Sprocket Size vs. Performance

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I only posted this in this section because I run a LO206 and this is an age old question. Two gear sets, 16/56 and 18/63 both are 3.5 ratios. Anyone want to give their opinions / insight as to relative performance benefits if any?
 
16 is going to spool you up faster coming out of the turn, 18 will give you that oompf further on down the straight away. Its part the gearing game of which one will work better, its all how you want the power to come in for your own setup and driving. Seen people gear theres both ways and be fast.
 
3.5 turns of the 16 will spin the axle once, it will multiply the torque to the axle 3.5 times. 3.5 turns of the 18 will spin the axle once and multiply the torque by 3.5 times. Both the 16 and 18 are on the end of the same crankshaft nobody has ever explained what happens between the front and rear sprocket to change the physics that makes one gear set faster than the other. There is the minute difference in the diameter that the chain travels around the front sprocket that may improve efficiency but I don't believe this is a measurable difference. Some will say try it, we have, we have used #35 and 219 chain, we found no measurable on track performance difference. With your 6100 limiter your 3.5 ratio will spin your axle 1743 times/min. and it will multiply your torque 3.5 times and I don't know how you change the facts of that.
 
Both gear sets have the exact same torque multiplication coming out of the turns. On the top, both gear sets will be going the exact same miles per hour with the same rpm.

I wonder if, on a paved straight line track, like a dragstrip, you would see any difference in elapsed times. The only way to go faster on the top is to turn more RPM. From what I read here, there are just too many variables in the dirt and the tires to compare, (and don't forget the air density) times between practice sessions or practice sessions to the main.

Has anybody compared the air density between their last practice session and the main? If you start racing in the late afternoon, and then the main is after the sun goes down, there's going to be a difference in the air density. If your jetting is perfect in the afternoon, if you don't change it, it's going to be off when the sun goes down. Think about this; if you're a little rich in the afternoon, you might have the perfect ratio when the sun goes down.
 
I agree 100% with both kart43 and Al. The only thing I can add is the possibility that the bigger gears are a little more efficient due to a reduced chain wrap angle but then on the other side its a bit heavier. All the testing we have done over the years we could never find any difference. I think time is better spent in other areas.

Steve
 
S&MRacing said:
16 is going to spool you up faster coming out of the turn, 18 will give you that oompf further on down the straight away. Its part the gearing game of which one will work better, its all how you want the power to come in for your own setup and driving. Seen people gear theres both ways and be fast.
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I used to be a die hard "ratio is a ratio" guy, not any more.
I;ve experimented enough, run fast enough to agree with S&M above.
While it doesn't change your overall lap time noticably, it changes WHERE you are stronger. It's about race-ability, not outright lap times
I have some theories why this happens, but that's all they are, theories.
If you're not already running 99% fast, there are probably bigger cats to skin.
 
I agree that a ratio is a ratio, however...


I think of it akin to using a heavy vs light flywheel.

Yes, there is a difference, and when you would want one over the other.
Momentum (big oval) racing really likes larger drivers up to a point of diminished returns.

Also think of the clutch driver in extremes -- a 2 tooth driver would be like the crankshaft being a screwdriver twisting to pull the chain.
Now change to a 400 tooth driver and you've got the effect of a cheater bar on the end of a long wrench pulling on the chain.
Now consider that the heavier reciprocating weight not only accelerates slower, it decelerates slower due to kinetic energy being released.

Probably not the best explanation, but some things to get your mind thinking. ;)


-----
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
www.CarlsonMotorsports.com
27 years of service to the karting industry
Linden, IN
765-339-4407
bcarlson@CarlsonMotorsports.com
 
CarlsonMotorsports said:
Also think of the clutch driver in extremes -- a 2 tooth driver would be like the crankshaft being a screwdriver twisting to pull the chain.
Now change to a 400 tooth driver and you've got the effect of a cheater bar on the end of a long wrench pulling on the chain.
Now consider that the heavier reciprocating weight not only accelerates slower, it decelerates slower due to kinetic energy being released.


bcarlson@CarlsonMotorsports.com
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The length of the lever at the crank shaft acts upon the lever at the axle. Therefore when you change gear sets the front size affects the rear size, thus remaining the same. A large or small dia driver acts with a small or large driven gear.
 
You may think it's the same, but did you ever try it? I don't think so , because if you did you'd know that there is a difference, because a ratio is a ratio might make sense, but that don't take into consideration the difference in power and friction of roller chain over different size sprockets.
 
At our last race we played with gearing to see what would happen. Now, I can't say if weather, rubber on track etc had any affect but... We switched up from a 17/61(3.59) to an 18/64(3.55) and picked up .3 I wouldn't think .04 in the ratio is going to offer up 3 tenths.

This was a sprint track with more flowing momentum type corners. My thought process was, the bigger driver would keep the energy "spooled up" and help us out. It seems to have worked. Didn't have the chance to try going back to the 17/61 and see what happened.

We will be back on track at a different course this weekend. Gear ratio is similar but has more of a tighter layout in the corners. Starting off with our 18 driver in practice and will drop it back down and see what happens after we get a baseline on the 18.
 
Look, about all I can say is after 5 years of math, physics, and engineering at Purdue, I can tell you that a ratio is a ratio.
Nearly 50 years of racing chain driven karts, I can tell you that there IS a difference on big momentum oval tracks.

There is no contradiction.
The physics aren't incorrect, it's the interpretation of the data that is incorrect.
This stuff can be argued until cows fly over the moon and it still won't settle the debate until you have seen it, tried it, and felt it (seat of the pants) on the track. ;)
 
Kart43 said:
The length of the lever at the crank shaft acts upon the lever at the axle. Therefore when you change gear sets the front size affects the rear size, thus remaining the same. A large or small dia driver acts with a small or large driven gear.
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You seem to understand, if you're going to use a 400 tooth driver, and maintain the same ratio, let's say 4 – 1, then you would need a 1600 tooth "driven" on the axle.

It seems that, sometimes, in these gear ratio discussions, there are people talking about driver sprockets with no consideration for the driven sprockets. You've seen them, people talk about a 16 tooth driver giving you more top end, while a 14 tooth driver gives you more pull out of the corners. Although this would be true if you didn't change the axle sprocket, that's not what the discussion is about. There are times when I have trouble thinking about two things at the same time, maybe that has something to do with it.
 
jack burroughs said:
You may think it's the same, but did you ever try it? I don't think so , because if you did you'd know that there is a difference, because a ratio is a ratio might make sense, but that don't take into consideration the difference in power and friction of roller chain over different size sprockets.
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Did you read my original post? I didn't think so, I clearly stated we have tried it. Now that is for sprint road course type racing. We found no difference, that is using identical ratios, and comparing #35 to 219 chain also. It is evident results vary, and yes I also have many years of post secondary training and study, with a respectable number of years Kart racing.

Thanks Al, it is not that I seem to understand, I have not yet seen anyone put forth a legitimate explanation, and I am willing to look at and consider all explanations.
 
Kart43 said:
Thanks Al, it is not that I seem to understand, I have not yet seen anyone put forth a legitimate explanation, and I am willing to look at and consider all explanations.
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I hear you, we've been discussing this for many years now and I've never heard an explanation of how, or why, this happens. All I hear are a lot of stories about what they did and the results, but never an explanation of why it happens.

I was at a Sprint race once, not racing, just wrenching for somebody, and I had my air density gauge with me. It was an afternoon, early evening race. When the sun went down, the air density went up six points. That's easily adjusted for with a KT or Euro engines, but that would be one jet size plus for a fixed jet carb. Say .038" to a .039" That's just a little over 5% increase in jet bore area.
 
At this point I don't care how it's explained and I normally want to know why things work. But, I know on oval racing, it makes a difference from experience. However it works, I don't care, it just does. More momentum and better grip the karts like bigger drivers.

It all depends on kart setup and available grip and momentum.
 
Someone should just throw a motor on an inertia dyno and end this debate. there are plenty here with dynos it seems. Plot the rotational acceleration vs time. the smaller driver should have a higher accel at the beginning and slow relative to the other towards the end. Another check is if both setups have the same time to complete the same change in rpm. if the theory is correct that is. Till then i don't think you can disprove physics because an uncontrolled environment says so.
 
alvin l nunley said:
You seem to understand, if you're going to use a 400 tooth driver, and maintain the same ratio, let's say 4 – 1, then you would need a 1600 tooth "driven" on the axle.

It seems that, sometimes, in these gear ratio discussions, there are people talking about driver sprockets with no consideration for the driven sprockets. You've seen them, people talk about a 16 tooth driver giving you more top end, while a 14 tooth driver gives you more pull out of the corners. Although this would be true if you didn't change the axle sprocket, that's not what the discussion is about. There are times when I have trouble thinking about two things at the same time, maybe that has something to do with it.
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Al generally when someone says a 14 gives you more pull out of the corner or a 16 gives you more speed at the end of the strait, they are referring to changing the driver gear and driven gear both while keeping the ratio the same. They aren't talking about just changing the driver and leaving the sprocket the same, that would be a huge change in rpm and would usually kill speed on a stock type engine like the 206 or clone engines, especially jumping 2 or more driver sizes like from a 14 to a 16 without changing the driven gear. Just trying to give you some insight on what people mean so you understand better.
 
Anybody who has had high school physics knows that no matter the number of teeth, if the ratio is the same, it is identical. Period. You can argue over perceived gains, minimal changes due to chain friction or minimal change in rotating weight if you want. This is a misconception I have heard for years, and I know the physical laws are not going to keep people from arguing the point. I would however suggest you read the article written by Todd Spade in their TSracing catalog. It may be on their web site as well.
 
I've heard the same thing about coming off a corner better with a smaller driver, but have never personally experimented with it.

I raced bicycles for many years, did many miles. Mostly road racing, but some track (velodrome), cyclocross, MTB and BMX. On a bicycle the thoughts were similar in that a smaller chainring (front sprocket) was easier to accelerate for a given ratio than the same ratio with a larger chainring. Also a larger chainring was easier to keep it rolling once you got it up to speed. Not sure this adds anything to the discussion, but on a bicycle we were only dealing with about 3/4 horsepower motor. So everything was taken into consideration. Personally I can't say that I ever completely bought into this way of thinking, but it's interesting to hear it in karting also.
 
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