Here’s an easy but accurate way to look at it - 10% bigger rear sprocket means 10% more umph at low speed at 10% less top speed. So for me in the 206 and a rear gear of 58 teeth, each tooth I add is about 2% more torque and 2% less top speed. No excel sheets needed. But as said above, you can’t expect to just show up with the right gear unless you get knowledge from someone who’s been there with the same setup
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It is going to be hard to gauge before you get there. The other thing to consider of course is that at some point your kart loses its battle with the atmosphere. I think 119MPH is the most I’ve seen with CIK bodywork, in a draft.
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Thank you, thats all I want to do, get pretty close to a workable ratio before I drive 3,555.3 km to the track with the kart on my trailer
I take it you’re driving from Perth. You should probably reach out to the Victorian Superkart Club and ask some questions of those who have run a stock KZ at Phillip Island. The track is brutal on engines, particularly as you’re at WOT for over 45 seconds per lap. I usually run a Stock Honda there using a CR125 and running Elf Race 102. I’m pretty sure on a 19T engine sprocket and a 23T rear hitting at just over 180 km/h into T1. I do use a front long circuit nosecone through. The prevailing wind, particularly the predominate southerly up the main straight, will make a big difference to what the engine will pull.
I haven’t run a Stock KZ but have tested my 175 Supershifter. In that case I was on a 20T front and 21T rear. I have however had carby and air box issues with a standard sprint front end, so something to think about. My next outing there I will be running the main jet leaner - pretty sure I’m copping a rich bog in G6 with the airbox inlets now facing backwards. More testing required.
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Yes from Perth, almost a week’s daylight driving. I have contacted VSC and hopefully they’ll respond. I also use a long circuit nosecone.
I will be running for the first time, a 38mm SmartCarb so I’m hoping for a bit more reliability.
I purchased my Superkart from a guy in Melbourne and had it shipped to Perth.
Holy 
You sir are very committed. Please keep us posted. For sure if there is anyone else at the event that has gearing info I would defer to that. If it happens for be for a different engine you can probably use the sheet I shared to extrapolate a similar gearing.
I’m still not 100% sure what engine you have, but here is the full list of sheets. If your model is not listed let me know and I’ll create a new one with the ratios from the homolgation sheet.
This thread inspired me to see how simple of a model I could make that is still detailed enough to be useful, and here is what I came up with:
Inputs:
- engine power curve
- gear ratio
- drag coefficient
- kart mass
- braking decel
- list of track “straights” including their distances, start speeds (prior corner exit) and end speeds (corner entry speed at the end of the respective section). For this, I used GoPro and poor memory, so consider it to be a fictional track for now
Assumptions:
- negligible friction
- constant braking Gs
- assumes that cornering speed is the same (or slower if you can’t reach the cornering speed) for all gear ratios
The result for my 206 and the fictional track:
The bottom line this exercise taught me is that, near the best gear ratio, this track is not terrible sensitive to being off a tooth or two. I also learned that the optimal gear ratio is heavily dependent on what I input as the drag coefficient and the cornering speeds. There can also be competitive reasons to go with a tooth or two away from optimal - for example, the “optimal” gear ratio can leave you as a sitting duck at the end of the straight. It might be worth it to go with a higher top speed to defend at the end of the straight, even those this might give a theoretically higher lap time by the tiniest of margins.
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This is worthy of its own topic. Nice work.
I probably have some friction models from my roller dyno. What kind of format or units would that need to be in. There’s some aero stuff on the forums too for CD and front area etc.
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I got my drag coefficient from fitting my own data, but the reality is that there is a ton of noise/uncertainty. Just changes in body position, air density, headwind, guy in front of you, etc, all make a bigger difference than getting the second digit on drag correct.
For friction, it just gets dwarfed by drag, so there’s no added benefit in including it, especially since it also is very subject to a lot of variables like tire temp and pressure and track temp/grip.
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Good points well made. Better to just stick to know values without variables sounds like.
It is useful to see what variables are more sensitive than others, and a model can sometimes teach you that far quicker than laps can (I can’t just add 10% drag and see what happens in real life). Recently, for example, I ran laps and there was a strong headwind on the back straight, and I noticed I was struggling to hit the limiter like usual. My corner exit speed was good, acceleration was good, just had no top end. With what I see in my model, the headwind could be entirely to blame, and I could’ve added two teeth to make up for it. I also cannot hit the rev limiter in my model like I do in real life unless I also account for the elevation change at the end of the straight. That small drop makes a large difference in such an underpowered class, so even elevation change can influence gearing selection.
So, to the topic of this whole thread, it is very hard to answer “I have a straight this long and a kart this fast, what’s the gearing I need” even with intimate knowledge of the track. Looking at the model does help the second question of this thread though (what do gear ratios really mean). That one is far easier to address than which ratio is best.
Thanks for providing this exercise to the karting community, that’s awesome.
The Phillip Island track plays host to the Australia MotoGP each year. The main straight is almost a kilometre long, a destroyer of little KZ10 125cc two stroke engines. If the ratios are not accurate to start with, your first practice lap could be your last.
I’m not in it to win races, for me it’s about equipment reliability and participation.
Thanks James, your spreadsheets did include my TM KZ10 125 with “Fun 56” upgrade
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Fantastic.
I created this post to try and inform/educate myself in the complexities of gear ratios. Installation of the correct gears at my workshop before I set off was paramount. I don’t want to be spending my time stripping down kart trackside to change sprockets So, before heading off on my long journey across Australia the calculations need to be mathematically achieved to find the optimal ratio.
That does simplify things a lot. James says the highest he’s seen is 120mph, so you’ll want a gear ratio such that you won’t exceed max rpm at 120mph, whatever max rpm is for longevity.
For example, let’s say max = 10,000rpm (just for this example), and you normally hit that at 100mph with your normal gear ratio, and let’s say that’s with 19T rear gear. Now you want a rear gear = 120/100x19 = 22.8 which I’d round up to 23. 23T would put you at 10,000x23/22.8 = 10,088rpm @ 120mph.
Just swap all the numbers out with what you think fits best.
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That model looks sick!
I will be racing next week at the Rotax Challenge in Canada in a racecar track and I was trying to “theoretically” see what would be the best gear.
I have done a couple of runs with the gears that I have and I definitely feel maxed out, and have to change gear but then the question is, to what gears should I change…
With my supposedly sound mechanical engineering background I was dreaming od doing somehting like what you are showing up!
Would you be open to share???
The overall concept is very simple; I think it is a good balance of detail without requiring an insane number of inputs. Basically, I break a track into “straights” using google maps where a “straight” is any zone you are accelerating. For the turns, I give it a max entry speed and an exit speed. The exit speed for a turn becomes V0 for the next straight. It accelerates according to power, drag, and mass. At each point, it calculates the breaking distance for the next turn and sees if it can accelerate more or slow down.
Then it just loops through the track. There are still some areas of the code I’m not happy with, but I might share it if I get everything dialed in.
Gearing for full size tracks tends to not vary a lot because you’re basically racing to top speed of around 95-100MPH.
@Tony_Z can you recall what gearing you ran with the Rotax in road racing?
WUT? I don’t see how you obtained this figure, regardless of the order of precedence of the operations. Is there something being left out?