On Thu, 15 Mar 2007 14:11:19 -0500, Mark Olson wrote:
> Zaphod Beeblebrock wrote:
>> On Wed, 14 Mar 2007 10:19:43 -0700, chateau.murray wrote:
>> If you will think about it rationally for a moment gentlemen, before
>> simply *assuming* that I am exaggerating, consider what I said about
>> changing out the rear sprocket.
> I don't care what you did to the sprockets, without a dustbin fairing on
> that 360, or lots of very expensive engine work, you don't have an
> icecube's chance in hell of getting it up to 120 mph. There are certain
> basic facts that you can't weasel out of-- and there's always somebody on
> Usenet who knows far, far more about a subject than you or I do...
>> Picture a multi-speed bicycle with different sized sprockets on the
>> front and rear. Consider your legs to be the two cylinders. Pumping your
>> legs at a given rate (RPM) will propel you at a given speed. If you
>> shift the rear derailleur to a _smaller_ sprocket, then pumping your
>> legs at the same rate as before will propel you at a *faster* speed.
>> Granted, the smaller rear sprocket will change the torque(?) ratio, but
>> at my relatively light body weight the sacrifice is an acceptable
>> trade-off for the higher top speed.
>> There is nothing magical or delusional about this:
>> Smaller rear sprocket = higher top speed.
> No. The faster you go, the more power it takes to get there. If your
> engine doesn't make the raw power it takes to force you and your bike
> through the air, no amount of messing with gearing is going to make it
> faster. I learned this way back in 6th grade when I hit on the idea of
> putting a large sprocket on the centrifugal clutch of my doodle bug and a
> tiny sprocket on the back wheel. Doing the simple math told me I could
> achieve hundreds of miles per hour with a 5 hp Briggs and Stratton engine.
> Thankfully the laws of physics have remained intact despite my high hopes
> and it isn't going to work any better for you than it did for me in my
> long-ago thought experiment.
> How much you weigh has *nothing* to do with top speed on a level surface.
> It greatly affects acceleration, but air resistance dominates your
> eventual top speed.
Actually, it does. With a lighter load and less torque required for
acceleration, more of the engine's horsepower can be used for overcoming
wind resistance, etc. The less available horsepower you have, the bigger
Take your 5 horse B&S motor, take two of them actually, and build two
identical go-karts. Put me on one and a 300lb guy on the other and we do
10 laps around a quarter mile track. Who do you think is going to win the
Granted, in this case wind resistance isn't much of an issue because the
top speed doesn't get high enough for it to matter much. But, put 12"
tires on those carts instead of 10" tires (we'll change tire size since it
accomplishes basically the same thing as changing the gear ratio) and what
If we do another 10 laps, I'm going to win by an even bigger
margin...assuming that my opponent can get moving at all.
Now take me and that same 300lb guy and put us both on identical stock
CL360s...I'm guessing that they might put out about, oh, 24 horses? We run
a straight 10 mile stretch of level highway- I'm faster off the line
because I have less load to accelerate, it requires less energy to
maintain my momentum AND, because I'm skinny and he's fat, *he* needs more
energy to overcome wind resistance than I do, which becomes an increasing
factor as speeds increase.
Now take those same machines and change out the rear sprockets for
sprockets with six fewer teeth. What's going to happen? Acceleration
becomes a greater factor, but more for him than for me. With a lighter
load and less wind resistance I'm going to beat him by an even bigger
margin. The change in gear ratio doesn't affect me as much as it affects
him because of his greater weight and larger size.
Give him back his larger sprocket and set us off again- I'll bet I'm still
faster off the line AND, because I have the taller gearing and less wind
resistance I'm going to go a LOT faster than he is.
On the other hand, if you were to put us both on bikes powered by 454 Olds
Rocket motors, my size and weight wouldn't mean diddly because of all the
excess horsepower available. With limited horsepower size and weight can
>> This is the first bike I've owned with a shaft-drive. I am constantly
>> wishing it had a sixth gear in it. I'm not as much as a speed-freak as I
>> was when I was younger, I'd just like to lower the RPM for cruising and
>> improve the fuel milage...especially since it seems like the alcohol mix
>> I'm getting for fuel now has put a drastic hit on it. I used to be able
>> to get 165-170 miles before the low-fuel light would come on, I ran dry
>> the other day at 136mi.
>> I experienced the same phenomenon on a trip to Texas last year. Straight
>> regular gas gave me the expected milage, the alcohol mix reduced it,
>> requiring more frequent fuel stops.
>> I have my doubts about the feasibility of changing the gearing where the
>> shaft meets the rear axle, which means that I'd probably have to change
>> the transmission gears. Is either one of these ideas feasible, without
>> going to extreme effort or expense?
> You might be able to swap in the rear end from a different bike. Other
> than that, there's no easy or cheap way to change the final drive ratio on
> most shaft drive bikes.
Thanks, I had a feeling that might be the answer I was going to get.
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