206 on a 2-Stroke Chassis - Seat Stays

Chassis Setup, Tires & Handling Forum
#1

I recently bought my first kart - a used Praga Dragon with a 206. From what I read (and please correct me if this is wrong) the big reason for a different chassis for a 2-stroke vs. 4-stroke is that the 2-stroke chassis tend to be a bit stiffer due to the higher speeds. Along these lines, I looked at each of the Briggs classes this weekend at my local track (GoPro, now Trackhouse), and I did not see a single one running with seat stays, and I was told this was also because the stays make it too stiff.

I bought my kart with a Jecko D7 seat, and I recently did my first morning with a C5 plus two seat stays going to the bearings (one per side, the previous two mornings were in the original seat and no stays). The kart rotated SO much more readily, I looped twice on my warmup lap. Once I acclimated though, I did my best times yet by about 5 seconds which really doesn’t mean much. Without the stays and with the larger seat, I could feel the chassis flexing aft of the seat, a sort of roll axis oscillation after catching a slide. Predictably, that went entirely away with the new seat and stays. Overall though, once I got past the initial lap, I really thought it felt good

So now my question: is this just a case of a negative times a negative making a positive? The two negatives being a bad driver in a bad kart working together better than said bad driver in a better set up kart? Or is there something I am missing? It seems other 206 karts run seat stays at other tracks when I google it, so is this just a GoPro-specific thing? If so, why? I find it very strange that, if it is stiffer due to being a 2-stroke chassis (30mm tubing, 50mm OD axle), why did it feel like a wet noodle? I am much smaller than the original owner, and I am not running lead yet, so if anything I’d expect it to be overly stiff as I received it, but it certainly hasn’t felt that way.

My best guess is that, with me being 30lbs under weight, I need more ‘help’ transferring load to the outside rear, and the stays help me do just that. But then again, the Jr class is my weight and also did not have anyone running stays from what I could tell. The bottom line is that I liked how it felt, and I am nowhere consistent enough yet to play with tuning. I only included them in the first place since I thought they were supposed to be used for a baseline, so I plan on keeping this as my baseline for a while. Still, I want to understand what’s going on here.

PS: This is somewhat a continuation of this older thread: Masters [email protected], CHASSIS HANDLING ISSUE - #8 by CrocIndy

#2

When your sessions have 5 seconds of variation, you’re not nearly at the limit yet to where the fine tuning of seat stays is going to mean much to you. If the kart is more stable and easier to drive with struts on, keep running them until you develop the consistency needed to really assess the handling differences.

Generally you probably don’t need them in a 206, but there are multiple ways to skin the cat.

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#3

Give me a little credit :joy: the 5 seconds of improvement stuck, so the actual variation between sessions this last day was actually much tighter. Each session, I dropped about half a second consistently, so it isn’t like I have a 5 second swing each way or anything that wild. I really think a lot of the improvement came from just reading up on here what it takes to make a kart turn. Body english and shoulder position were big improvements, especially now that I am not flopping around in the seat. But yeah, I do totally agree that I still need to keep what I have got a for a while longer before I try tuning anything. The big reason I ask is that a lot of my improvements have come from just gaining an understanding of what the kart wants and/or doesn’t like.

I’m an engineer by birth and then by trade as a result. I will always want a fundamental understanding of something, so that’s all I am going for here. Purely a thought exercise at this point. I just can’t figure out why it would feel so bouncy without the stays when I am putting less weight on it. My times are on par with the previous owner now, but he was heavier and therefore loading it harder. Is it normal to set these up to be ‘boingy’?

#4

Sorry, wasn’t trying to discredit too hard! Just noting that you likely aren’t at the limit of the kart yet if you’re still finding .5 from session to session, so the kart isn’t going to function as it would if you were driving at the limit.

Think of the chassis whole chassis as a spring. With the struts on, you stiffen up the relationship between the rear section of the kart, so it won’t flex as much. Removing the struts helps the kart flex at the expense of actually transferring load into the tire and generating side bite. You want the kart to be springy enough to be reactive and responsive to your inputs, but not so springy that it becomes unstable and unpredictable.

Driving a kart is a bit like driving a downforce car in the respect that if you aren’t at the limit and pushing hard, the vehicle isn’t going to behave properly. A downforce car needs you to attack the high-speed sections to generate downforce and grip. The faster you go, the faster you CAN go. A kart requires you to be driving hard enough and inputting enough force that you are unloading the inside rear wheel in the corner, so the kart can rotate. If you aren’t driving it hard enough, that inside rear won’t release and the kart will never function correctly.

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#5

This is one of the MANY lessons I have learned from the keyboard (largely from your posts, so thank you!) that has contributed to a lot of improvement. I’ve even used the downforce analogy with my wife a couple days ago, haha. One of the big excuses for getting myself a kart was because I was at the track anyways with my 4yo and wanted to learn more for helping her. And it looked fun. She is currently fighting this exact issue - she doesn’t get enough heat in the tires to make good grip, and then she leans into a tight corner harder and harder when it doesn’t turn. It seems so simple to say “just go faster”, haha. The promise of a chocolate bunny did result in her getting a PR though…

This concept of the kart being unhappy goes back to that “double negative” concept, but I don’t think I am terribly far off now. I would’ve qualified top ten out of thirty two days later if I compare my best times on Thursday to the race on Saturday. Obviously it isn’t an exact comparison and there are tons of variables, but the weather and time-of-day at least were comparable. I’m 30lbs light, so I am not claiming to be top ten pace by any means, but at least the kart can’t be that far out of its element, I hope. The are only one or two turns that I feel like I was getting right, and most of them I find myself getting on the throttle before the apex, so supposedly these means I am over-slowing. In other words, there is a lot of potential time to be gained from removing very obvious driver errors before the kart becomes the problem.

I guess here’s an easy question - am I right or possibly right in my assumption that a chassis should act stiffer with a light driver than with a heavier driver? Treating it like a spring tells me the oscillations I felt would only get worse, but it isn’t quite that simple.

#6

To put it concisely - because I am both underslowing and underweight, I need the extra stays to transfer enough weight across the rear to the outside. As I pick up pace and add lead, I’ll need the stays less and less

Sound right?

#7

You should almost always be on throttle before apex, especially in a 206. But it’s very likely you are over-slowing, that’s a super common thing in the beginning and a 206 requires almost very little braking anyway, especially at a fast flowing track like GoPro.

That’s quite possible. With less ballast up high in the CoG, you aren’t able to flex the chassis as hard as a heavier driver would so you won’t have the same flex properties. As a small driver, I almost always run seat struts, but I know many guys taller than me who never do.

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