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?
James,
Gingerman: Top speed = 86 MPH, Geared at 16/75
Gratton: Top speed = 90 MPH, Geared at 16/73
Blackhawk: Top speed = 90 MPH, Geared at 16/71
Road America: Tops speed = 95 MPH, Geared at 16/66
Real data from Race Studio
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Thanks guys for the input and comments.
I will try my own model John and see if it works.
Are you using mathlab or similar? I was just planning to use Excel and a discrete model with time step of 0.5s (Alfano allows you to download CSV).
The main key thing I am still struggling is the calculation of the acceleration, as it is a function of the power curve of the motor (I have the theoretical as a function of RPM) but can’t find the flow of inputs to extract and put that into the model (sorry for my very low level physics modeling, not proud of… used to be really good at phsyics in uni but it seems I am getting too old now…)
BTW, another quick question. For this track, the lowest speed in the couple of runs I did is 75 kph, 80% of the time I am doing 85+kph and maxed at 125kph because of the gear. As per the longest gear of DD2 and the manual, I think we could hit 135 kph.
Do you think that I should look at aero efficiency of some sort?
For another road circuit baseline, our Rotax Max drivers at Phillip Island use a 16/68 and hit 158 km/h (98 mph) at the end of a very long main straight into T1. They are also using a road circuit/superkart front nose cone. In this example the driver is just over 90 kg (200 lbs).
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