I’ve been riding 650’s using a 53/39 chainring and 11/21 cogset. A buddy advises that I change the chainring to 54/42 to better correlate with 700 gearing. I don’t ride 700’s, not even in training, so I’m not really interested in how they correlate.
Assuming the same intensity and cadence, would I be getting free speed with the bigger chainring, or would I simply be moving my gearing up a cog in the back?
Scott, I ride the same thing as you and don’t see the sense in going with a bigger chainring if I’m not running out of gears. I was riding as fast downhill as people with chainrings 2 teeth bigger than mine this past weekend. When I start finding that I am riding in my 53-11 and need more speed, then I’ll consider a bigger front chainring.
The ratio is important of the number of teeths is important. So one more teeth of the cogset in the back does not translate into one more teeth of the chainring in the front. It is the ratio of front:back
e.g front 54 teeth, back 12 teeth = 54:12 = 4.50
e.g front 53 teeth, back 13 teeth = 53:13 =4.08
at a given cadence, e.g 100rev/min, you get further with the 54/12 than the 53/13. The higher the ratio, the harder the gear. this is true when comparing the the numbers between bikes of equal wheel size. You cannot simply look at the ratio when comparing 650s to 700s.
when comparing 700s to 650s, you also have to take into consideration the circumferances of the two wheels. Assuming you have the same ratio (chainring, and cogzize) and equal pedal rpm’s, the bike with the 700s will travel further than the bike with the 650s wheel because of the greater cirumferance of the first wheel.
if you have sufficient gears and you never run out of them, don’t worry about it. otherwise you can get a bigger chainring in the front or a smaller cogset in the back.
daniel
Scott, can you spin out a 53/11. Not many of us can. If you can’t then don’t worry about it.
Scott I agree with what has been said. If youi don’t spin out in 53-11 don’t change anything. If you have a 650 and 700 with the same cogset but 650 has 56-42 and 700 has 53-39 you have roughly the same gearing at the same rpm but if they have the same cogset and both have 53-39 the 700 will be significently faster at the same rpms. I have a friend that rides a 650 and when he is in his 53-11 I can walk away from him ( I ride 700) and have 3 gears left. Your talking gear inches ie how far you travel with 1 revolution of the wheel. Yo compare enter gear inches in a surch and several websites will appear where you can enter gearing and wheel size and get the gear inches to see the comparison of wheel size to gearing.
Thanks. I’m not worried about spinning out, but wonder more about whether I’d be racing at 54/17 instead my current 53/16. The 16 is nice to have with the 15 and 17 on either side. Racing in the 17 with an 11/21 cassette would limit my ability to fine tune within the big ring alone. I might have been be willing to trade off this inconvenience if changing to the 54 simply buys me some speed without the proportional increase in effort. I guess my concern was that the original 53/39 set-up was plain ol’ wrong and costing me speed!
Scott…
could have been. On my tri bike, I run a 56-44/11-21 on my disc for most races. I got a 56 tooth big ring for a great deal so I thought I’d just take the plung. However, I needed to work it for a full season as it was so different from whatI had beore. (53) And two years later, I still can’t spin this out. If I had my choice to do it again, I’d spring for a 55 or a 54 tooth ring instead.
It took me a full season of heavy riding and one additional leg workout with weights to really adjust to this. But, I did a lot of research about gearing ratio’s and such, but the bottom line is…Just Ride. For most races, flat and or rolling, I keep the gearing between 56 front switching back and forth from the 14 and 16 on the rear cassette. My cadence is lower, but it pulls really nice. The other gears are rarely used.
Just my 2 cents…
Fran
The other issue to think of is not so much being able to spin out a 53/11 (or 12) but what chain lines your are running. If you are constantly riding in the big ring with 53/12, then I’dsuggest you go to 55 tooth ring so youcan ride a 14 or 15 tooth cog at the back )whichever works out to the same ratio) and keep a straighter chain.
I you ride mostly flatish courses, then this is more important than if you ride hills and are constantly using 39/21 (or 23 or 25), switching thru 39/14 then up to 53/19 down to 53/12. Hilly courses (training or racing) mean you’re gonna be all through the small and big ring and using most of the cogset as well.
I don’t worry what gears I have and what gears my mates have. I worry about achieveing my goals of 95RPM as much as possible and I shift as necessary to achieve it, with the straightest chain I can achieve it with.
TriDork
p.s. I ride lotsa hills, with long flats in between. I have 53/42 and 12-21 cogset and that gets me though most things except my knees can take a hammering on the hills. When it comes time to change, I will probably go for 52/39 up front and 12-23 cog togowith my 650 wheels. I might occassionally spin out on long downhills, but I am so fat I RULE on downhills anyway 
I ride a QR Kilo with 650s (when I bought it in '99, no one made Tri bikes with 700s). Out here around Vegas, there is really no flat ground. In fact, you can find grades as steep as you want. As far as gearing, it all comes down to gear inches, ie. the inches the bike travels with one revolution of the chain wheel. It is useful to know when comparing your road bike gearing (700s or 27s) to a 650 tri bike. The best set-up out here for me on a 650 tri bike (I am bigger than most; 178 and 6 foot) is a 39/55 and a 11/25. The 39-25 gives me 40.5 gear inches for climbing steep grades and the 55-11 gives me 130 gear inches (I will spin-out at 40 mph on a good downhill).
Sheldon Brown has lots of good stuff about gear inches. The formula is simple: chainring teeth x wheel diameter (inches) / rear cassette cog teeth. For example, my top gearing is 55 x 26 / 11 = 130 gear inches. The granny is 39 x 26 / 25 = 40.5 gear inches.