The axle bearings in every kart we’ve owned in recent history have been a very tight fit in their cassette. How tight or loose should they be? And does the bearing tightness in the cassette effect kart?
I have taken emery cloth to the cassettes to allow the bearing to be seated in the cassette with minimal force. Have I been wrong in doing so? Karts come with an axle bearings that fit very snuggly in the cassette. Should I just leave them alone and struggle with getting the bearings in and out?
Do you have pics of the cassettes your are dealing with? Some have pinch bolts and some have a slot for . Whenever I can, I set the outer run of the bearing free to move inside inside the carrier. Not laterally, but enough so that the chassis rails can flex independently of the axle.
James,
You seem to be doing the same exact thing as me. The Italkarts we owned had pinch bolts as well as grub screws to hold the pinch bolts. The last chassis we had was a Tony kart. It’s been same with all of them. My theory is the bearing should be free enough to be installed in the cassette without the use of an axle or any other tool, yet not have any lateral movement. When the bearing has to be rotated in place by using the full length of an axle, I see to issues. One is aligning the bearings straight enough to pass the axle through. Second is a question of whether the bearing in the cassette deflects first or the chassis being twisted first. Third (I know I exceeded my two), with the bearing unable to freely move in the cassette, isn’t the characteristics of the different axle stiffness being masked?
I would think that they should be able to rotate in the cassette but I’m definitely not an expert! I’d love to hear more options on this.
Very interesting topic. I have never thought of this, but have always been very careful to get bearings lined up across the chassis rails so that there was no torsional pre-load - I believed they were stiff by design, but admittedly thought little on the matter. I’ve been experiencing some minor rear end issues with my kart since our control tyre moved to the Dunlop DFH and this topic has got me thinking.
My view with regard to bearings is that if the chassis wants to flex it is going to, regardless of whether there is a tight or location fit between the bearing and cassette, because there is enough torsional play in the bearings to allow for this. However a location fit between bearing and cassette could result in longer life for the bearings and less power loss due to reduced torsional loading… the questions remain: what is the upside to a tight fit? what compromises do we make when selecting either option?
P.s. not a mechanical engineer so the above thoughts should be taken with a pinch of salt. I’m prone to overthinking things I know little about 
I ACTUALLY HAVE A (semi) ANSWER TO THIS!!
Again, this is just based off of my experience:
Prior to the 2007 Grands (recall, back at that time chassis were stiffer than today), we spent a long time honing the races in our OTK rear bearings. This involved a ball hones and they had a beautiful polished finish at the end. Another thing we did was lube the outer race surface that interfaces between the bearing and cassette. We left the pinch bolts relatively loose.
What resulted in testing for the Grand Nationals that year was an exceptionally fast kart down the straights. It wasn’t super noticeable in practice as we ran by ourselves, but the mid-range of our engine during closer practice on race day was noticeably stronger. The kart also had a propensity to ‘roll’ slightly in the middle of the corner but felt very, very strange on corner exit. Still, it was fast, so we left it be.
Worth noting, this ‘rolling’ feeling was in tigher technical sections, and was unlike anything I had ever felt in a kart before or since. The best way to describe it is similar to how a taller driver in a kart feels when they put a ton of negative camber in the front spindles and causes the kart to ‘roll’ over the spindles because of how much jack they induce. I’m not sure I would recommend this sensation to most drivers, as it makes sensing weight transfer tricky.
Ultimately, I think if you have enough grip in the track and the kart the pinch bolts loose or tight will still cause chassis flex and pick up. I wouldn’t recommend leaving them loose on principle, however.
I think you hit the nail on the head with wondering what the advantages of them being tight. I would think you want the bearings free to rotate in the cassette but not sloppy?