Is gearing just gearing? Smaller front..

So I have a question. If lets say a bike came with a 16th sprocket and you wanted to bump down..... -2 instead of going up 5-6 in the rear.. Is this frowned upon? Somewhere I have hear that it is. Any truth to it? Meaning is it worse for the drivetrain with going down two in the front rather than 5-6 up in the rear.

Or does the bike not know the difference. Now before you guys ask the reasons would be chain length and sprocket availability.

 

The final ratio determines the end effect, so if -2 up front gives the same ratio as going +6 in back, same end result.

That said, I've been told not to go smaller than a 14 up front, or bigger than a 16 or 17 if I can avoid it as you burn energy making the chain wrap around that small a pivot, and I don't recall the loss that can come from going to big, just that 14 to 16 was the 'sweet spot'.

 

The bike definitely doesn't know the difference. Only the chain and sprockets do. You might have marginally greater wear on the chain due to using a very small front sprocket, but it also reduces the length of chain needed slightly and your rear sprocket will be correspondingly smaller to result in a given final drive ratio so there is less rotating mass.

 

Pretty much agree with what has been said, but keep in mind that sprocket selection can and does affect the way the rear suspension reacts...to whatever extent.

 

gearing is not just gearing. I ran more gear at Tally Sunday, with smaller sprockets, than I did down there in Jan, and the bike wheelied less and was more stable on the power.

http://forums.13x.com/showthread.php?t=293803&page=2

 

Ok..... so lets say you have a bike with extact same ratios.... Much like a airplane on a treadmill.

14-43 and a 15-46. Both 3.07ish Would shift points be the same?

 

shiftpoints exactly the same. Torque on swingarm, Much different.

 

I'd be surprised if it was that different, but I can see how moving the chain further away from the swingarm pivot would increase the torque somewhat. Moving to smaller sprockets should reduce the effect that the chain torque has on the suspension.

 

its more complicated than just that, but i did go faster there using much taller gearing than everyone else used to use.

just because a track is short doesnt always mean you should use lower gearing. hell, we have a bunch of tracks that don't use 6th gear. gear it for the track and usable gears that give you the fastest laptime. that was my rule of thumb.

 

Squat, anti-squat effect and overall chain/sprocket combined weight are a few things that factor in.

 

The main difference in how the suspension works is because of the difference in height of the chain because of the rear sprocket.

 

Yeah I typically gear for what part of the track matters the most. It's always a trade off it seems. I'll gear it for the best drive off of the most important corner rather than just gear the hell out of it. Some times I may go the other way if I am too high in a gear in a particular corner. Road Atl turn 7 comes to mind.

 

:wow: WHAT?? Your telling me "the bike doesn't know the difference' if a racer runs say a 18/53 or a 14/41 ?? (basically the same ratios.. 2.93 v 2.94)

I would love to hear your thoughts on this since it is the tech section.... please discuss swingarm angles and "chain torque/squat/anti squat" (pull) on the bike and how a 41 tooth rear and a 53 tooth rear dont effect the chassis or as you say "bike wont know the difference"

 

I've been told not to go smaller than a 14t front due to the radius that the chain has to turn while under stress being so small. Causes excessive chain wear.

Again, it's what I've been told

 

dropping teeth on the front:
- increases chain angle -> more anti-squat
- increases swingarm length -> flatten squat curve, ie less squat

theres an article on Sportrider from a while ago that goes over most of it
http://www.sportrider.com/tech/146_0404_motorcycle_traction_geometry/index.html

 

The bike meaning the motor. The final drive ratio is virtually the same. I believe I agreed (above) that the torque on the swingarm would be affected, though I doubt it's as big of a factor as some people think. Basically if your name isn't Checa or Stoner you probably won't notice much.

If you want, do some math and show us how much of a difference it is going to make vs rider weight or spring rate. Pretty sure the sprocket size differences will be the smallest force factor working on swingarm movement unless we're talking about ridiculous, theoretical sizes. But I could be wrong.

 

final drive is final drive .. 3.00 in one configuration is still 3.00 in another configuration. You may have wheelied because you moved the axle way forward or something with another sprocket combo.

I agree it is best to gear a bike for the best drive of most of the corners. many tracks you won't use 6th, or even 5th (barber).

Now how things affect suspension? like the example Ed points out .. that is another discussion .. and with no disrespect to my fellow racer dudes, i'm not sure how many of us would ever feel or know if better/worse if a 15/45 would work better than 16/48 if you could have the axle in the same place

OP: you fail to mention what bike, but i would never go smaller than a 15. Doing so will have the chain riding on the to of the swingarm guide pretty hard.. and i don't have scientific data to back it up, but any tuner/excelent mechanic i have worked with/talked to would not want the chain making that severe of a turn.

 

+1

Dave Moss talked about this in one of those Unsprung videos of his. The larger the rear sprocket I believe it was, the more torque it exhibits on the rear suspesion/swingarm.

 

I went from a 16/47 (2.9375) to a 15/45 (3.00), shortened chain 2 links, as to keep axle in almost same spot. I am sure I could have went to a 15/46, and still been more stable than before. I am a believer.

 

You can always complicate it further by compensating for the ride height change associated with the eccentric adjuster of a single sided swing arm.