Swift Clutch Stall Speed 5000 RPM

My son runs in the micro class and we got our first tech DQ (hooray!). I understand the technical side of what may be happening in the engine by having different engagement RPMs and the powerband, etc. I guess what I don’t get is why the magic number for this is 5000RPM? And what advantage did we have if our clutch stalled at 5100 RPM? Any driver that is getting tech’d for this is keeping their lower end RPMs much higher than this anyways, so am I safe to assume the clutch never disengages if we have a good run? I could potentially see a minor advantage on the start (would have to check to see what RPM they are at then), but even that is questionable. What am I missing? Why are all others 6000?

SKUSA rule:
402.5 Clutch: OEM as per engine manufacturer PDF. Clutch engagement must not exceed 6000 RPM (Swift 5000 RPM). Slip must not be adjustable

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Not sure why Swift is different than other classes for spec RPM, but slipping the clutch higher will definitely help on the start and can help yank the kart off the corners if it’s at a higher RPM. Not uncommon for people to spritz a little WD or Tri-Flow on the clutch drum to get it to slip higher and get a boost in the low-speed sections or the start, which is illegal. We used to have to do clutch tests before every session in Komet at bigger races.

At 5-6k it’s going to be locked up all race, so yes, the idea is that these low-speed clutches somewhat replicate direct-drive characteristics but allow the kart to idle and start on the grid without a bump-start.

Thanks TJ, I have heard of all the clutch greasing. I think it is a little ridiculous. I learned my lesson on this one, I will be checking before qualy and all following sessions. I was a bit upset as it was a rental and thought I didn’t need to worry about it. It really didn’t matter much though. After the dq, we started in 10th, but were able to get to the front passing a lot of fast kids. In the end, it looks like going from 5100 down to 3800 doesn’t matter much. Still wondering why 5000RPM…there was a ton of meat left on those shoes.

I have yet to see it done in tech, but how is the stall speed checked? Rev the kart on the stand holding the brakes until the clutch grabs?

That’s what the rule book states but we always had a small metal bar we had to roll over on the grid while the tech official monitored the revs.

They used this method for one of the sessions at USPKS Orlando this year. If I recall correctly, the tech officials had their own rpm reader that they clipped onto the coil wire.

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Interesting. This does not seem straight forward to tech at track. In a control environment with the engine on a dyno maybe, but the fact that OP got DQ’d over 100rpm seems like any easy thing to argue. We are talking an accuracy of +/-2% if they said it stalled at 5100 and not 5000rpm.

I didn’t argue the accuracy of the measurement as I was looking at our mychron which was also somewhere in the 5100 range. Race director stated that tech dq was not protestable.

They used some kind of device where the put it in the vicinity of the plug wire. The one other time we had this test done, they used the clip on tach. Both times they give her a couple of revs and pull on both pedals at the same time and monitor the RPM. They did let the engine cool for 10 minutes or so saying that sometimes the heat affects the measurement (it did not in our case).

What a tough lesson to learn over 100rpm. At least they tired to work with you on it.

We have been karting for 2.5 years and my son and I learned so much last race. And I am still learning! Like right at this very moment I discovered this rule -

109.5 Technical Protests: Any protest involving technical items must be submitted prior to the component involved being removed from the Technical Inspection area.

I could have protested…

Its tough to remember what you can and can’t do per the rules. Every series has is own list of rules.

I just always question the ability to measure things accurately at the race track. I understand that everyone is being measured the same way and it’s your responsibility, but I would never think to stop by tech and have them double check my clutch stall speed.

Had to protest a squish measurement once. The tech was using old no name calipers and trying to DQ me over 2 thousands. I had to go grab a calibrated micrometer and remeasure. Even the battery starting to die a small amount will drift caliper measurements of cheap units.

That’s why I question the accuracy of the tachometer or Mychron, I doubt it’s accuracy is +/-2%.