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Old 05-16-07, 07:10 AM   #1
George
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How fast can I go?

I didn't want to hijack jppe thread, so I thought I better start my own. I looked at Sheldons way of figuring out the gears until my eyes started to glaze up. I got to where I could figure how many inches I could go with one turn of the cranks, but I still didn't know how to calculate the speed. I'll have to dig into it more.
I was wondering how much speed I lost going from 28mm tire to a 32mm tire on a 700cm wheel, with 175mm cranks. I like the differenence as far as comfort goes, but I was wondering how much speed I lost changing over to the larger tires.I added a rack and trekking bars so that probably be a factor as well. I'm waiting for a different stem adjuster so I can get more aero-dynamic and get some weight off my hands to help with speed and comfort as well.
Now if I was in shape to do this, how fast could I go if I had no wind and no grades. Not just me, anybody, a good sprinter or whatever. I know mine would be different because of my weight and power of my legs. I'm talking about someone in top physical condition with all the conditions above, being in 48 up front and 11 in the back, how fast can this person get the bike going.
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Old 05-16-07, 07:51 AM   #2
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If you can calculate the inches per revolution, then multiply by 100 for inches per min at an assumed cadence of 100.
Multiply by 60 for inches per hour.
Divide by 12 for feet per hour.
Divide by 5280 for miles per hour.

Convert to metric to loose your mind.

This is all based on the assumption that you can pull at 100 rpm in the chosen gear. Eventually you will find a gear that "bogs" you down. That's it, thats as fast as you can go unless other conditions change.
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Old 05-16-07, 07:52 AM   #3
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http://sheldonbrown.com/gears/

Use the drop down menu for "gear units". You can choose MPH@ various RPM to see how fast those gear combinations would go. Your combination would result in 31.5MPH @ 90RPM or 35.1MPH @ 100RPM.

I don't know of any way to calculate the drag effect of fatter tires. I would think that at the same air pressure, the effect would be minimal if any. Thankfully, I do not suffer from engineer's disease so I couldn't care less what this may calculate to be. I would just ride.
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Old 05-16-07, 07:59 AM   #4
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That's a bit of a complicated question there. One's speed in a specific gear (you wondered about a 48x11 combo) is determined by your cadence. So how fast can you spin? 120rpm is smoking for me although that's not so impressive for many track racers. If you go to this site: www.analyticcycling.com they can calculate everything out for you. 120rpm on a 48x11 is about 41mph. HOWEVER to spin 120rpm in that gear requires a LOT of power output from your legs. Something around 950 watts. That assumes a light bike that is reasonably aero. The larger tires, rack, and a higher stem all cost you. More power will be required to spin that 120rpm. The pro's can probably do it for a short time. Us regular humans might be able to do that for a few seconds?
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Old 05-16-07, 08:01 AM   #5
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Thanks for the replys guys. It's just something I've read on some other forums or threads and I wanted to know how they did it. I have my cadence up to 90rpm and I'm working on getting it higher, and it is a lot easier on the knees, thanks again.
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Old 05-16-07, 08:08 AM   #6
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Thanks for the web site Stevie, that helps for a lot of my other questions, I have bouncing around in my head.
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Old 05-16-07, 09:05 AM   #7
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George needs all of these complicated calculations.

All I need is a measured mile and a sundial.
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Old 05-16-07, 09:14 AM   #8
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Engineering unit = Furlongs per fortnight.
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Old 05-16-07, 09:18 AM   #9
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Quote:
Originally Posted by George
Thanks for the web site Stevie, that helps for a lot of my other questions, I have bouncing around in my head.

Wow. George, you might want to consider getting a helmet with straps that aren't so tight. (I hope you realize that I'm just kidding.) I'm actually impressed. I have almost no patience for such things. On cold winter evenings I wish I did.
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Old 05-16-07, 11:16 AM   #10
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I ride 48/11 on the tandem and with 26" x slick 1.4 tyres we have a speed of 30 mph at a cadence of 100. Put the MTB tyres on, and they are 26x2.1 and we go faster for about 1 mile. Then the drag sets in from the tyres and we can't reach a cadence of 100 in that gear. Same offroad. Bigger tyres cause a lot more drag so I use a narrow 1.8. May not give me an advantage on speed on the others initially - but definitely ups my milage before I start tiring.
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Old 05-16-07, 02:39 PM   #11
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Quote:
Originally Posted by stapfam
I ride 48/11 on the tandem and with 26" x slick 1.4 tyres we have a speed of 30 mph at a cadence of 100. Put the MTB tyres on, and they are 26x2.1 and we go faster for about 1 mile. Then the drag sets in from the tyres and we can't reach a cadence of 100 in that gear. Same offroad. Bigger tyres cause a lot more drag so I use a narrow 1.8. May not give me an advantage on speed on the others initially - but definitely ups my milage before I start tiring.
I would expect a lot more difference in drag between 26 X 1.4 slicks and 26 X 2.1 knobbies than between the 28 X 700 and 32 X700 tires that George asked about.
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