Binding mid-corner

#1

My 2 karts are TAG’s – Zanardi KZ2 with Rotax Evo and CRG KT2 with VLR 100. There are numerous references to “binding” online but I can find no clear definition or, better yet, description of what actually happens.

What I have experienced when I turn in too tightly is a quick BUT slight mid-corner oversteer shift (sometimes with a tire chirp) with a quick decrease in RPM’s. No hopping, just slow out of the hole. I’m looking for feed-back or push-back on what I think is happening (besides driver error – see “turn in too tightly” above) .

The fronts are turned in and have come up to maximum grip with a defined turning radius. The rear is over taxed and breaks loose, dropping the inside rear tire on the track.

This does 2 things: (1) It creates a sudden push effect without actually pushing the front tires which are already gripped-up; (2) It puts the inside rear tire on a smaller, slower radius arc than the outside rear tire which slows down the entire axle/sprocket/chain/engine assembly all at once. The kart is “bound-up” and the RPM’s have suddenly dropped.

I find if I hit the sweet spot in a turn, not too slow not too fast, then my turn is smooth ,and I start gathering speed as I squeeze the throttle at or before the apex. By the way, these are 35 to 40 minimum speed turns, if that adds anything to the discussion.

What say you?

#2

“Binding” a common term used to explain things that are not explainable. It’s like; “I have no idea what’s happening, it must be binding”. “Something is happening that’s keeping me from going as fast as I think I should”.

The cure is often as hard to find as the explanation for binding… It’s often used to explain why you’re not getting through a turn as fast as you want.

#3

You pretty much summed up what’s happening. They key is as you mentioned driver error - you need to be smooth on both gas and steering not snappy and sudden.

There is no differential on the rear axle, karts get through the turn by lifting the inside rear. If you smash the gas too early for example the outside rear loses traction, slides and plants the inner back on the tarmac, unless your wheels are straight the kart will bind because outside is trying to turn faster than inside rear.

The key is firstly smooth driving, then you can tweak front and rear track, front caster (minimal effect), axle stiffness and others to maximize the traction and geometry.

#4

There are two things that could be going on: 1. Snap oversteer and 2. Bind of kart chassis on exit.
Typically, as a kart track builds up rubber a kart will tend to start to understeer into the corner because the rear tires have more grip which causes the driver to turn the wheel harder to get the same steering response. This sharper angle of steering puts the kart into a zone that is putting more twist into the frame that eventually overcomes the grip and then makes the rear end break into a loose condition. This all happens from entry to mid-corner. On exit is where the bind comes in as there is not enough geometry in the front end to keep the inside rear tire released or free so the engine ends up fighting the chassis to accelerate out of the corner. I have several different brands of karts that I race and the typical set-up that I have found that frees up the kart is 1 box of negative camber and 1 box of toe-out on the Sniper laser gauges. If you find that the kart rotates too quickly at apex, then close up the toe setting to neutral or half a box of toe-out. Caster setting I will usually keep at neutral to negative(kingpin standing up more) as I don’t like a kart that is too responsive. If none of this works due to high grip and track rubber build-up then an axle change might be needed. As well, experiment with tire pressures to find a balance and timing so that the tires come in at the right time of the race.
John K

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#5

Our track is almost always green; only once a year when we get the Challenge of the Americas event do we begin to get rubber – and that is maybe the day before they actually “own” the track for their 3 day event . Otherwise, the teams that come early and practice clean up our track to put it in its best green condition that we have all year. Over a number of years, my fastest times ever (by a full second) have come only under those conditions. John, I have learned about snap oversteer, mostly from reading, but my karts are not set up in a way to cause that problem – and our track is never too grippy when we club race. My question about binding was to understand the term – it is thrown out so frequently in various posts and on various websites that I finally thought I’d throw out my own understanding and see where I was off. Alvin, in the first response to my post, echoed what I had been experiencing, i.e., a term used frequently but, in many cases, not understood. My CRG KT2 with VLR 100 was set up by Ron White and I find it a pleasure to drive. At the front, on the kart stand, he has me at 2mm (one sniper box) positive toe each side, 2mm negative camber (although I’m experimenting with 1mm negative as 2mm does not seem to get the entire MG Yellow contact patch in play) and 2 1/2 spacers out. More than any other kart I’ve ever driven, this set up on a 30/30 frame feels alive and nimble. Due to the lower HP compared to my Zanardi KZ2 with Rotax Evo, I’m less prone to jerk into a turn or over-cook the entry speed. Hence, less binding per track session. But I agree with Richard – it is the driver causing the problem. By the way, I’m still confused when I see film of karters throwing their karts into corners (usually trail braking) and then getting on throttle mid “slide” without apparent binding. Any idea how this is possible? For reference, I try to “arc” my turns as wide and as smooth as I can.

#6

Rotation. At braking they start the back coming around and catch it at the inflection point before it exits slip angle and becomes a slide. Throttle goes down.

#7

I doubt you’re looking at modern, fast drivers when you’re looking at that footage.

Not nearly as common today as it used to be. Modern karts require more precise and smooth inputs compared to the balls-out style you could get away with a bit more a couple decades ago. Smooth has always been fast, but there used to be a few different ways to attack it.

#8

I started karting in 1966, it was wrong then, it’s wrong now.

#9

BIND: to answer your question, bind can happen even on a non-rubbered up track. It is like you said in your original post that the kart is not raising that inside rear wheel continuously around the corner and then you are trying to finish the corner and that wheel is effectively causing drag for the motor to try to power through which is then bogging down your exit speed. Sometimes when this happens you can hear a slight chattering of the rear tires fighting each other with their different rotational speeds.

I have owned several CRG and GP karts over the years. When they went to the 25mm spindles with the 10mm kingpins they ended up being very responsive on the steering feel which for me was difficult to re-learn to drive after many years of kart chassis with 17mm front spindles and 8mm kingpin bolts that required much more body english to get them to turn in. The best solution I found was to find the tire pressure that gave the optimum grip in the rear and then drop the front pressure until I found a nice balance to drive with. With the negative camber set-up described above and this lower front tire pressure you will get even tire wear on the front tread.
The MG Yellow front is a square shouldered tire which has a lot of front grip so dialing out more front steering should require you to turn the wheel more which will put more geometry into the chassis which will keep that inside rear wheel released and not causing a bind.
Hope this helps as it is difficult to explain and may only be part of the set-up changes needed to make your kart faster.
John K

#10

Thank you guys for all the input. I’m afraid I’ll get (justifiably) kicked over to another subtopic on tires, but John’s comments on MG Yellows and pressure leads me to a statement which I’m guessing will get LOTS of push-back. Over the years I have consistently found my fastest laps driving TAG’s on our green track with about 14.3-14.5 psi hot all around (immediately off track at pit in where I store my gauge when driving). This has been true for all kinds of weather (except rain, of course) and with either a Rotax or VLR100. Obviously I go out with staggered cold pressures. Equally obvious, I am paying no attention to managing my tires on our 8/10ths mile “momentum” track . Our club races usually consist of qualifying, two 8 lap heats and a 12 lap final, although I’ve been out for 23 lap practice sessions with the same results, clockwise or counterclockwise. And not infrequently my fastest lap is near the end of my session, not, as you might expect, in the 3-5 lap qualifying “window”. Reading other posts and reviewing YouTube footage from our Australian friend, my tire grain looks small and good – all the way across – when I come in. Occasionally my fronts look over-worked, with the grain too big, but this usually happens when my pressures are in the high 13’s right at pit-in. I’ve tried mid 12’s and mid 13’s but I’m always faster with mid 14’s. Thoughts?

#11

Good work keeping track of that data.

My first question would be, what tire pressures are the fastest guys in your class running?

Something to think about, is that especially on a grippy tire like an MG Yellow, some drivers may not be able to get to the limit of the tire consistently at the correct pressures, because the tires provide lots of grip and it can be easy to greatly upset the kart with the softer sidewall when on lower pressure. Bumping the pressures up a bit might take some of that sidewall flex out and make the tire a little easier to drive, and that extra comfort might give you more ability to push.

So with the higher pressures, the tire might not be in its optimal grip window, but it might be more comfortable and easier to drive it.

It’s also probable that your track’s lower grip surface requires slightly higher pressures than you’d see at a track with more rubber built up.