Yes, with the smaller sprocket up front, remember the pully system from school, you get more pull on the chain (wear). That is why you get more acceleration.
Also, a foot pound of torque is one lb at one ft. One lb at 1/2 ft is 2ft/lb torque. More torque with less diameter at the power side.

I forgot about the speedo. Good point.

BTW, I had that plastic part between the sprocket and speedo drive crack and my speed reading was all over the place, mostly low.

 

I have broken one, and rounded off the part that goes on the sprocket on two of them, so I am on my fourth one. But they are only five bucks. I should buy extras.

 

Actually 1lb at 2 feet = 2 ft/lb ... 1lb at 1/2 foot = 1/2 ft/lb

 

The easiest way to look at this is to convert the sprocket change into a percent of change. All related items will be changed by that percent. 1 tooth on a 17 tooth front sprocket is about 6%. Therefore the chain load in tension goes up by 6%, the torque to the wheel will be increased by 6% for a given RPM, and the speed of the wheel will be slower by 6% for a given RPM, hence the speedo will be in error by 6% (higher because it comes off the front sprocket).

Changing the rear if it's a 42 will be 4.75% for two teeth and 7.15% for three teeth. The one thing that changes is that the chain load does not go up because the change was made past the chain. It is now all at the rear sprocket bolt-up and wheel. Wherever the change is made, all downstream elements see the load change, upstream does not. Does that help? Also, three up at the rear may require a longer chain, not sure.

And yes, changing the front does increase the hinging of the chain and that, with the additional load due to leverage change, is not the optimum place to do it for chain life. I have no specifics for what real world life expectancy problems will occur but the load is higher. It's best done at rear for life based on logic but, at the same time, I don't know anybody who broke a chain (that wasn't already junk) from a front sprocket change. With that, make your own decision.

 

[quote]ORIGINAL: chainstretcher

Actually 1lb at 2 feet = 2 ft/lb ... 1lb at 1/2 foot = 1/2 ft/lb

[quote]
I stand corrected. I was thinking in reverse. It would be true at he rear wheel with the chain pulling on the sprocket.

Good writeup dad.

My original point was that by dropping up front or going up at the rear the bike will accelerate faster. To accelerate mass to a constant velocity in less time requires more work. The chain is the means of delivery of the extra work.

 

Hey, I'm an engineer too! Except I'm the civil sort... not nice civil, but civil as in roads and infrastructure... but enough about philosophy!

From the mechanical engineering subject I had to do (8 years ago now... wow I'm feeling old at 25) the concept of torque was described as "force" x "lever arm" and it was also called "moment" when describing a static load.

I know it's dangerous and I'm most likely wrong but I'm going to argue with Dad now... on the basis that I think any change in any drivechain ratios would be transfered through the whole system... from the gas explosions in the pistons to the friction of the tyre on the bitumen. Seeing as how there's this thing called equal and opposite reactions (Newton's something) and there's mechanical interlock... ignoring clutch slip. If you change one ratio, say a sprocket, you are effectively changing the ratio of the whole drive chain and the "work" that the engine has to do to get a given amount of acceleration. In other words, if you have varying setups to get the same acceleration, the engine works harder with a touring setup than with a stunters setup.

Now that I've thought it through like that, the point of Dad's example makes more sense to me... if the engine provides a set amount of force or torque, a change in ratios will mean a change in work which explains why touring setups are slower off the mark than stunt setups.

Dammit my brain just exploded!

This thread about ratios and spreadsheets reminds me of a thread I made about ratios and a spreadsheets. If I was at the right computer I'd upload my spreadsheet again to get a comparison, oh well.

What would be the limit on + teeth on the rear sprocket before the chainguard started being struck?

Edit:
Thanks Dad, it's one of those arguments where no ones wrong, just misunderstood. I think by the end of my post, I had realised that I agreed with you anyways. Always a good thing!

And TimBucTwo, thanks for your input on the engineer scene... I was just refering to the line in the first post about engineer's never being wrong! I wish it was true!

 

I am not an engineer but.....I have stayed in a Holiday Express before.

I hope this clears things up.[sm=smiley23.gif]

 

Triangle, yes A change occurs front to rear in all parts that spin. It's at least RPM but not necessarily the load the parts saw originally. My point was where the load change occurs, the parts that are seeing the additional strain from the increased leverage that the lower gear affords. For each mile travelled, the whole package turns more revolutions, except the rear wheel, but the parts that see a load change at wide open throttle are those that are after the leverage change.

In the case of changing the front sprocket, that means the chain sees the load change because the leverage change occurred at the front sprocket. All elements from the output shaft splines back to the piston dome see no change in their load at wide open throttle. Whereas changing the rear sprocket, the additional leverage is in the sprocket's larger diameter. The chain pulls with the same force as it always did as there is no change to the elements upstream of it. The additional force is accomplished through the rear sprocket's increased leverage.

Any better or just more confusing?

 

Having read all this stuff and needing to replace chain/sprockets before winter hits and having fiddled with gear calculator programs/spreadsheets i've determined to go -1 on front sprocket and +2 on rear. I dont do very much long distance riding - mainly bashing down country roads and a few A roads. I really fancy the extra ooomph - i'm looking forward to it a lot :-)

Thanks

 

dad, I agree. I just could not get it in words. Thanks! A smaller sprocket up front will have a smaller radius and pull on the chain harder. Going up at the rear will give quicker acceleration with the same pull as the stock setup but with higher RPM.

So, to answer Trips original question is to go up at the rear and maybe the larger sprocket radius will give the chain longer life, and it may have more links to spread the wear, but the chain will also have to travel more for a given distance and will wear out faster, off-setting the larger radius. Well at least it wouldn't be prone to stretching the chain.

I think it’s a crap shoot.