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Hi Everyone, 

Just got back into Karting, Running a Rotax.  I would conisder myself Rotax Heavy, as I push about 99kg.

I'm having a strange issue, and I seen a golf Thread (4years now), about a guy who had a similar problem.

Is the same Gear ratio really the same?!?!

11-77=7.000

12-84=7.000

Kart struggles to rev out, and pop's and splatters at the higher RPM, when running 11-77.

Any explanation for this ?

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A ratio is a ratio is a ratio, doesn't matter how it's achieved. 

There might be a tiny bit more lost power when wrapping the chain around the smaller sprockets. More link movement as the chain wraps around a tighter radius will require more power to overcome friction, lost as waste heat (i.e. chain will get warmer, maybe). Minimal would be overstating it IMO. If any significant affect existed (I doubt), it would cause a very slight loss of power, not "pops and splatters". I think your problem lies elsewhere. 

Regards,

John.

For what it's worth I agree entirely with John. The bigger the back sprocket the less drag there is on the front sprocket, thus a mick hairs power plus.As your ratio's are the same the problem is elsewhere.

Hi, 

i have had this debate  my self. I think even though the final ratio is the same it is the way the power gets there.The torque will be different as different rpm, so it depends at what rpm your engine delivers its max  torque and horsepower.

Ian,

An emphatic no.

There is no magic here. It's not about torque vs power at whatever rpm (please lets not go to torque vs power). It's a constant ratio regardless of rpm or whether it's a smaller pair of sprockets vs a larger pair of sprockets (assuming the same numeric ratio). It's all about mechanical advantage (leverage). A sprocket pair will have a degree of leverage defined by the larger number of teeth divided by the smaller number of teeth, and if the result of this is X then the leverage will be X regardless of the sizes of the sprocket pair (or gears, or whatever, assuming identical ratio).

For there to be any difference in the "way" torque / power is transferred from the crank to the axle (with differing sizes of sprocket pairs of the same numeric ratio), there would need to be some 'slippage' in the system, and there just isn't any. Same for gears and toothed belts as for sprockets and chains. 

In any case, even allowing the fanciful notion that there might be some affect on "the way the power gets there", this still would not explain the manifestation of this problem. That is, it doesn't explain why the engine "pops and splatters", rather than just suffers from some form of power / torque 'modification' (which could be either a decrease or an increase in this alternate universe). "Pops and splatters" will be related to how the engine is running, not how it is transferring its' power to the axle...

Regards,

John.

I agree with the equal ratio gives equal outcome theory,  but that is purely in regards the ultimate speed obtained.  Given a 7:1 ratio regardless of what size sprockets are used will give you a rear axle rotational speed of 2,000rpm for an engine running at 14,000rpm.  Hence for the same ratios the "speed" will not vary.

And I also agree with .....  "Pops and splatters" will be related to how the engine is running, not how it is transferring its' power to the axle...

But there are two things to consider.  In engineering design practice it is considered that a toothed sprocket driven by a roller chain should be a minimum of 13 teeth in size.  This simply is allowing for the chain links to wrap around the teeth in a most optimal way , any smaller size and the extra bend required of every link is wasting energy and causing more wear on the chain and sprocket.  Of course in karts we almost always need to use sprockets in the 9 to 12 tooth range simply to get the gearing we require.  But its worth a thought as regards the choice of an 11 tooth versus 12 tooth sprocket.


Then next consider the torque applied by the chain pulling on a sprocket of a given diameter.  Ignoring the gear ratio or the numbers of teeth on either of the sprockets and simply seeing the drive of the engine being transmitted via a lever arm at the axle that is longer or shorter ( ie an 84 versus a 77 tooth sprocket ) it has to be said that the larger sprocket must deliver a better result to the rear wheels.  Any one of us has used a spanner that is 200mm long on a nut and we could not budge it,  but add a 300mm long tube on the end of the spanner and we could !!

The engine may produce 20hp and 10ft/lbs of torque,  and regardless of front sprocket size this is what the engine is delivering thru the chain to the rear axle.  So if you supply that to a rear sprocket that is 50mm bigger or smaller in diameter, then you are applying it thru a lever arm that is 25mm longer or shorter.  That just has to make a difference of some kind ?? 

I have always aspired to the idea that so long as I can obtain the required ratio , the larger rear sprocket I can use the better the outcome because the larger front sprocket has to be better.  But I have never had anyone address the rear sprocket used as a varying sized lever arm idea.

Anyone ???   

The number of teeth determines the diameter of front and rear sprockets and the diameter determines the number of teeth.

The circumference of a circle is Pi x Diameter - double the diameter and you double the the circumference and number of teeth - increase front diameter and teeth by 10% then you need to increase the rear by 10% to get same ratio

Ian's question on "lever arm" - When the front diameter is increased by 10% (for the same ratio) the rear diameter is also increased by 10% and no change in leverage - on a seesaw 50kg at 1 meter will hold 100kg at 2 meters

On the track use the biggest front sprocket you can without too many kerb strikes and grounding of the LARGER rear sprocket

In the old days clubman usually ran a 10 tooth front, but a 9 tooth at the old short Coffs Harbour track otherwise the rear would be too big - 9 tooth did rub the shaft of most sprockets just inside the teeth

So, in fewer words, Smithy you agree 12/84 is preferable to 11/77. I spannered for a mate at Coff's over 40's for several years in Clubman Heavy.We always reduced the front to a 9 because a 10 was about a 90 rear.

On second thoughts - does anybody check what is posted ? - on a seesaw 50kg at 2 meters will hold 100kg at 1 meter

Yeah. 50kg at 2 metres will nicely balance 100kg at 1 metre.

The way that I see this is that as already explained, 77/11 exactly equals 84/12 for transmitting the engine power to the rear axle. Since power is equal to torque times RPM, both result in the same outcome.

What changes is that the larger sprockets result in a slightly lower pull on the chain.

As can be inferred from this, a slightly lower pull (load, tension - whatever way you care to express it) using a slightly larger radius on the sprocket gives exactly the same result.

The benefit of the slightly larger sprocket, i.e. a slightly reduced load on the chain and sprocket teeth is likely to result in slightly longer chain and sprocket life, all else being equal.

But with every benefit, there is bound to be a down side. In this case, it is the slightly higher velocity of the chain, in particular when it goes round the engine sprocket. You see, the links and pins of the chain would much prefer not to go round the sprocket at all, but instead would rather go straight ahead. It cannot however, as its mates have already turned the corner, and would very much like the rest of the chain to do likewise. For that matter, so would the driver.

A phenomenon we know as centrifugal force comes into play. In this case, it can be calculated by either of the formulae of mass multiplied by velocity squared, all divided by the radius of the engine sprocket, or alternatively by mass multiplied by the radius of the engine sprocket all multiplied by the angular velocity squared of the engine sprocket.

So we now have some energy sapping additional loads applied by the forces of nature.

Whether this is better or worse than the situation of the chain waltzing abruptly around a smaller sprocket, I do not know.

I have seen suggestions that larger sprockets are better at lower revs, and smaller are better at higher revs. Again, I know not.

I do know that an engine that peaks out at 16,000RPM will use a larger sprocket than one that peaks out at 20,000RPM. But I believe that has more to do with the size of the rear sprocket. Something about keeping the axle sprocket off the track and ripple strips.

I have also observed that no one at the top of their game uses sprockets any larger than they need to be.

12/84 would be my choice. Being on the heavy side I always found the motors torque range worked better with a bigger front sprocket. Why and how??? Back to if it would stop the popping its a rotax and it could be a number of things. Gearing won't change the problem.

Hi Guys, 

Thanks for all the in-depth Answers!!

But the original post I found, and went looking for, is because everything else has been IRONED OUT !

-New Fuel Filter

-New Fuel Lines

-New Plug

-Correct Jetting for that location and weather.

-Top End Li-Po Battery, kept on trickle always.

I use a Digital Ultrasonic Cleaner for the Carb and Power Valve, after every track meet.

The original Post I found states:-

"My usual setup of 12/84 works very well, and with the adjustment of plus or minus a tooth on the rear, it works well in most situations.

However, as we know, running large rear sprockets like these, we run the inherant risk of the sprocket comming into contact with curbs if we run wide.

 

To mitigate this risk, I have fitted an 11/77 setup which essentially equals the same final ratio of 7.00 but using a smaller rear sprocket.

This all looks great on paper- on track she's no good.

On track it has the same or similar performance bottom to midrange pickup, but the top end is all but gone!

Just as she would really start to scream, it would start popping/ missing and the power curve would drop off sharply.

It was pulling the same RPM range (13200-13400).

Obviously I went up in jet size to try and remedy the lean feeling top end and popping, but to no avail.

I ended going up four jet sizes, in the end it solved the top end, but then it really felt boggy pick up.

I eventually went bach to my initial jet setting and gearing of 12/84....... and what would you know, running perfectly again! - I really cant figure out why the different setups with the same ratio behave so differently.

If any one has experienced a similar scenario, or can understand the potential problems and soultions, I would like to hear."

All the explanations I have read, don't explain the issue here.  The guy goes back to original 12/84 and usual JET size, and problem goes away.

I'm having the issue this guy is having, 11-77 Pops and Splatters at top end.  Everything else is in order.

Next time I go out, at Todd Road, going to Run a 12-84, and see if this issue goes away.  If it persists, then I'll be digging elsewhere.



Ian Williams said:

"But there are two things to consider..." etc. 

We agree that the 'pops and splatters' have nothing to do with the gearing. 

I agree that the larger the engine sprocket the better, almost purely due to the mechanical issues related to wrapping a chain around a small sprocket. These are primarily to do with wear and tear on the sprocket and the chain, though there is a very very minor issue of parasitic losses (if it were a big problem then the chain would get very hot over a number of laps, maybe even burn the grease off). We do tend to be limited in engine sprocket size (to smaller than ideal) because we don't want the axle sprocket coming into contact with the track, so using a smaller engine sprocket to achieve X ratio is purely a practical consideration IMO. If a larger engine sprocket can be used for a given ratio, then that is theoretically better, though I expect it wouldn’t show up in lap time (never having actually tested this).

"Then next consider the torque applied by the chain pulling on a sprocket of a given diameter.  Ignoring the gear ratio or the numbers of teeth on either of the sprockets and simply seeing the drive of the engine being transmitted via a lever arm at the axle that is longer or shorter ( ie an 84 versus a 77 tooth sprocket ) it has to be said that the larger sprocket must deliver a better result to the rear wheels.  Any one of us has used a spanner that is 200mm long on a nut and we could not budge it,  but add a 300mm long tube on the end of the spanner and we could !!"

Preface all I say here with ‘to the best of my understanding’. I make no claims to actual expertise with this.

Yes, I think it's helpful to think in terms of lever arms created by the sprocket diameters (or more specifically, the radius). A sprocket with X teeth will be half the diameter of a sprocket with 2xX teeth, so there is a direct correlation (I checked with an online sprocket size calculator, just to double check my assumption). But your analogy is ignoring the front sprocket, and we can’t consider the rear sprocket in isolation because both sprockets are equally important.

"The engine may produce 20hp and 10ft/lbs of torque,  and regardless of front sprocket size this is what the engine is delivering thru the chain to the rear axle.  So if you supply that to a rear sprocket that is 50mm bigger or smaller in diameter, then you are applying it thru a lever arm that is 25mm longer or shorter.  That just has to make a difference of some kind ??"

No, it’s not regardless of the front sprocket size. The size of the engine sprocket does substantially affect how hard it can pull on the chain (converting torque into linear force). The larger the engine sprocket the longer its’ ‘lever arm’ and the less mechanical advantage it has, and the less hard it can ‘pull’, so less ‘torque’ is transferred through the chain to the axle sprocket. But, this is reversed at the axle sprocket, the larger the sprocket the longer the lever arm, and the more mechanical advantage it has (to convert linear force into torque). The result is that for a given ratio (no matter the combination of sprocket sizes used to achieve it) at any given engine rpm the torque seen at the axle will not change. With both sprockets the length of the lever arm has a big affect, but it’s an oppositely opposing affect at each sprocket.

Imagine two cases where the ratio is 1 to 1 with both the engine and axle sprockets being the same size. Arbitrarily let’s say case one uses two sprockets that have 20 teeth. If the engine sprocket rotates one turn then the axle sprocket rotates one turn, obviously. If case two uses say 40 teeth on both sprockets, then one rotation of the engine sprocket still turns the axle sprocket one rotation (so no change in kart speed at any given engine rpm). In case two (40 teeth) the chain will experience only half the linear force as it will in case one (20 teeth), i.e. in case two the force exerted at the edge of the larger diameter engine sprocket is half that of the force exerted at the edge of the smaller sprocket of case one. There is a difference here, at any given engine rpm the chain will be moving faster with the larger sprockets.

Regardless of whether the pair of sprockets have 20 teeth or 40, no torque is lost or gained at the axle because the affect of the longer and shorter ‘lever arms’ is reversed at the rear axle. Even though force acting through the chain is reduced by half with the larger sprocket sets, it is multiplied x2 by the length of the lever arm at the rear sprocket, so zero net change of torque seen at the rear axle. This is true at all engine and axle rpm, and is still the case if the ratio is not 1 to 1, so long as the ratio is the same.

You mentioned power (“hp”). This is also (almost) unaffected at the rear axle because power is a product of torque X rpm (i.e. torque X time, which of course is the arbitrary fixed time factor of ‘Minute’ in R.P.M.). If the ratio is the same, then no matter the combination of sprocket sizes that create that ratio, the torque and rpm seen at the rear axle will be the same at any given engine rpm. There will no affect on acceleration of the kart because time is a constant (don’t tell Einstein I said that, I mean it’s a constant for all practical purposes related to this issue).

This begs a question, if our larger 40 tooth engine sprocket can only pull half as hard on the chain as our 20 tooth sprocket, then force acting on the chain is only half with the larger sprocket. What then happens to the unaccounted for force while it’s still ‘in’ the chain? Some of the force becomes ‘power’, i.e. it can no longer be measured in an instantaneous moment of time (as torque can be). With the larger sprocket pair the chain must move more quickly at any given engine or axle rpm. There is less force ‘in’ the chain, but it is acting more ‘quickly’ in a given time period. When this ‘power’ arrives at the axle sprocket, some of it is ‘converted’ back into torque. The net result comes out in the wash, i.e. torque and power is still the same at the axle so long as the ratio is not changed, no matter how that ratio is achieved. Simple (reaches for the aspirin...).

But what about torque convertors in automatic transmissions I hear someone asking? It’s not quite the same. The convertor does multiply torque, but some power is ‘lost’ due to turbulence in the fluid inside the convertor. Since power is energy, and energy can “neither be created’ or destroyed”, some ‘escapes’ as waste heat (caused by friction between fluid molecules). The same thing happens to the kart chain, some energy is ‘lost’ as waste heat due to friction in the chain links and rubbing contact with the sprockets, and the faster the chain moves the more energy is lost in this way. Chain drives are very efficient (more so than gears), and I very much doubt it’s a significant amount of loss, unlike with a torque convertor (or a slipping clutch for that matter).

Regards,

John.

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