Direct Drive Gearing Theory

[Flatspot]

We race Rotax and I have some questions on gearing that I would love to get your input. It concerns gearing differences, if any, based on the countershaft versus final drive gear.

Assuming the overall gear ratio is the same, what advantages or disadvantages are there in adjusting through the countershaft gear versus your final drive gear? Example: If you have a 11/77 or a 12/84 set-up the overall ratio is 1:7. Theoretically the same but are they really? Are there advantages to one over the other. Is the centrifigul clutch in play here?

Any input you have here is appreciated.

[Maxkarter]

Other than the one having a much smaller chance of loosing sprocket and chain over a kerb, they arent exactly the same. What I mean is if you check on a calculator the difference is something like x.xxxxx1233 which really small. TBH, I can't expalin this properly...

[speedy king]

Too complicated for me

[jjordan]

If you don't mind a Yankee dirt tracker muddying the water: always believed that the bigger the front driver the slower it will accel/deccel, the gear/chain combo has more inertia than the smaller front driver. The smaller front driver will accelerate quicker, decel quicker. but will carry less momentum. This may not be a factor in the tracks you run but on our dirt ovals it really depends on the track shape: ie, some have short straights and big corners while some have long straights and tight corners so momentum/enertia is more of a factor. In this type of racing getting a good start is critical so we always try and get away with the smallest front driver we can, as long as the engine doesn't go to the top(or beyond) of it's power band halfway down the straight. A factor for us is that the rear gear cannot get too big or it gets outside the chain guard, also the really scientific among us would probably weigh the different gear/chain combo's to find the one that weighs the least (overkill?). We don't want the engine to go through it's powerband too quickly but PULL through the powerband to build kart speed, reduce the chances of wheel spin.

[Ali Rushforth]

Shouldn't make any difference really. As someone said, the only real difference is the chance of losing a chain/sprocket if it is too large.

A difference may/may not be found, if using to combo which uses the shortest chain?

[jjordan]

Yea, the simple answer is:it depends. Depends on your track, (on driving style?), the only real test is the stopwatch, try them both back to back. Sorry to complicate it, it's all theory till you put it against the watch.

[Flatspot]

Thanks for the input. In the past I have been under the same impression as jjordan with regard to the small driver up front gives some acceleration benefits. Particularly in the form of grabing revs quicker. This may be important in Rotax as getting the kart up off of the corners is in my opinion the single most important thing posting quick lap times in a Rotax (well any kart for that matter). The issue of keeping the sproket off of the kerbs is important too. I hate mechanical DNF's more than just about anything as they are usually preventable.

Next step, as jj points out, is to get it against the clock. I am going to do some testing and if there is a discrernable difference I will post the methodology and results.

Thanks again!!!!

[jjordan]

What's neat about your racing is that if you have a track with significant straights, you could set up your timing in sections and and see what effect it has on the twisty bits vs. straights. But then again, I am probably over thinking this. The mind is a terrible thing..........

[Alex Hodgkinson]

Shouldn't make any difference, whatever gearing you're using to obtain a certain ratio as the engine will have to do the same ammount of work to move the rear tyres the same ammount..

[Flatspot]

jjordan: What's neat about your racing is that if you have a track with significant straights, you could set up your timing in sections and and see what effect it has on the twisty bits vs. straights. But then again, I am probably over thinking this. The mind is a terrible thing..........

You think you're overthinking this thing? .....

[jjordan]

....what do you think?

[tkj17]

Another thing to consider is the tension in the chain. For the same torque at the driving wheel (or power at a same engine revs) the chain tension or force in the chain will be higher with the smaller sprocket and gear, due to the shorter lever arm. The smaller set thus would likely wear the chain and sprockets a little faster than the large set.

Though I would run with the smaller set for less risk of a chain kerb incident.

The advantage of the lower rotational inertia of the smaller set would be very very small indeed. Think about the amount of weight difference compared to the weight of other items on the engine side and the wheel side.

Anyway my 5 cents worth.

[Revracing]

Running different drive/driven gear combinations but keeping the overall ratio the same can in some instances make a noticeable difference.

Over the years we used to play arround with this alot and found (in some cases) depending on track conditions as much as 0.5 seconds a lap improvement.

The gains come from the inertial losses in the gear train system (resistance to movement).

Basically the lower the inertia the greater the power at the wheels. It is difficult to predict and is influenced by the condition of the axle bearings and sprocket and chain wear.

So have a play, you could be pleasantly surprised.

[philneast]

A couple of years ago KartOz did some tests on a dyno with different front/rear sprocket combinations.

They found that the larger gear sets were more efficient, ie more power measured at the wheels for the same overall gear ratio.

The explanation related to the greater amount of turning the chain has to do around the smaller gears.

[Alex Hodgkinson]

Originally Posted by philneast

A couple of years ago KartOz did some tests on a dyno with different front/rear sprocket combinations.

They found that the larger gear sets were more efficient, ie more power measured at the wheels for the same overall gear ratio.

The explanation related to the greater amount of turning the chain has to do around the smaller gears.

Surely the ammount of energy you're saving is minimal though.

[jjordan]

Minimal? You bet! Our Stock Briggs 4-strokes are all about minimal hp. You are fighting for hundredths, every little advantage. I always thought that was part of the appeal. The drag racer Sneaky Pete Robinson said it best, When asked why he spent hours milling shavings off of a valve train component he said: "see those shavings on the floor? They aren't on the ceiling."

[Flatspot]

Well we finally got to put this question to the acid test and I can't beleive nor do I understand the result. We were using an alternative configuration on our new track and the conventional wisdom going around the paddock was a 12 front and 72 rear sprocket (exactly a 1:6 ratio). With out telling the driver we swithched to a 13 front 78 rear which is also exactly a 1:6 ratio. We made absolutely no other changes to the kart, tempurature/density altitude/everything was steady. On average he was .7 seconds per lap faster. That's right .7 per lap. We couldn't beleive it so we changed back, again without telling the driver and he ran an average .7 slower. Funny thing is he came in complaining that his engine had just quit pulling. We told him that we had adjusted his carb and we would reset it as it was. Back on with the 13/78, he was happy again and again .7 faster. I don't get it but I wont knock it.

A racing guru friend said it saps off power to wrap the chain tighter. Doesn't seem logical to me but nevertheless we had a great weekend. The funniest thing is that a neighboring team sent a 13 year old spy over who saw the 78 on the rear. They immediately changed to a 78 just before qualifying but left the 12 on front. 2.5 seconds off the pace they were. Who would have thought?

[speedy king]

Wow, thats interesting. Will look at working mine out

[jjordan]

Flatspot, that just blows me away. I was always convinced that there was a difference but 1) have not taken the time to test (middle of our season) and, 2) on an oval the difference would be a lot smaller and might not show up as clear cut as your test. Good work, and thanks for sharing your info so unselfishly.