How does sprocket size effect performance?

timeconsuming

timeconsuming

Foolish until proven intelligent.
Hello gentlemen

My chain has been loosening itself a bit as of late and things are generally pretty worn (no clue when [if] the previous owner ever updating the sprockets and chain) so I was thinking I need an overhaul.

New front sprocket and rear sprocket, along with a new chain.

I believe on my bike ('75 cb400f) the stock sizing is 17t front, 39t rear...

I've noticed first gear is relatively short, and I top out at about 90 on the freeway.

What I'm curious to know, as a layman, is how altering the sizing of either the front sprocket, rear sprocket, or both, would effect the performance of my bike, both on the low and top ends...

Thanks for the wisdom!
 
It's all about gear ratios. Take the sizes you listed (17 front, 39 rear) as an example. Divide 39 by 17 and you get around 2.29(rounded off). A higher gear ratio (as in greater than 2.29) will give you the feeling of more torque, but usually at the cost of top end speed. A lower gear ratio (less than 2.29) will give you the feeling of less pulling power and possibly higher top speed.
 
Oops - let me clarify some things. There's a reason I used the words "usually" and "possibly".

Just about any bike's top speed is dictated not by it's gearing ratio, but by its aerodynamic drag (wind resistance). So if yourtop speed right now is 90mph with tthose sprockets, and say you bump the rear up to 40 teeth, it doesn't necessarily mean your top speed will be 85mph or anything. That little bump in gearing might give you enough usable power to go 95mph instead.
 
What are you looknig for? More bottom end or more top end? Going down in the front or up in the rear will increase your bottom end at the cost of reducing your top end. Also, going down 1 tooth in the front is like going up almost 3 in the rear (Exact ratio is about 2.75).
 
Like Big Rich said, though. The CB400f has about the same HP as the CB360t. I am not gear limited on top speed as much as HP limited.

Unless you fully fair the bike, you aren't going much faster.

If you put a gear ration that is designed for 110 MPH, you may find you run out of steam at a lower speed then with the numerically higher gearing.

So at 90MPH, how many RPM are turning? what is the redline? If you are at redline, maybe a numerical reduction in ration may give another MPH or 2. However, if you aren't into the red, you may need a numerically higher ration to allow the engine to be in a better spot.

Your best bet is to get a dyno run and see where your HP and Torque peak. Then you can see if you are HP limited on speed or just RPM.

Aerodynamic have a large effect on your top speed on a stock, unfaired bike, You are pretty much driving a parachute. So HP is probably limiting you...
 
Also - it's easier to change the front sprocket than the rear if you want to experiment with different gear ratios. If you are buying new chain and sprockets you might want to grab another different sized front to have a play around with.
 
Thanks guys, definitely lending some clarity.

Basically I wouldn't mind having a bit more juice on the top end, but I also wouldn't mind being able to get up to a little more speed in first gear before having to shift to second, as it seems pretty short as is.

I'm not sure if that's something that is attainable simultaneously, but I'm hoping so that giving myself a smaller rear sprocket might do the trick.

Also I think I had a typo, as I'm pretty sure the stock sizing is 38t-17t. Making my ratio about 2.24.

My thoughts as of now are to drop sizes down to 34t-16t. Making my ratio about 2.13.

Is that too drastic of a change? Not sure how much the decimals will provide a noticeable difference in performance.

Curious to hear your opinions!
 
mydlyfkryzis said:
Like Big Rich said, though. The CB400f has about the same HP as the CB360t. I am not gear limited on top speed as much as HP limited.

Unless you fully fair the bike, you aren't going much faster.

If you put a gear ration that is designed for 110 MPH, you may find you run out of steam at a lower speed then with the numerically higher gearing.

So at 90MPH, how many RPM are turning? what is the redline? If you are at redline, maybe a numerical reduction in ration may give another MPH or 2. However, if you aren't into the red, you may need a numerically higher ration to allow the engine to be in a better spot.

Your best bet is to get a dyno run and see where your HP and Torque peak. Then you can see if you are HP limited on speed or just RPM.

Aerodynamic have a large effect on your top speed on a stock, unfaired bike, You are pretty much driving a parachute. So HP is probably limiting you...
Click to expand...

Redline on my bike is 10k, and I'm hitting my topspeed of about 90 (on flat ground, though the speedo goes to 120 I believe) just at or under 8k, given my tach is functioning properly, though when I'm pushing it like that, it tends to rattle and bounce a bit.
 
So then given that line of logic I might be better off heading in the opposite direction, 16t and possibly a larger rear, 39 or bigger?
 
I'm going to go with Hillsy - do the front sprocket first.

Both sprockets and the chain should be replaced as a set (as I'm sure you know). But when experimenting with gearing changes, order an extra / different sprocket as well. That way you can change it out if you'd like after 20 miles or so.
 
This video is a bit lengthy, but it's a pretty clear explanation. The first 16 minutes are basic sprocket and ratio theory. The last 6 minutes are more real world examples and also how to calculate the a theoretical new top speed based on the old to new ratios.

http://youtu.be/O35AwCp-6b8
 
If your bike only manages to hit 90 mph at just under 8,000 rpm, it should do around 103 at 10,000. The fact that it doesn't do that suggests that the motor is down on power or one of the dials is inaccurate. A good CB400f is overgeared stock and runs to a higher top speed with lower gearing (higher ratio).

I'd strap it to a dyno and see where it peaks and how much power it makes.
 
Things are kinda all over the place, but listening to what your goal is, you should go up one tooth on the front sprocket. That will give you a taller first gear, which is what it sounds like you want.

8,000 RPM at top speed is probably just about right. Going up one tooth on the front sprocket is likely to make your top speed a little slower in your top gear. So, drop down a gear and get your R's back up there to the peak. A lot of bikes are geared for a more leisurely RPM when in top gear, not maximum top speed. Many bikes that I have had will go faster on the top end when I drop down a gear, because as was said, the limit is not topping out engine RPM, the limit is horsepower vs. drag.

So, you may like the slightly taller gearing for a little longer first gear, and lower RPM at highway speed. You can still get that top speed by dropping down a gear.
 
As already stated, change the front sprocket to play with ratios initially, then fine tune with rear if you really think its necessary. This is a really handy site for playing around with ratio's
Don't forget that rear tire OD plays a big roll as well... Generally guys will try putting a wider tire on a stock rim, which effectively only makes a taller tire, not wider one. This site accounts for this as well.

http://www.gearingcommander.com/
 
I think this is going to be one of these combination things. You have an older bike, it could be down on power a bit. I don't know how big you are, but at 210lbs. I can slow a bike a bit as my aerodynamic shape isn't optimal, and your clocks could be off a hair. You have to remember back in the day, they used small guys to test these bikes and a 400f is pretty small. I think there is a video around where a guy bought a junk Kawasaki and took it to Bonneville, his clocks said he was doing approx. 130m.p.h. but his official slip was 118m.p.h.

A lot of bikes had a top speed of 100 or less. and that's including some pretty legendary bikes. So, if you're say 130lbs. with a 24" waist and chest, and your bike is at it's optimal performance, I think the Honda 400f had a top speed of 114 m.p.h. for 37 h.p. I think that's pretty stout... I have a FJ600 that supposed has a top end of 135, with my fat ass on 30 year old bike, I get a little over 110m.p.h.
 
Probably the easiest way to know if your bike is geared properly is how it accelerates in top gear with you riding it... Ideally, you want to be able to reach the redline but not too quickly, if that makes any sense?
I prefer not to have to downshift out of top gear to climb a grade while maintaining speed as well...
 
Thanks guys. Perhaps the easiest solution is to spend the extra $ to grab a 16t and an 18t front sprocket, allowing for a little bit of experimentation.

I do notice that sometimes I have to downshift to fifth from sixth when climbing a steep hill on the freeway, in order to maintain decent rpms.

Also I'm relatively small and light (145lbs) but my bike could be down on power. Really need to replace some gaskets because my top case is leaking a bit of oil onto the fins, and I'm pretty certain my carbs may be out of sync... all god damn four of em.

Thanks for all the insight guys, you're an invaluable resource!
 
timeconsuming said:
I do notice that sometimes I have to downshift to fifth from sixth when climbing a steep hill on the freeway, in order to maintain decent rpms.
Click to expand...

It's a 400 - you're asking a bit much of it here IMO.
 
timeconsuming said:
I do notice that sometimes I have to downshift to fifth from sixth when climbing a steep hill on the freeway, in order to maintain decent rpms.
Click to expand...

Totally normal.
Sixth gear would be a tad tall, and you will probably get a few MPH more top speed in 5th than in 6th. Just about any small displacement bike I have ridden was that way.
 
hillsy said:
It's a 400 - you're asking a bit much of it here IMO.
Click to expand...


Yeah I definitely don't baby it. I have been considering saving up some dough for a commuting machine so I don't have to keep punishing my bike everyday.
 
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