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PostPosted: Sun Sep 02, 2012 2:28 am 
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Due to gearing restrictions, I'm forced to choose a gearing ratio that will give a rollout of no more than 26 feet. Out of my options, the best two would be either a 52x14 or a 45x12.

Now, a 45x12 will give me exactly 26 feet, and will probably weigh less with a DA 7900 12-15 cassette and a Specialites TA chainring. I'd run a 34 inner as i'd rarely have to shift out of the big ring.

A 52x14 will only roll out to 25' 8", and i will be forced to use an older ultegra 6600 14-25 cassette, as that is the only one available. However, with a 52, i could be able to put on Q-Rings, but i don't know if that is worth the 4 inches that the 45 has to offer.

Any thoughts as to which would be the better gearing ratio?
Thanks


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PostPosted: Sun Sep 02, 2012 4:12 am 
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Is there a rollout test on your bicycle, and if so, is it after the race, and if your bike fails, are you disqualified? If so, you might not want to cut it too close, as a minor variation in rollout could get you disqualified. How confident are you in the 45/12 passing the rollout test, if indeed there is a test?

A 45/12 is 0.96% higher than a 52/14. You may have to decide whether you want more top end or to take a chance on disqualification.

On the other hand, if 45/12 is pre-approved with no rollout test, then that concern goes away. But if it's USA Cycling juniors, then I believe that the rollout test after the event is what counts. Any changes in tire or inflation pressure could change the rollout. USA cycling lists 52/14 as highest (recommended) gear to meet the 26 foot rollout test, so a 45/12 would be pushing another 0.96% beyond that recommendation, and may be somewhat risky.

In fact, for a 45/12 to pass the rollout test, the effective circumference would need to be 2113.2 mm or less, and a 700c tire could be bigger than that, although the smaller the width, the greater the likelihood of the tire being less than 2113.2 mm circumference. So I say forget it, unless you get a really small circumference 700c tire and decide you want to take your chances. A 52/14 will pass as long as the effective circumference does not exceed 2133.6 mm, which should give you a fairly comfortable margin for most 700c racing tires.

Is the rollout requirement 26 feet or 7.93 m? They are not the same. 26 feet is 7.925 m. A circumference of 2114.6 mm would just barely pass a 7.93 m rollout requirement, as opposed to 2113.2 mm being the largest to pass a 26 foot rollout requiremwent.

I wouldn't even be confident as to how accurately the rollout is done. If you use a gear (45/12) higher than the "usual" one, then even if it would pass an accurate rollout test, you're taking your chances that it will pass the actual (inaccurate) rollout test, and you may not find a sympathetic race official if you complain about the rollout test result.


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Posted: Sun Sep 02, 2012 4:12 am 


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PostPosted: Sun Sep 02, 2012 7:52 am 
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I agree with HammerTime - I've been there and had to re-roll out a number of times to prove my roll out was actually less than the limit when I was very close to it. The other thing I would say is that 52x14 gives you more options when you move up a gear restriction (?) level next year (though I don't know where you are in the whole process)

The only time I would consider trying to get smack bang on it, was if you feel like the only way you are EVER going to win a race is in a sprint.


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PostPosted: Sun Sep 02, 2012 12:24 pm 
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Try a 21 or even 20 m tyre
Or you could take out a little air before the test


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PostPosted: Sun Sep 02, 2012 4:42 pm 
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Thanks for all the insight, but i've actually been using 45x12 for about 1.5 seasons and have never had any issues with rollout, including two years at Nationals. 45x12 gearing is actually one of the specified gearing ratios in the USAC rulebook. I'm asking because i'm in need of another 45t chainring, and before i drop $70+ on a specialites TA ring, i want to make sure that a 52 might not be better. Putting all the disqualification and rollout matters aside, which one would be the way to go? Also, are there any other alternatives to the specialites ring? I can find a ton of 46t rings, b/c of cyclocross, but 45s just dont seem to exist unless they are for track, which obviously wont work.

What i've found so far is that with a smaller ring like a 45, i rarely ever shift down to the small ring, so i've put on a 34 as a bailout gear for really steep climbs.

Just recently 45x12 became a more popular option, as more and more riders around my area are now using this option.


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PostPosted: Sun Sep 02, 2012 10:01 pm 
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45x12 is the way to go. I used 52x14 when I raced juniors but we were allowed to have unrestricted gearing in senior races at the time. Since you'll be racing with restricted gearing all the time, get as much rollout as you can. There is a noticeable difference between the two IMO.


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PostPosted: Mon Sep 03, 2012 6:12 am 
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Apples for apples, the 52x14 combination will be more efficient on a mechanical efficiency basis, probably about 2% more efficient. That would well and truly dispel any losses due to the minor additional weight. The main issue is not so much the size of the big ring on the front, but the small cogs on the back; the difference in chain rolling angles between a 14 and 12 is quite significant. There is an engineering paper on it somewhere, haven't looked at it for a while though.

You could always run a modified Dura Ace Cassette. Here's the tech docs for 7800 and 7900: http://techdocs.shimano.com/media/techd ... 608753.pdf

http://techdocs.shimano.com/media/techd ... 728250.pdf

If you remove the 12 and 13 rings (and might have to replace the 14 with an inner ring type with built in spacer), then you can add other rings on the back. I'd start with a 12-23 cassette so that you have an 18t in there, and then add a 24 and 25, or say 25 and your bail out gear or whatever-easy to change for each race depending on conditions. Miche make individual rings to do this.

The other advantage to starting at 14, is that your ratio's will be closer together where they matter. The % difference between 12-13-14 cogs is great that 14-15-16, etc. Going to 11 is crazy I reckon given the drivetrain efficiency losses and

For the record, I'm looking to build up a 44-30 or 44-33 crankset, with a 14-15-16-17-18-19-20-21-23-x (x=25, 27, 28 or 30, depends...) set-up for long audax/rando multi-day/week rides. I'm more interested in maximum efficiency and endurance over all-out speed for those though, I'm happy to tuck into an aero position and freewheel on steep descents, saving calories. I'll build my cassette up from Ultegra cassettes and Miche cogs. I don't mind that the set-up could weigh slightly more (rings and chain will be lighter, cogs will be heavier) because the power losses are much more significant. Also, I can happily run no front derailleur, cable and DT shifter on that set-up if things are pretty flat, and only change manually down to the bottom ring if I get to a decent hill.


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PostPosted: Tue Sep 04, 2012 1:25 am 
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oysters wrote:
Apples for apples, the 52x14 combination will be more efficient on a mechanical efficiency basis, probably about 2% more efficient. That would well and truly dispel any losses due to the minor additional weight. The main issue is not so much the size of the big ring on the front, but the small cogs on the back; the difference in chain rolling angles between a 14 and 12 is quite significant. There is an engineering paper on it somewhere, haven't looked at it for a while though.

You could always run a modified Dura Ace Cassette. Here's the tech docs for 7800 and 7900: http://techdocs.shimano.com/media/techd ... 608753.pdf" onclick="window.open(this.href);return false;

http://techdocs.shimano.com/media/techd ... 728250.pdf" onclick="window.open(this.href);return false;

If you remove the 12 and 13 rings (and might have to replace the 14 with an inner ring type with built in spacer), then you can add other rings on the back. I'd start with a 12-23 cassette so that you have an 18t in there, and then add a 24 and 25, or say 25 and your bail out gear or whatever-easy to change for each race depending on conditions. Miche make individual rings to do this.

The other advantage to starting at 14, is that your ratio's will be closer together where they matter. The % difference between 12-13-14 cogs is great that 14-15-16, etc. Going to 11 is crazy I reckon given the drivetrain efficiency losses and


I can say that I have in fact heard of people mentioning that 14 is more efficient, and the 14t does indeed offer a smoother transition between gears, but what about the "harder gear" that 45 is able to offer? Does the 2% make up for the 4" per full revolution of the crank that i gain on a 45?


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PostPosted: Tue Sep 04, 2012 2:42 am 
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Depends on what the limit is-max speed at max rpm, or do you even get near max pedalling rpm? If the races/course says any attack is at max rpm and you cant catch him bc youre topping out, better lose some efficiency (in that gear) so you get the gear. Besides, as mentioned, youll be losing that efficiency in that gear. Youll also lose a little in all gears smaller than about 16t, to proportional degrees.


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PostPosted: Tue Sep 04, 2012 5:36 am 
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thisisatest wrote:
Youll also lose a little in all gears smaller than about 16t, to proportional degrees.


That's right. So if your top is a 12, you are more likely to be in one of those inefficient gears for more of the race. Which may take its toll on you and make it harder to be there for the sprint at the finish...

At least with a 14 top, you would be rarely in the 14 and 15.

If someone told me I could choose between a smidge more top speed in an all out sprint at the end of a 200km slog, but at the cost of say 2-3% overall loss of power for the 200km, I'd pick the efficiency option every day. If you are a little bit extra knackered trying to keep up with the other guys, then they will cream you in the sprint.


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PostPosted: Thu Sep 06, 2012 3:10 am 
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Looks like i'll be putting on my old 52 back on, and eventually ask for help in possibly building a DA cassette for races. I'm looking at the 52t Q-Ring, however... says it will feel like a 54 on the down stroke. Could this make up for my 4" loss over the 45?


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Posted: Thu Sep 06, 2012 3:10 am 


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