I know there is several posts here about gear ratios and how to do the maths, which I sort of know how to do, but I don’t understand how to interpret the numbers.
I’m running a KZ 125 gearbox superkart with a setup of 20t engine side and 19t on the axle. This setup is fine for my local track, but I’m planning to race at Phillip Island, a 4.4k track in Victoria which has a main straight of 900m.
It won’t be possible for me to swap and try different sprockets, I want to arrive at the track with the correctly calculated gear ratios and sprocket sizes installed.
Can anyone help or point me towards a document/spreadsheet
That’s a tall ratio. What KZ engine do you have, the ratios vary.
TM which has the “fun” upgrade, taking it to 144cc
Which model of TM? They have different internal ratios depending on what you have.
Here’s a sheet for the older models. Make a copy first from the file menu, then make changes as needed to tire circumference, shift RPM and overrev RPM as needed.
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Sorry James, should have been more specific, TM10
Thanks
You could calculate different ratios by dividing rear sprocket with the engine sprocket.
20/19=1.05 which is very tall.
For example at Kristianstad in Sweden which has a main straight of 170m I use 26/18 which give me 1.44 in ratio. This is with a TM KZ10C
For 900m your current gearing should work, how long is the main straight on your local track?
Thx Lucas.
As I said, it’s the number ratio I find difficult to interpret. If I change a sprocket, the number ratio will change to something else.
But how does a ratio convert into a reduction or increase in top speed or acceleration and what’s the reasoning behind it.
In an earlier response above James provided a spreadsheet which does calculate top speeds using different sprockets but doesn’t tell me
-
where top speed is achieved relative to the length of the main straight, or
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what effect do the different ratios have when pulling out of a corner
Perhaps I’m asking the impossible
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Without having some prior data, it’s not reasonable to expect to show up to a new circuit and have the gearing perfect. You need reference numbers for roll speed in critical corners, as well as top speed on the straights with a reference ratio. It’s really just a fine-tuning item that you’ll need to test when at the track. Run a session, change gearing, run another session, adjust accordingly.
Generally you can estimate pretty closely, and expect to make ratio changes in increments of ~0.3. For example, I may start with a 1.50 and find that I’m tapped out before the end of the straight, in which case I would adjust down to a 1.47 (taller ratio). This will of course affect gearing throughout the entire track, so you’ll need to manage the balance between gearing correctly for the corners and gearing for the end of the longest straightaway.
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.