Third bearing conflicting info

Hi all
I need advice on 3rd bearing fittment. When to use or lose. Kart is an exprit noesis.
Thanks in advance

Running the same kart, we keep it loose as our baseline setup, and rarely tighten it.

Tightening will effectively stiffen the axle. This will reduce flex in the rear of the kart and eliminate some of the inside rear wheel lift. If the kart is hopping from being too soft in the rear, the 3rd bearing is a good quick adjustment to help eliminate that.

We usually use it as a quick test adjustment to see if we need to go stiffer on the axle. If we tighten it and the kart gets better, then we know that we should probably go to the next stiffest axle if the track continues to grip up.

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Thanks for quick reply.
What would you suggest as a base setup?
Rotax, dunlop DFH, 165kg (364lb). Tracks low to med grip.
Trying to free up off the turn.
Thanks

Like I said, we run it loose as a base. I believe that is the OTK recommended setup as well.

I always start with the OTK baseline setup and rarely stray too far from it.

If the kart turns in well but isn’t getting off the corner properly, I would take a look at reducing some caster. If the kart isn’t free off the corner, the inside rear wheel isn’t staying lifted long enough. The lifting action is happening too quickly. Reducing caster will help slow down that lifting action and keep the inside rear up longer, helping the kart roll off the corner better.

There are several adjustments you can make, but I would start there. Any time you can reduce caster and keep the front end working well is good.

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Welcome @Marin_Vujcich!

Don’t be afraid to experiment and try changing one thing at a time.

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Thanks for the responses

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Hey Marin,

Did you do some more testing yourself on this? I’d be interested in hearing your findings/thoughts on running the bearing loose vs tight on the rotax senior application

Yip have done.

In short the 3rd bearing in on the OTK makes the inside wheel drop to quickly and more likely to get an exit push.

Never run it now. Might be ok in the wet, but havnt experimented with that.

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Thanks for that, i am going to experiment with it loose on my 30mm birel chassis. Is there any difference to running it in place with zip ties vs having it completely removed?

No difference. (20 char)

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As far as loose or zip tied, our club rules specify if it is not secured with bolts it must be zip tied or wire tied. Not sure why though. Not like it could fly out. I’m pretty sure I’ve seen the same rule for other events also.

Sorry to pop up this thread again, but want to validate my thinking, guessing its just a terminology issue on my end.

OTK Chassis, with N axle.
I’ve always thought that on a green track (dirty, low grip) you run the 3rd bearing tight as it acts as a harder axle which would produce more grip. Once the track grips up or evolves you loosen the 3rd bearing as you start to get to much grip causing other issues.

Based on how I read @tjkoyen comments, I’m backwards. He prefers to run the 3rd bearing loose at the start, as the track evolves he tightens the 3rd bearing to determine if they need to a harder axle, which would produce more grip. Wouldn’t you want to lessen your rear grip once the track evolves as the rear will be over powering the front?

Assuming I’m wrong and I should be doing everything backwards :slight_smile:

In an OTK kart, the harder axle “frees” the rear up. It’s the axle you go to when the rubber starts to build. So putting the third bearing in will stiffen the axle and free the rear of the kart up.

It can be confusing to think in terms of “grip” and what changes produce or remove “grip”. Since the tire (which is providing your grip/traction) stays constant (unless you change wheels or tire pressure), you aren’t actually changing the level of “grip” in the kart when you’re making most chassis adjustments. You’re changing how the kart uses the grip available. I always advocate for thinking in terms of “inside rear wheel lift” or “load” or “chassis flex” or “rate of lift” or similar when talking about chassis handling terminology. Almost every adjustment we make on the kart is adjusting how quickly and how much the inside rear wheel unloads, by altering the chassis flexing dynamics.

This is an important distinction because a given adjustment may provide a feeling of “more grip” or “less grip” depending on the other characteristics of the chassis setup. This is where people start getting confused I think when trying to figure out how to fix a chassis problem. For example, adding caster will “free” the rear of the kart up as it transfers more weight and induces more rear wheel lift. However if you go too far and add too much caster, it could transfer so quickly, that the inside rear wheel will set down prematurely, making the rear of the kart “tighter” off the corner as it is no longer unloading the inside wheel. One example of the same adjustment providing two opposite outcomes.

Some karts go to a softer axle to get more rotation, some karts go to a stiffer axle. This is probably down to different ranges of material stiffness between brands (Kart A’s “stiff” axle might be equivalent to Kart B’s “medium” axle) and down to how the axle and chassis flex in tandem to make the kart work. Different tubing in the chassis frame might require a different methodology on how the kart uses the axle.

But bottom line, with the OTK range, you go with a stiffer axle when the track grip level goes up, and you go to a softer axle when the track grip level is down (rain).

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