Let's talk about Seat Position

(James McMahon) #63

How is the kart actually handling and how do the tires look. You don’t always have to obviously visually lift the wheel, rather you’re looking to unload it.

Like anything other stopwatch has final say. Broadly speaking you add caster or front track width for entry to apex and control the unloading from apex to exit with the rear track width.

(TJ Koyen) #64

Tend to agree with James. What tire are you on? I’m assuming something harder if you’re running an STVK. On a harder tire, a kart isn’t going to jack weight like it will on a softer tire. If your weight distribution is as it should be, and it sounds like it is, then I wouldn’t worry too much if your pace is fine.

On the harder tire you’re not necessarily going to see it truly lift the inside rear, sometimes just getting the tire light enough that it isn’t scrubbing is all you can do. But if you need more jacking, a stiffer front bar or adding caster will both achieve that. It’s pretty much guaranteed to lift if you add caster and stiffen that front bar.

(Liam Sergeant) #65

That sounds like the seat is set quite low for someone so small? I’m not giving advice, more looking at what people think. I’ve seen kart set ups whilst I’ve been researching that had small drivers spaced way up high.

I think James and TJ are providing great actual advice though. I’m just learning and find different approaches interesting.

(Kevin Pitta) #66

I’m curious about that too. I probably should raise the seat some more, but how do I know when it’s too high?

(James McMahon) #67

You’ll find your driver will be chasing the rear as the inside wheel unloads excessively, perhaps even to the point that the outside tire isn’t sitting square on the track.

Before you change anything, be clear on the problem you want to fix? Is it on exit, or entry. Perhaps both. But either way, figure that out and make changes accordingly.

Also… are you sure the chassis is square?

(Matthijs Hofman) #68

Since it’s been quiet for three days in this thread, I dare to ask a new question.

In my experience, normally placing your seat forward means more grip to the front wheels, means less understeer. But I read somewhere that Australian racer Col Fink from Kracer mentioned that (in certain cases?) you should do the opposite: put the seat back when experiencing understeer and the other way around.

The basis of this theory (out of my head, because I cannot find the original article) is: when you are understeering you already ask too much from your front tyres, so by placing the seat forward you put even more pressure on the front tyres making the understeer worse.

Can you guys enlighten me when Col’s theory works? Or is my experienced skewed from spending too much time in rental karts? Normally the only thing you can adjust in a rental kart is the seat position, so that’s why the answer to this question is so essential for rental karting.

(Eric Gunderson) #69

I think the best way to figure this out is to test, ideally with a tire temp data logger…move the seat back and forth and note how the tires heat up and fade over multiple tests to see if there is a certain ‘tipping point’ where you simply exceed the load limit of the tire.

It makes sense from a race car standpoint…but when you factor in that the chassis ‘hikes’ and flexes, I’m a bit more skeptical. However,_ the fact that karts can hop or bind on corner exit does suggest that there is a lot more at play with all of these factors than we may understand…or rather than I currently understand.

My suspicion is that this might be a possibility with heavier drivers in particular, or with taller drivers, as they tend to ‘roll’ the chassis more around the front tires.

The more I think about this, the more intrigued I am to hear other’s thoughts!

(James McMahon) #70

It’s like any adjusment, you just reach a point of diminishing returns. Of you’re already close to that point before making the adjustment… then it might make the situation you’re trying to remedy, worse

(Marin Vujcich) #71

Been meaning to reply to this. I have experienced this in my kart when pissing around with extreme set positions when I was in the “blaming the kart” phase of my learning. The rationale is that a kart centre of rotation at the start of the corner entry is very much near the rear axle until extreme slip angles of the rear tire move it forward until the kart “takes a set”, albeit this set can be almost non existent in very short corners. In simple terms the front needs to swing across/move side ways. If you have more weight further forward the side load on the tires can be somewhat significantly higher and overload them. More to the middle of the corner having the weight further forward affects the dynamic jacking of the kart. Always think of extremes, what would happen if you could mount the seat on the front axle centre line? Pretty obvious it would affect the jacking in the rear massively, but the effect is the same just at a smaller degree. So 2 very different mechanisms that can be proven mathematically, but give the same understeer affect. When I found this out I had to do some numbers to get my head around it. That is also when I discovered how complex the mechanics of a kart is. Race cars are simple in comparison!

(Matthijs Hofman) #72

Thank you Marin, I’ll take the time to read your reply a second (or third!) time later, because it’s rather complex to fully understand.

(Jonas Kvist) #73

The original article by Col Fink.

(Liam Sergeant) #74

Revisiting this topic and reading through all the responses again as I just had a (very short) session in my kart after installing my front extension.

I found the rear end much, much ‘looser’ i.e. much keener to break free or slide under braking/corner exit. I also found that some of the lines that felt right felt much smoother than my first/previous dry track day.

Is this my kart behaving more like a normal kart, or have I shifted the weight forwards and now don’t have enough weight on the rear? I haven’t moved my seat at all.

Marin, your mention of the jacking and affect of forwards weight balance on that makes me wonder. Perhaps my kart was too ‘bound’ previously and I wasn’t jacking much at all?

Please forgive my quotations etc. I’m not sure if any/all of my terms are correct.

(Liam Sergeant) #75

Does anyone have an opinion on how much a 15mm shift rearward of my seat will have on weight balance/set up etc?

Is 15mm too much? I was going to go straight for 20mm, and I’ve read 10-12mm is a good change to trial. I only have enough space to drill one set of holes for the seat stays so kind of want to get it right first time without being too conservative.

Should I just go for the 20mm?

(James McMahon) #76

Do you have a set of scales?

What’s the problem you’re trying to solve?

(Noah Koenig) #77

I dont know how much in a number on rear percentage, but I can tell you that you will put more weight on the rear end.(obviously:wink:) Depending on the chassis, in general as you put more weight on the rear end you will have more rear grip. Granted this is not always the case. I know that in cars when you move weight back the car actually wants to over steer more due to more weight being transferred to the rear tires which allows the car to rotate. Sometimes with taller drivers, teams will move the seat back to get weight off the front tires and allow the chassis to rotate more. Telling us what the issue is would help as well. Scales are gonna be a key after changing the seat placement.

(Liam Sergeant) #78

Thanks Noah, (and James).

I’m still chasing comfort mainly due to height/long legs.

I installed a front end extension kit. And that was great for a little bit more room at the front. I’m still a little bit cramped and as I’m starting to push things a little bit more the rear is feeling a little bit more lively which is what made me think I might get away with putting my seat back which would make me a lot more comfortable I think?

(Noah Koenig) #79

When you say “lively” what do you mean? You could move the seat back some more if the kart is lifting too aggressively. You could also get a longer steering shaft and longer steering wheel hub to get more room there.

(Liam Sergeant) #80

I just mean I’m starting to get some more oversteer from apex to exit again. I’m very new and so am not sure. Perhaps that’s exactly the balance I should be going for. When I first installed the front end extension I seemed to lose quite a bit of rear end grip, but it also took quite some fiddling with tyre pressures to come back. so that may not have been due the extension at all.

It may also be quite reasonable to suggest/think that I ought to be now working on getting comfortable/used to the kart. My lap times are progressing well still. I’m certainty not limited by my kart.

(Noah Koenig) #81

Oversteer from apex to exit can be caused by two different things:

  1. The inside tire is being held in the air too long(kart overrotates) and when it sets down the kart slides.
  2. The outside tire is being overloaded on entry which causes it to slide from apex to exit. (Very bad for Rotax as they have no bottom end)

What is your setup if I may ask?

If you aren’t yet on the limit with the lap times and kart it may be better to leave it, so that once you do plateau. Then you can say for sure what the issue is.

(Marin Vujcich) #82

Hey Liam.

What track you running at?

Before you make big changes. Play with you entry line a bit to make sure you have rotated enough before you apply power. Apex later essentially. Having a loose rear end in a rotax with DFH is not common, unless you have pressures to high. We typically struggle with too much exit grip with these tyres.
Hope this helps