3km gearing help

[InPursuit]

I am looking to try the track in 2 weeks at Trexlertown at the "Regionals".

I am confused about gearing and will not really have the opportunity to try it out to figure out which is correct until race day.

On fixedgearfever.com there is a program (http://www.fixedgearfever.com/module..._op=getit&lid=) that says a 03:45 3k with 49/15 is 114rpm which is a tad fast for me and it says that 49/14 is 106 RPM which is a tad slow for me.

FWIW I have 14, 15 and 16t and 49t chainring. I don't want to buy a new chainring this year.

I figure that 03:45 3km comes out to about 30.6mph once you are up to speed (last 2666 meters at 30.6mph and 30 seconds for first 333 meters).

http://home.earthlink.net/~mike.sherman/shift.html says something very different, which is that 700x23 with 172.5mm cranks with 49/15 is only 29.3 mph @ 115rpm which obviously is a huge difference between the above.

It also says that 49/14 @ 115rpm is 31.4mph and 49/14 @ 112rpm is 30.6 mph, 112RPM being my sweet spot right now.

I'm comfortable around 115 RPM but not higher... again 112RPM being my sweet spot at the moment... and trying to decide between 49x14 and 15. These are 2 conflicting resources and I am not sure if the first is an "average" RPM vs "Cruising speed" RPM (more important to me because it indicates my highest RPM assuming my lap times are similar).

Thanks

 

[swampy1970]

On fixedgearfever.com there is a program (http://www.fixedgearfever.com/module..._op=getit&lid=) that says a 03:45 3k with 49/15 is 114rpm which is a tad fast for me and it says that 49/14 is 106 RPM which is a tad slow for me.

FWIW I have 14, 15 and 16t and 49t chainring. I don't want to buy a new chainring this year.

Unless you can magic up a 14.5 sprocket you're going to either have to pony up and get something smack in the middle of the 49x15 and 49x14 - which would be a 51 x 15, or you could get a bit more aero so you could go a little faster and just end up around 110rpm in the 49x14 that you have, or just ride with what you have and figure out which of the two gear options you have works best.

It's just a matter of deciding which of the three options you figure works best for you...

 

[InPursuit]

Well....can you confirm either one of these calculators is accurate?

I am happy to use one of the 2, I just don't know which is correct.

One calculator essentially says 49x15 is 30.6 mph (by virtue of it saying that 49x15 @ 114-115rpm is 03:45, assuming a constant RPM for the final 2666 meters), the other says 49x15 @ 115rpm is 29.3 mph.

One of them must be wrong as these are not even remotely close speeds @ rpm in the same gear.

 

[swampy1970]

Well....can you confirm either one of these calculators is accurate?

I am happy to use one of the 2, I just don't know which is correct.

One calculator essentially says 49x15 is 30.6 mph (by virtue of it saying that 49x15 @ 114-115rpm is 03:45, assuming a constant RPM for the final 2666 meters), the other says 49x15 @ 115rpm is 29.3 mph.

One of them must be wrong as these are not even remotely close speeds @ rpm in the same gear.

I would say the latter is 'the more correct' of the two. Actual speed depends on the acutal roll-out size of the tire used. As tires vary slightly in construction due to design differences, so will speed.

Sheldon Brown's speed/gear calculator has been pretty acturate - it says that 120rpm in 49x15 is ~30.5mph