Front Sprocket Size

[yellowesty]

I've been running a 15 tooth front sprocket on an '08 GSXR. On track I never use 1st gear and don't use 2nd very often (probably should use it more often). Reading as much as I can about driveline efficiency, it seems like I might see a bit more real-world, rear-wheel power if I moved to an 18 tooth front sprocket and utilized more 1st and 2nd gear on track. The issue is NOT overall gearing -- it's driveline efficiency.

Does anyone have any experience with this issue? Opinion? Or even (gasp) data?

Thanks.

[formulasuper]

Your sprocket ratio really depends on what track you're running on. At Rd Atlanta I run a 16/47 ratio with 22 X 9 rear tires, however at a shorter track like Barber I wouldn't even get into 6th gear, barely able to use 5th.

[Daryl DeArman]

I believe what he's asking is that if he can produce the desired overall gear ratio is there a power to the rear wheels benefit to the larger primary sprocket?

Random thoughts about this:

If you run your chain drive ratio tall enough that you are now using 1-4th instead of 3-6th you now will have larger splits between each gear as the gear ratios are closer in the higher gears.

Higher chain speed at same road speed should consume more power. In other words a 30 tooth rear sprocket and a 45 tooth rear sprocket at 140mph are tuning the same RPMS but the 45 tooth has a much higher chain speed and more chain links. How much difference accelerating and decelerating that chain to higher speeds consumes...I have not a clue.

That larger primary sprocket results in a larger radius for the chain to go around which should reduce friction. Is it enough to offset the losses from higher chain speed and more chain mass? Again, no surprise, I have not a clue.

Going with a larger sprocket allows you to get closer to optimum rear sprocket size than a small sprocket will. In other words with a small sprocket a change of +/-1T on the rear makes a bigger difference than that +/-1T change would with a larger front sprocket.

The only semi-real data I have is that larger drive sprockets last longer than small diameter ones. That's got to provide a clue as to their efficiency improvements. Is it enough to overcome the larger gear ratio splits? Let us know when you find out

[Northwind]

I'm not sure chasing efficiency with front sprocket size variation would be as critical as getting the proper gearing for the key corners that will give you the highest corner exit speeds. I usually will gear for top gear to be near peak rpms at the fastest part of the track. I may adjust off of this to get the car laying down the best power in the critical corners. My experience and data has shown a 1 tooth difference on a rear sprocket is approximately 4.5mph change at top gear max rpm. 1 tooth on the front sprocket is approximate to a 10 mph change at top gear max rpm.

[Daryl DeArman]

I'm not sure chasing efficiency with front sprocket size variation would be as critical as getting the proper gearing for the key corners that will give you the highest corner exit speeds. I usually will gear for top gear to be near peak rpms at the fastest part of the track.

When you take this common approach are you finding that you use only 3rd-6th gears at your tracks?

If so, and you chose to gear so that 4th was your top gear (used 1-4th while racing) do you think there would be gains to be made in gear train efficiency?

I'm thinking the wider splits would offset any drive train efficiencies, never mind not having 2nd gear available for a slow paced start.

[Northwind]

The tracks I run, Blackhawk, Road America, I get down to 2nd gear and run up to 6th. Brainerd has the slowest corner in North America from what I'm told, and you have to drop down in mid revs in the F1000 first gear. It is slow. BIR also has a very long straight that nearly equals Road America top speeds. So all the gears get used to full range. I don't have a track I run on that has very little speed variation. If we want to add another dimension to this thread about sprocket size and friction. Let's also factor in the additional rotating mass caused by larger sprockets and the additional chain required to go around them. Added weight to the car too if you cannot achieve minimum weight. So there are a lot of variables that come into play for absolute performance. Interesting question from the OP and we'll see if anybody that has hard facts chimes in to enlighten us.

[yellowesty]

Thanks to all for your thoughts on this -- so far. I'm still not sure how to go forward.

The only tracks I run regularly are Sears Point and Thunderhill. My current rear sprocket (50T) gives me a bit over 13k RPM at the end of the main straight at TH. That's as fast as I ever go (my power peak is at 12k).

It's true that (at least on a GSXR1000) the higher gears are closer than the lower gears -- but not by much. The ratio of ratios (lower gear divided by higher gear) goes 1.248, 1.197, 1.143, 1.103, 1.072. So adjacent ratios of ratios differ from one another by no more than 5% (going down to 3%). More important, if I plot rear wheel torque (ignoring driveline losses) as a function of road speed for various front sprockets (keeping the same rear sprocket), the curves lie on top of each other except at speeds below 55 mph where, if I were to use 1st gear with a 15T front sprocket (which I don't), I would (for a very short time) generate over 1000 ft-lbs of torque at the rear wheel.

I'm not comfortable saying that a higher chain speed at the same road speed consumes more power. It certainly has more kinetic energy, which it did acquire by consuming power, and which it must lose via braking (engine or wheel), but the chain is not very heavy and the chain speed difference is 18/15=1.2 or less, so it's not a big effect. If I could pick up 2hp with a larger front sprocket, it would overwhelm the power needed to accelerate the chain by an additional 20%.

The sense I'm getting from the list, however, is that the power losses from driving a 15T front sprocket (vs. an 18T) may be minimal. I can believe that -- though all application notes from chain and chain drive manufacturers say that large sprockets are more efficient than small ones. But they DON'T say by how much!

Has anyone tried running different front sprockets when dyno testing engines?

Inquiring minds want to know.

[racerdad2]

Can't help with dyno #'s. However, from karting experience, running the same ratios, the larger front sprocket has proven quicker by a tenth or two depending on the track.

You may have to spend a test day changing sprockets to see what works best for you.

Happy hunting !

[Daryl DeArman]

Given your current engine choice and you hitting 13K with a 15/50 in 6th, changing to a 18T front you will not reach 12K in 5th at the same speed, so now you are adding teeth to the rear sprocket------do you have any clearance concerns?

[Stan Clayton]

Can't help with dyno #'s. However, from karting experience, running the same ratios, the larger front sprocket has proven quicker by a tenth or two depending on the track.

You may have to spend a test day changing sprockets to see what works best for you.

Happy hunting !

I can believe this. Although the lower gears one would use are slightly further apart than the taller gears, on tracks that emphasize acceleration off the corners rather than raw mph speed, the higher torque multiplication of the lower gears might actually make a measureable difference.

[ReynardF1000]

Another way to view the splits between the lower ratios and higher ones is to divide the split percentages. Therefore with 5% splits in lower gears and 3% splits in the upper gears, the splits are closer by 40%. This is substantial!
Stay with the 15 tooth driver and make small changes to the rear. Try one less tooth in the rear for starters. This will provide a small reduction in maximum RPM's at the end of the straight(s) at TH. Look to your times, back-to-back for true improvements.

[yellowesty]

Mea culpa: I shouldn't have gotten this discussion derailed into "splits" and "splits of splits."

My concern (at the moment) isn't overall gearing, it's efficient transfer of power from the countershaft to the rear axle.

So a better way to restate my original question may be:

Does using a 14T front sprocket (and the appropriate rear sprocket) waste more power than using a 18T front sprocket (and the appropriate, but obviously larger) rear sprocket? For the moment, let's ignore clearance issues and inertia issues and just focus on power transfer.

If the chain has parasitic power loss because it bends around sprockets, one might expect that smaller sprockets (with tighter bending) would cause greater power loss.

Do they? And is it significant (i.e., > 1%) ?

[Daryl DeArman]

Does using a 14T front sprocket (and the appropriate rear sprocket) waste more power than using a 18T front sprocket (and the appropriate, but obviously larger) rear sprocket? For the moment, let's ignore clearance issues and inertia issues and just focus on power transfer.

If the chain has parasitic power loss because it bends around sprockets, one might expect that smaller sprockets (with tighter bending) would cause greater power loss.

Do they? And is it significant (i.e., > 1%) ?

Yes and I don't know.

You need a redneck chain sprocket dyno.

Mount two parallel shafts on some pillow blocks with decent bearings.

Mount up your current 15T/50T combo. Install a chain, measure the chain tension. Spin it up with a drill motor until the drill speed is at max rpms, release the drill motor and time how long it takes to coast down to a stop.

Repeat a few times to account for heat in the chain and bearings until you can get some degree of repeatability.

Now do the same thing with a18T/60T combo.

Compare the results.

You put in the time and effort, you can decide whether or not to share your findings.

[Blah]

From engineering my daughter in karts, we went down this track trying less teeth on the front sprocket to reduce drag, without boring the *^R& out of you, the larger circumference provided by the higher tooth sprocket enabled us to deliver the power from the engine to the rear wheels with less loss of hp and a better lap time. Sounds a bit unusual but I have 2 years of data to quantify the choice.

[Brett Lane]

Your sprocket ratio really depends on what track you're running on. At Rd Atlanta I run a 16/47 ratio with 22 X 9 rear tires, however at a shorter track like Barber I wouldn't even get into 6th gear, barely able to use 5th.

Scott is running just under 3:1(2.93). I run a 15/45, which is 3:1, and works at most tracks. Going to a 16 tooth front sprocket would be good when using a 47 or 48 tooth rear, depending on the track. Those two should cover just about anywhere except Road America or Daytona...

[yellowesty]

I posted the same question (but worded better) on the DSR Forum. Here's a response posted by Hasty Horn which includes (gasp) data:

Back in the days when Yamaha first started building shaft drive bikes we took some interest in recording crankshaft HP compared to rear wheel HP for both chain drive engines and shaft drive engines. Our chief interest was in knowing how much the shaft drive was costing us in frictional loses vs. drive chain losses. In the process we changed ratios of chain drive many times and did not see any appreciable difference from ratio to ratio on chain drive engines...we never went to extremes and our "rule of thumb" was never to go below 14 tooth on the countershaft sprocket and preferably higher for chain durability.

For us the interesting thing was that chain drive (and all the other internals) had a frictional loss of about 17% (with new sprockets and well lubed chain vs. the 20% loss we were looking at for shaft drive. HOWEVER, the shaft drive got more efficient the more we ran it and the hotter the system (and the lubes) got. To the point that the shaft drive would fall to 19% and the chain drive percentage ALWAYS grew....sometimes up to and past 19% (with a really nasty dry chain and waaaay too loose).

I hope that answers your question but I must say that our measuring tools were pretty crude (except for the million dollar Horiba dyno lab that collected all the exhaust gas in giant tubes for analysis) and theoretically a high turnover countershaft setup should be less efficient. The only reason one should use less than a 14 on the countershaft is if there is an unavoidable clearance problem.

Yamaha's initial drive shaft components were all built by Getrag the German company that builds BMW's driveshaft pieces. The in-line fours with drive shafts had one more right turn then the comparable BMW boxer twin.

Hasty Horn

[ReynardF1000]

Chain drag increases (significantly and life is reduced) by the number of degrees angle a chain bends around its smallest radius during the circuit of its path. Small sprockets create higher chain angles and more power loss.
However, measuring the loss difference will be very difficult for most of us. I like the xneck test noted above. Measure the current drawn on the power leads with an Amprobe for some degree of accuracy. Also, note that friction increases with the square of speed.
As aero drag increases with the cube of speed, it's so very difficult to gain real advantages... However, I do make a point of using the largest sprockets possible, dictated by the largest sprocket on the engine.

[Daryl DeArman]

You need a redneck chain sprocket dyno.

I like the xneck test noted above.

For future reference; as a descendent of a poor farming family I find neither redneck nor okie offensive. I apologize to anybody offended. Please accept the term in the spirit it was intended; a compliment. I'm quite proud of the work my grandparents and great grandparents did to survive and the ingenuity that their means demanded they possess.



As to the crude dyno mentioned above, I would suggest that an improvement might be to mount both the 15/50 and the 18/60 and just swap the chain as to minimize the inertia variable as much as possible.

[Zcurves]

I have been pondering this very question. I can only offer this bit of data. I run 17/47 and 17/48 and lately the 17 is the stock sprocket. I have found that it is more durable than the Vortex. Plus they usually come with the engine. I haven't compared them on a set of scales but I doubt there's a significant weight difference. Anyway, I'll probably get my hand slapped but I get 12-15 races out of a good quality chain (with no catastrophic failures).

I think any performance gain would be in the weight reduction of a shorter chain. How many links can I reduce if I go to a 16/44? I may be wrong but a five link reduction doesn't sound like much. I'll have to pull out the scales this weekend and find the reduction percentage.

[Daryl DeArman]

I have been pondering this very question. I can only offer this bit of data. I run 17/47 and 17/48 and lately the 17 is the stock sprocket. I have found that it is more durable than the Vortex. Plus they usually come with the engine. I haven't compared them on a set of scales but I doubt there's a significant weight difference. Anyway, I'll probably get my hand slapped but I get 12-15 races out of a good quality chain (with no catastrophic failures).

I think any performance gain would be in the weight reduction of a shorter chain. How many links can I reduce if I go to a 16/44? I may be wrong but a five link reduction doesn't sound like much. I'll have to pull out the scales this weekend and find the reduction percentage.

You are probably looking more like 3 links.

If your original chain was 100 links for example, given the same centerlines you are now going to need 97 links, assuming using stock chain pitch. That's a 3% reduction in chain weight.

Additionally you are going to save about 15% in the weight of the sprockets as the rear sprocket diameter is about 3/4" smaller.