Larger Chainrings - I wanna go fast!



jarodwinn

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Sep 2, 2009
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While on a training ride today I got into a sprint on a shallow descent and noticed that in my highest gear I could only get my top speed to about 28 mph while my cadence was around 100-110 rpms. I'm now thinking of upgrading my large chainring to something bigger, the one I currently have is a 50T. I live in a relatively flat area here in Michigan and am wondering what size would be best suited for a rider built more for sprinting?

Thanks in advance for the help :D
 
You don't say what sprocket or wheel size you are using, but assuming you have a 700c and 23mm tyres, then calculating from your speed and rpm you must only have a 15 tooth sprocket.

This sounds wierd to me, as the vast majority of cassettes run 13 12 or 11 small sprockets. Either you have a 15 or your computer is out.

110 rpm with a 50 tooth chinring, 13 sprocket and 700c = 33mph
110 rpm with a 50 tooth chinring, 12 sprocket and 700c = 35mph
110 rpm with a 50 tooth chinring, 11 sprocket and 700c = 39mph
(all assuming a 700c wheel and 23mm tyre)

A 53 chainring is standard fare, but changing a 50 to a 53 will result in only a 6% increase in speed at the same rpm.
However, a 20% increase in speed is possible if you change from a 15 tooth to a 12 tooth sprocket. A smaller sprocket is probably a better option first up.

Also I woudn't say that 110rpm in a sprint is really spinning out. 130~140rpm in the dash to the line isn't uncommon (another ~20% increase in speed).
 
tafi said:
You don't say what sprocket or wheel size you are using, but assuming you have a 700c and 23mm tyres, then calculating from your speed and rpm you must only have a 15 tooth sprocket.

That's a very good point. Over the winter and early spring I've upgraded quite a few parts on my bike, one of which being the cassette. I just checked and the Mavic cassette that I've got shows a 14 tooth sprocket... that makes sense because the sprocket I had on before was a 12. Tires are the standard 23mm.
I guess I'll hav to pay better attention to what I'm buying next time.
 
As I said above, a one tooth difference in the sprocket size makes a proportionately greater difference to the gear ratio than one tooth on the chainring.

I guess it comes down to cost but changing from a 14 to a 12 gives you 14% greater gear ratio, whilst going from 50 to 53 is only a 6% difference. Whether or not you can actually get a 53 for your crank may be another question.

You can calculate this stuff yourself from cadence, gear ratio and wheel circuimference.
 
Thanks for the info Tafi. I noticed while out on a ride yesterday that my computer is acting up as well, in another sprint my speed went from 25 to 18 and then as I was winding down it registered back at 29... on top of that my HR monitor stopped working, so it looks like I have a few more bugs to work out.
I spoke with my LBS and it sounds like it would be cheaper and smarter to change up the cassette than it would be to replace the chainrings. I think I'll ride this cassette out and replace it in the fall.
 
jarodwinn said:
I noticed while out on a ride yesterday that my computer is acting up as well, in another sprint my speed went from 25 to 18 and then as I was winding down it registered back at 29... on top of that my HR monitor stopped working, so it looks like I have a few more bugs to work out.
I spoke with my LBS and it sounds like it would be cheaper and smarter to change up the cassette than it would be to replace the chainrings. I think I'll ride this cassette out and replace it in the fall.
How did you end up with a cassette whose smallest cog is a massively-large-by-today's-standards 14t?
Did YOU buy-and-install the cassette on your bike or did you have your LBS install the cassette?
If you still have your old cassette, then I recommend that you think about restacking the current cassette with the 12t and/or 13t from the older cassette.

FWIW. Check your hubs, too ... not all hubs are created equally ... or, they may simply need maintenance.

OR, think about changing your hubs/wheels.

IMO, DT's hubs have the best cartridge bearings ...

Also, change the battery(-ies) in your bike's computer(s) and or sensors! Bad/(tired) batteries could result in erratic readings, particularly with wireless connections (in case that is what you are using).
 
I bought this cassette on bonktown.com. It seemed like a good deal at the time. Now I know why they had it marked at 90% off. My old cassette was due for replacement, so I'll go ahead and use this one for the year. Now I'll have another excuse as to why I'm so slow :D
 
If you wanna go REALLY fast you could get a 67-tooth chainring just like Graeme.

Graeme Obree's Amazing Bicycle | Cyclingnews.com

If you're running a 50-tooth chainring I recommend that you get an 11-tooth small cog. Anything bigger and you'll find yourself maxing out in situations where you need the 11, such as descents or TT's with a tailwind.
 
jarodwinn said:
I bought this cassette on bonktown.com. It seemed like a good deal at the time. Now I know why they had it marked at 90% off. My old cassette was due for replacement, so I'll go ahead and use this one for the year. Now I'll have another excuse as to why I'm so slow :D
BTW. I still recommend that you think about restacking the current cassette with the 11t and/or 12t & 13t from another cassette ...

You can certainly buy an LX cassette off of eBay for under $20.
 
alfeng said:
How did you end up with a cassette whose smallest cog is a massively-large-by-today's-standards 14t?

Maybe it's a junior gearing cassette? USA Cycling limits the gearing juniors are allowed to race, and some buy larger cassettes rather than blocking the highest gears on a typical cassette. They look like pineapples. ;)