experiment a litte.

I would say go for a 11-23 cassette, but don't be too keen to go straight down the block - you don't have too

Compact to double for TT?

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