TM R2 vs Vortex VTZ vs Iame S4?

For clarity re-paddle shifts:

Paddle shifts are allowed in KZ, assuming the link that operates the shift is mechanical. Seamless shifting mechanisms (ergo, a system which does not require an ignition cut to change gear) is forbidden. Other forms of electro/pneumatic actuating shifting mechanisms are also forbidden.

KZ2, you only get the trusty metal stick between your legs to play with (Any innuendo is purely your interpretation :stuck_out_tongue_closed_eyes: )


I feel like paddle shifting in a KZ wouldn’t really add to the experience. If we were talking something with 65+ hp and stickier tires then paddle shifting would perhaps be more of a necessity, and quite badass. But I feel like the current “metal stick” approach is suited quite well to the existing platform.

Ohh you have me thinking

Single cylinder 250 with an electronic powervalve
6" wheels
Paddle shifter with flat shift

That would be just bonkers. I’m luckily to be fit enough that I can wheel a KZ without feeling like it’s trying to kill me, but what you described would be a “hold on for dear life” scenario. Sign me up :smiling_imp:

I remember seeing something years ago about a class in the UK, something like 250 national. Used 250 2 stroke bike engines on sprint style kart, though I think some were built special for this class. I believe they used more reclined seats compared to standard sprint karts. Be quite a gnarly ride I’d assume.

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I think that’s still a thing. @Alan_Dove did some coverage on it in the last year or so.

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One of my Superkart colleagues in his 250 (I think this is his International rather than National, but he has both). They still in general use a mechanical stick shift but there are some examples of electronic paddles (which are allowed under our open class rules, just rare). Six inch tyres, not five. When I’m doing just over 110 mph down the main straight at Phillip Island in my Stock Honda and these guys go past me as if I’m standing still, it does give you some read on just how insane they are (kart and driver). As for their cornering abilities, another world.

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The history goes back to the early era of British karting. iirc there wasn’t a push against adopting the new wave of 250 engines coming from outside the UK and they wanted to stay with 200, but it came anyway. Anyway, the fundamental of the class is 250 single-cylinder road-derived bike engines. This goes back to the 60s/70s. IN the last decade or so there was an influx in ‘bespoke’ engines approved for use within Motorsport UK competition. Reason being the supply of engines for the class was no longer enough to support the class properly. the CR250 is still widely raced though.

The class is raced both on short and long circuits.

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Simone, is it forbidden for me to change gear in a KZ without lifting off the accelerator? I can reliably flatshift a TM without too much abuse, and my jailbreak starts with a 175 rely on slamming 3-4 and 4-5 without lifting.

As long as you maintain all homologated components pertaining to the gearbox and do not have a dedicated mechanism that acts upon the ignition and geartrain with the purpose of allowing a shift to take place without lifting off the throttle, you are technically allowed to shift gears “without lifting off the accelerator”.

How are you managing that on stock homologated components though? The entire transmission requires the torque demand to cease so that the forks can shift gears. How are you able to shift under load?

It’s definitely possible in the higher gears, just requires a strong pull and is most certainly harder on components. I don’t like doing it personally.

Act like you’ve decided the chain is on too tight.

Using the gear lever, provide enough force to try to slide the engine back to reduce the chain tension. Time it so that the engine has revved up enough to fall off the pipe when you do this.

This is easy on a CR125, harder on a KZ.

Charles is likely speaking of my TM R2 he has at the moment. The gearbox was treated with Isotropic Superfinishing. (a mechanical/chemical surface finishing process) Final surface finish was approx. 6 RA.

I can’t say it’s faster as I didn’t bother doing any benchmark testing before processing, however it will indeed flat shift without much protest.

The initial goal was simply to reduce the amount of metal coming out during oil changes during the first few hours of running. However as per usual there was still a bit.

Also worth noting, we had the gearbox analyzed by a partner company (they specialize in gearbox design and manufacturing for the military aerospace industry) to see if we could find any improvements, however short of completely remanufacturing the entire gearset, there’s no real interference to remove between the addendum and dedendum, there’s actually some backlash.

I’d be curious your thoughts on polished/treated gears, as I’m unconvinced. I’ll continue to do it to my own engines because I have access to all the equipment/facilities, and it can’t hurt, however I’d have a hard time telling someone they need to spend their money on it.


Thanks a lot for the inputs regarding the analysis you conducted. Considering the regulatory stability for such a long time, i suppose everyone has converged towards a pretty well developed geartrain. I also understand why the removal of material or modification of the cogwheels themselves doesn’t make much sense. I would suppose the design goal is to seek the smallest tolerances between the primary and secondary gears, with the lowest coefficient of friction possible, which is where most of the benefits of treating the gearbox comes from.

To your questions specifically, and as a driver myself, the only noticeable differences in shift behavior between a stock gearbox and a treated one is the relative smoothness of the shifts, which is augmented with a treated one, and the amount of force required for the shift to take place. I have also noted that there is a scale to this, with some tuners having better feeling gearboxes than others.

Nevertheless, i have never encountered one that allowed a flat shift to take place (i must admit i haven’t necessarily tried either…)

I suppose you could theoretically make a case for a faster shift with a treated gearbox, just because it comes that much “easier”.

I would be very interested to know if there is any inherent tangible performance benefit though, through a test bench application, although i am skeptical about it at this stage.

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Curious how this compares to the factory “special” option. Can you share who treated the gearbox on your engine? I’ve considered the same process for the same reasons you mentioned, but the price tag and inconvenience of sending off to someone is not especially appealing.

I haven’t pulled apart a factory polished box, I started with a standard gear set.

Unfortunately it’s not a vendor who would entertain non-production work, and was done after hours so I can’t share. I never actually sought out the service, it all stemmed from a conversation I had with a representative of a well regarded engine builder who when asked about what type of processing they were doing to the factory polished gearboxes gave me an answer that sounded like the type of thing someone would say who didn’t know what they were talking about, but wanted you to think they did. I had called this friend of mine as he’s familiar with the actual engineering behind gearboxes, and he confirmed what I was told was a load of shit as suspected, and he offered to take a look.


Although it’s not difficult, it certainly doesn’t feel like you’re doing “the right thing” when you force it. That said, for how little you actually need to crack the throttle to shift without any resistance, I don’t see the benefits in flat shifting unless it were using the seamless concept T-Kart had highlighted.

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I wonder if anyone has done REM treatment on a KZ gearbox? Supposed to be superior to normal polishing.

Isotropic Superfinishing more or less is REM Polishing. One is a trade name to my understanding, but the process is functionally the same.

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My AK-USA gear linkage doesn’t let me flat shift your R2 - too flexible to give me confidence - so my statement is about the old K9Bs I’ve driven.

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