Bicycle Mechanics - Faster = Size of Front Cogs?

I was wondering....
(This may be stupid, dumb or kinda interesting, just wanted multi directional opinions)
If I made a custom front cog for my bike (front derailer right? Where the pebal is) which was alot bigger than the normal sized one, though it might be ALOT harder to turn the pedal wouldn't it make the bike go faster once up to speed?
We obviosuly shift down the gears to add more tension so we can drive the wheels harder to go faster.
Instead of stopping in 24th gear, couldnt we make that front cog bigger and effectively make a gear... 46 or ... 60+ something?

Or is this a stupid idea?
I was interested in it. Anyone who has a clue, write back.
Thanks
James

Ask yourself this: in the highest gear you have on your bike now, are you at any time able to pedal fast enough that you'd feel the need for an even higher gear?
If no, then you don't need that.
I know it's hilly in Wales, but I don't know if you're going fast down the hills...

Not to mention your poor front derailleur...

Think of it as simple multiplication: If you are using a 22 tooth front gear and 22 tooth rear cog, then every time you turn the crank, the rear wheel will go around once, (1:1 ratio). Then if you switch to a 44 tooth front gear and keep the rear cog the same, at 22, you'll be spinning the rear wheel twice for each turn of the crank, (2:1) ratio.

Then, when you get to 132 teeth (or thereabouts), on the front, you can kick anyone's butt, (if you can get moving at all).

http://www.speed101.com/images/sept/Mom_KyleEdge_Gears_1992.jpg

Basically the diameter of the front gear in relation to the diameter of the rear cog will determine how much mutliplication you're doing. So, a 50 front/ 25 rear setup will get you the same "speed" as a 22 front/ 11 rear setup, since they're both 2:1 ratios.

At least I think that's what you are writing about. In the real world, the manufacturers have it all worked out to gear ranges that work for everyday cycling, so there's no room for "reinventing the wheel," or so to speak, here, especially since the gear sizes are limited by other aspects of the bicycle, like chainstay clearance and derailer range limits.



Joe

Answer to CDCF: Yes.
I have a Giant OCR2 road bike. By the time you reach 47mph on a downhill (though I can do 45 on flats) I find that I'm pedling too fast and my weight is being thrown around too much, making it feel all a bit 'on the edge'
So generally yes, I believe I need that extra gear.
Thanks for the reply though.

Answer to Joe:
Yes that's what I'm talking about. Great reply, starting to make sense now. Never thought of the ratio's and number to teeth. Plus the picture you linked is what I envisioned.
So the best bet is to have a ratio of like 4:1 ? right? I'll research some stuff on gear ratios next I think.
Why dont the pro race bikes then have massive front gears? I could probably answer that myself, just wanted to throw it out here.You watch the final sprints to the finish on these races and they are going like the clappers! Wouldn't it be there you'd want that higher ratio, or gear and rather drive it home rather than hammering the crap out of it? I dont know, I'm just debating the issue and why we dont see bikes with massive front gears. There's obviously a reason.
Thanks for your reply though, much appreciated.

James

You'll go faster with a bigger front chainwheel PROVIDED that you're strong enough to turn the crank.

That makes sense yes, but obviously your not going to be standing still at first are you? You'll build up your speed so using such a bigger gear would be quite easy as you already have the momentum.
But generally that would be going downhill, unless you are fit and strong enough to maintain such a pace on flats.

The effort required to turn the cranks depends on the power requirement for your motion at that point in time.

That includes rolling resistance, efficiency losses in the mechanicals, gradient, wind resistance including headwind/tailwind...

This thread takes me back to the early days of posts from MERTON and 53-11_alltheway. :D

If you want to see really huge gears, check out some of the bikes used for land-speed records using motorpacing. Most of these have enormous "development", and cannot be turned over at all by normal humans below 50mph or so. They have to be towed up to speed by the pacing vehicle. Normally, the rider has an electronic control for the motorpace vehicle's throttle, so he can precisely stay in the "pocket" of no air resistance.

http://www.canosoarus.com/08LSRbicycle/LSR%20Bike02.htm

The lower of the bikes probably has something like a 25:1 gearing!
At a cadence of 100, assuming the wheels are 20", that's 240 km/h (150 mph).

That makes sense yes, but obviously your not going to be standing still at first are you? You'll build up your speed so using such a bigger gear would be quite easy as you already have the momentum.
But generally that would be going downhill, unless you are fit and strong enough to maintain such a pace on flats.

Air resistance is everything. All of that other stuff is small potatoes.

My little Ranger pick up truck will actually go faster in 4th gear than it will in 5th. That's because in 5th gear the engine is running farther down on the power curve and it doesn't generate enough horsepower to overcome the wind resistance.

The same thing's true on a bicycle. You can calculate gear inches and mph at 100rpm cadence all that you want, but if you don't have the muscle power to turn the crank that fast it ain't gonna happen.

Answer to CDCF: Yes.
I have a Giant OCR2 road bike. By the time you reach 47mph on a downhill (though I can do 45 on flats) I find that I'm pedling too fast

So the best bet is to have a ratio of like 4:1 ? right?


Sorry, I refuse to believe that you can hit 45mph on flats with a road bike. That's the top speed the world class trackies hit in sprint.
Anyway, gears do run out thereabouts, but that seems to be a good thing to me... Bombing down a descent at 50 mph you better have a clear mind and not pedal at all so you can stay in control.

4:1??? That's the ratio MTBs have... A Giant OCR surely has something like 52/11, ie. 4,72

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