# Larger Chainrings - I wanna go fast!

#### jarodwinn

##### New Member
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

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.

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

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
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.

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