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Compact vs Semi compact

by barrydocks
Sun, Jul 12, 2015 14:57
12

So 2years ago I rebuilt my old Principia after many years of being sat in the garage, I opted to go with a Dura Ace 7900 10 speed groupset to replace the old 9 speed Ultegra set. One of the things I did was to go to a compact 50/34 chainset with a 28/11 cassette as I really couldn't manage the hills like I used to. I also went from 170mm cranks to 175mm which made a huge difference to climbing as I have always been more of a low torque, high cadence rider rather than a grinder and this seems more apparent in my older age!!

I am now much fitter (I zipped around the recent Velothon Wales course) and now have the opportunity to upgrade to a 9000 11 speed set up (I have enough Evans Cycles vouchers through work). I am considering a semi compact 52/36 chainset with the same size cassette with 175mm cracks. One of the irritating things about the 7900 chainsets is that the rings are not interchangeable but they are on the 9000.

So 2 questions:
1. How much difference to the torque will where be in changing to a 36t chain-wheel compared with a 34t?
2. If I wanted to change the 36t to a 34t, for say a hilly sportif, will I also need to change the 52t to a 50t?

Thanks

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12 comments

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Batchy | 9 years ago
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I use 50/36. On 12 to 28 with 172.5 cranks. I originally had 52/36 6800 ultegra on 10s 6700 groupset but swapped 52 for 50 big ring. The cassette was originally 11-28 so I took off 11 sprocket and added a 16 which gives me 12.13.14.15.16.17.19.21.24.28 This set up works fine for me. I find that a 34 inner ring is just too low for my riding style. I did ride with a 52 outer for a while and whilst this was fine for really big speeds the 50 is much more manageable for general every day riding. Mind you the replacement 50 t Ultegra 6800 ring cost £90. The 16t sprocket was plundered from an old 12-25 cassette.

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2 Wheeled Idiot | 9 years ago
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For question 2,
Officially you would need to change to a 50t big ring, however in practice it will *probably* work. I think I saw some riders in a world tour race using 53/36 which officially wouldn't work.
It may not be quite 100% shifting but will still be pretty good when you swap in the smaller ring.

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Simon E | 9 years ago
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A compact is adequate for most people, 50x11 is a pretty big gear in anyone's book. You don't need the extra 2 teeth on the bigger chainring (in fact most people don't, they just like the thought of the 'man-sized' chainring) but you will probably still be glad of the lower gears when you hit a steep climb.

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crikey | 9 years ago
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I'd go with barry on that one; I'm aware that crank length is important to some people, but I never suffered any issues going from 175s to 170s and finally settled on 172.5s; all these choices were based on what I could get rather than what I would want.

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fukawitribe | 9 years ago
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Hi Barry, just a quick question - if you're more a low torque/high cadence sort of person wouldn't you be better off with a shorter crank, especially as you are quite a 'small chap' ? I'd have thought a shorter crank would help with leg angle at the top of the stroke as well but if you're happy with your setup then it's moot - not meaning to question what you like but I thought i'd say something. Hope you find the chainrings you like. Cheers

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barrydocks replied to fukawitribe | 9 years ago
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fukawitribe wrote:

Hi Barry, just a quick question - if you're more a low torque/high cadence sort of person wouldn't you be better off with a shorter crank, especially as you are quite a 'small chap' ? I'd have thought a shorter crank would help with leg angle at the top of the stroke as well but if you're happy with your setup then it's moot - not meaning to question what you like but I thought i'd say something. Hope you find the chainrings you like. Cheers

No, for any given gearing, the shorter the crank the lower your cadence is and the more torque you need to produce to turn the pedals.

I suspect 175mm cranks are too long for me but I used these in the past without a problem and don't get any knee pain if I get the seat height right

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fukawitribe replied to barrydocks | 9 years ago
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barrydocks wrote:
fukawitribe wrote:

Hi Barry, just a quick question - if you're more a low torque/high cadence sort of person wouldn't you be better off with a shorter crank, especially as you are quite a 'small chap' ? I'd have thought a shorter crank would help with leg angle at the top of the stroke as well but if you're happy with your setup then it's moot - not meaning to question what you like but I thought i'd say something. Hope you find the chainrings you like. Cheers

No, for any given gearing, the shorter the crank the lower your cadence is and the more torque you need to produce to turn the pedals.

I suspect 175mm cranks are too long for me but I used these in the past without a problem and don't get any knee pain if I get the seat height right

Aye, indeed true at the same gearing and power you'll need more torque however the 'spinning' aspect was something that came up as I was chatting to a fitter a couple of weeks back. My interior knee angle is quite acute at the top with 170mm, and my thigh quite flat, and discussing moving to 165mm. I think what he was getting at was the consequence of the shortening is that you gear down because you're effectively self-selecting for torque anyway, hence the rise in cadence for a given effort. Lower gear to keep the torque similar, higher cadence to maintain a given level.

That said, if 175mms don't cause any discomfort then no need to consider smaller - it was just a thought - and you get the advantage of a slightly better lever out of the saddle on the climbs. @crikey - yep, 2.5mm apparently isn't really that noticeable, maybe unless you're really on the edge of the long end and going longer (that bit my guess, might be bollocks) and it wouldn't have been recommended to try and chance for such a small amount if it had been possible.

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crikey | 9 years ago
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Have a look at the difference between 50-11 and 52-11 as a maximum gear.

I used to use 53-11 when I was racing, but never miss it now. Descending fast requires a fast cadence, so I'd work on that, plus by the time you get to spinning it out, you're better off getting into an aero tuck rather than thrashing about pedalling.

Spend a couple of months only riding in the 34, spin your legs off and work on higher cadence, and get yourself proper aero tucked in.

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crikey | 9 years ago
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I don't think you are using 'torque' in the way other people understand it.

The difference between a 34 tooth chainring and a 36 is small and wouldn't really be worth swapping the ring or the whole chainset for. Especially now you can have 11 speed cassettes; just choose the gearing that you want and ride it.

Essentially, if you can get up something with a 34 chainring, you can probably do it with a 36. You might have to dig in a bit, but that's the idea.

If you are a high cadence rider, why would you even consider putting bigger rings on?

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barrydocks replied to crikey | 9 years ago
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crikey wrote:

I don't think you are using 'torque' in the way other people understand it.

The difference between a 34 tooth chainring and a 36 is small and wouldn't really be worth swapping the ring or the whole chainset for. Especially now you can have 11 speed cassettes; just choose the gearing that you want and ride it.

Essentially, if you can get up something with a 34 chainring, you can probably do it with a 36. You might have to dig in a bit, but that's the idea.

If you are a high cadence rider, why would you even consider putting bigger rings on?

Yes, you're exactly right. As quiet a small chap, weight (of both me and the bike) makes a massive difference to my hill climbing - before rebuilding my Prinicpia I bought a cheap Carrera which was over 3Kg heavier and I struggled on even the slightest of incline.

The main reason of considering a slightly larger chainwheel was simply availability through Evans Cycles (they can't supply a 9000 chainset in 34/50) but I also found I am at my max cadence on some fast down hill sections with a 50 chainwheel - ie I want to go faster down hill after making all the effort to get up it in the first place!! I thought 36/52 might be a good compromise over say 39/56 or what ever a standard size is these days but I don't want to kill the climbing - being able to just swap the small chainwheel if needed seemed like a good idea?

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vonhelmet | 9 years ago
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Most double dérailleurs can only handle a 16t difference, I think. For the price of the chainrings and hassle of changing them all the time you might be best just buying a complete second chainset.

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CXR94Di2 | 9 years ago
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Use sheldon brown gear calculator for ratios comparison. You could try 34 t on the larger crank, but will have to check derailleur clearance at the extremes

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