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Discussion Starter #1
so, im running 17/34 gearing on my racebike. why should or shouldnt i go to 16/32. i was told it'll be less rotating mass, so its beneficial. like drilling brake cams. (every little counts, right jb?) im probably not going to waste the $$ and time to do it, but anyone have some good thoughts??

jc

i dont know shit
 

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off the top of my head you are decreasing the rotating mass (because the overall sprockets are slightly smaller) but at the same time you are sacrificing acceleration too. The smaller the overall diameter of the sprockets the numerically lower the gear ratio. you are talking about a very small change in mass so I doubt it would be beneficial enough to make a difference.

of course I could be way off on this but I think that is how it works.

if you haven't already, switch to aluminum sprockets. Also an old speed trick I learned from a motorcyle drag racer was balance your sprockets as accurately as possible.
 

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yeah, but the gearing ratio is still exactly the same, so there shouldn't be a difference? i mean with both set-ups your getting exactly two turns of the rear sprocket for every one turn of the coutershaft.

joe, how did you like that gearing for NHIS. i think it's right for VIR and Summit but i'd likely throw a 36 rear on for NHIS? maybe that's how mike and jam left you on the T12 exit?

-tt



Edited by - ttilghman on Feb 06 2008 1:47:09 PM
 

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In this case the overall gear ratio would be the same, 16/32 = 2.00, 17/34 = 2.00 so acceleration should be the same for both. Less rotating mass, maybe a couple of grams. Assuming the chain you use is the same chain in both cases the 16/32 should get you a slightly longer wheelbase since you'll have to move the wheel further back to take up the chain slack. This in turn should move the center of gravity of the bike slightly forward which in the case of the cb350 is a good thing.

But unless you need new sprockets or chain anyhow I can't see that it's going to make enough difference to bother with.

Craig
 

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Ok, so maybe I mixed terms. The rate of acceleration would stay the same because the ratio is the same. However the actual speeds woudl be different. basically it is the same as switching to a smaller tire diameter - the RPMS of the sprocket have increased but you are not covering as much distance so the speed suffers during acceleration (although theoritically your top speed should be higher).

that being said I have no expirence with this and I am interested if such a small amount of weight difference actually has an effect. remember rotating mass has its biggest effect when changing directions (like going from deceleration to acceleration) and reducing that means you might be able to accelerate slightly faster.
 

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I don't think that's right, Geeto. In the case of the tire analogy, because the tire circumference is longer, equal rotational speed translates to greater linear speed. In the case of the chain sprocket, the same thing is true - but for the CHAIN, not the wheels. That is to say, with smaller sprockets, the chain won't be moving as fast relative to the stationary frame, but the wheels will be moving at the same speed they would with larger sprockets (plus some negligible amount resulting from decreased rotational mass).

Another analogy: your bike's on a dyno. For some sick reason you decide to stick your hand in the spokes of the rear wheel, and it subsequently gets torn off. If you grab the spokes at the rim, your hand will fly at a considerably faster rate of travel across the shop than if you inserted your claw close to the hub. In either case, the wheel moves the same speed.

The only mechanical advantage to a larger sprocket configuration is decreased chain wear (less angular travel at the chain pivot pins), but, much like the decreased mass of slightly smaller sprockets, the effect is likely inconsequential.
 

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this is ridiculous.
our races are decided by seconds (and even minutes sometimes). and you're talking the difference between blonde and brown pubes. get your head(s) outa your arses. a better idea might be to eat some raisn bran the night before a race so you're less full of shit after your morning dump. or skip that last sip of coffee and make a real difference. maybe put some helium in your tires. what the "f".
-parks
joe REALLY don't know shit
ps. and geets...your "save" is like "that depends what the definition of "is" is".
 

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Discussion Starter #9
ive thought a lot about craigs comments before i posted this. when they change gearing in motogp, they adjust the chain length, or move the swingarm to retain wheelbase. like id ever know the difference. but the topic came up because i was talking about reducing weight. and the sprockets came up. like i said, im probably not going to change things, but im curious as ive always wondered. i think if you had a really well calibrated dyno, youd see the difference. every little bit help. every little bit.

i got left in t12 because i slowed down the entire lap prior! if i had mirrors, it would have never happened! i was too afraid to look back to see who was there!

that gearing seems good for nhis. but its been a long time since i had been there, and i never really had the bike dialed in. the issue i have with changing gearing is corner entries and exits. getting in at my speed, and getting out. i like to stick with whats close and comfy for me. but id be interested in knowing who has what for nhis. just for curiosity sake. especially for the stockers. what are you running tim?? i think my gearing might be a little short for vir. it seems short for summit too, but that front straight is soo long compared to everything else. ive heard people say gear for top speed on the longest straight. but i dont think that works every single time. i guess call it rider preference.


jc

i dont know shit
 

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Discussion Starter #10
parks, i know guys who have dynod oils! not that i care that much. im just curious what people think about the gearing thing. if it matters more than a speed issue.

jc

i dont know shit
 

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cheese up the gears. leave em. it's all psychological anyway. I got a carbon fibre helmet and I swore I was going faster because of the weight savings.
 

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Discussion Starter #12
me too evil! well, that and it looks cool.


jc

i dont know shit
 

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joe-

i've only run the 17/34 but have a 36 in the pit box. basically, since i've only had good compression for about 7.5 laps, it hasn't made a difference. i thought i was figuring things out when we were at VIR last spring, but then i found out it was the floating valve seat? but i think i'll run the 34 at summit and VIR and try the 36 at NHIS since i'll be wearing a heavy-ass Scorpion helmet.

-tt
 

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Xander knows what he's talking about.

Smaller sprockets is always going to be better assuming the same drive ratio. Less weight, and chain speed is less...meaning the chain doesn't have to be acclerated as much either. Fewer Feet Per Second of chain travel per turn of the rear wheel with smaller sprockets. So...you have less weight in sprockets, less weight in chain (if you removed unneeded links, and less energy spent accelerating the chain around the sprockets.

And think of all the situations where someone told you to just take a shit before a race rather than saving some weight on the bike....well add all those up and you better take about 20 shits before the race. Bottom line...light is better. Don't bother drilling this, don't bother shorting those bolts, don't bother using that alloy, don't bother putting on that glass etc etc etc. Just take a shit instead. Yeah....that's how races are won...doing stuff halfway.

Look at the bikes winning races consistantly...you won't find many heavy pieces of shit. You'll find very meticulously prepared bikes. And believe me...if those winners started losing...they'd start looking for more HP and less weight.....go tell them to take a shit instead and see if they listen.

No one single change rarely makes a big difference...but add up 10 little changes, add up 20 of them...that makes a difference. Park my bike on a scale next to a typical 175 and see it 30 lbs lighter...tell me that doesn't make a difference....well, that 30 lbs came from lots of little changes.

I'll take a picture the next time I take a 30 lb shit :)

JohnnyB
 

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it's amazing how fast shit accelerates out your ass.
-parks
ps why didn't you tell em about the principal of a smaller bend angle of each link going around larger sprockets/cogs and therefore lower friction as the chain changes direction from one end to the other.
 

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What Branson and Xander said, except there is more frictional loss running a chain around a smaller sprocket. But lighter is better, and the rear and half the chain weight is unsprung so double good. A smaller O.D. sprocket will accelerate faster even if static weights are the same.

I think you'd notice more difference in using sprockets to change the wheelbase though. I've played with that a bit on dirtbikes. How do you guys scale your bikes and what is a livable front to rear percentage?
 

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16/37, I think

Personally, I give myself an enema before every race weekend, plus I've got a carbon helmet.



Jeff
 

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Discussion Starter #19
wow, 16/37.....no way that can work at vir or summit. no wonder why you ran away from me at nhis!

that thing must sweet wheelies!


jc

i dont know shit
 

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Hate to be late to this discussion, but here's my thought; yes, you should gear for the straight to get the maximum benefit of top speed, but if the smaller countershaft sprocket also gives you an advantage in acceleration from some of the slower corners, and the final drive is the same on the straight and you reach the same top speed, then you have a better chance of making up time on the slow corners and potentially a better opportunity to pass in those sections. Also, if the 16/32 gets you to top speed quicker and you can keep it pinned there longer, I think your lap times should improve, 'nest pas?

I'm just thinkin' like a dirttracker though...


FR
 
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