Rebuilding an older kart to be competitive (with pics)

[tom]

I'm in the process of rebuilding an ebay bought kart into something that I will feel comfortable challenging for wins with at major meets. I'm going to take pictures and describe what I am doing along the way, hopefully someone out there can find something amongst the information I supply that will help them out!

I bought a KT off ebay for $1400, and did one race meeting with it, and a bit of testing at our farm track. It needs a bit of an upgrade, so here is what I am doing:

World, meet Lara, stripped of her seat and axle!



Side-pods aren't just for bashing people out of the way.. I feel that any kart handles it's best when you can keep the weight centred, so heavy side-pods hanging out at the edge of the kart aren't my thing. I like these rookie pods as they don't collect much mud, and stay light. (i'll explain later how you can have the exact same kart corner weights, but have a kart handle poorly as the weight is spread away from the centre of balance) Anything you can do to keep weight away from the extremities of the kart is a good thing. Here are the pods, installed just outside flush of the rear wheels, and with a decent strut back to the frame to help with pushy drivers.


I'm always amazed at how few people use heel stops, sometimes I push quite hard back into the seat to get weight transfer, I like to have a solid footing, otherwise my heel slides up over the front frame tube.


As you might have read in other threads, i'm doing a bit of KT development work, the digatron is great for logging all the engine data I need.


OK. now onto more important stuff. The most important pieces of a nice handling (and fast) chassis, are the seat position, and the axle set-up. Before I install a new axle, I took the time to check the chassis for twist. This is easily done by putting a straight edge across the tops of the kingpins, and sighting back to the rear axle. I have used an old axle (that is straight), as you can see in the pics. if you then sight forwards from the rear axle, you can easily see the smallest twist in the chassis. If it is straight, you will see the edges of the two axles align perfectly all the way across. If the chassis has a twist, you will see the front straight edge disappear on one side first.

Front view of kart with straight edge (old axle) across king pins.


Rear view from axle showing sighting of front straight edge.


And almost disappeared, this one is nice and straight, any twist will show up easily here.


If you are looking at a used chassis, this is a good thing to check, it can be seen with the seat on, but not as clearly. Combine this with measuring from the king pin back to the rear axle both sides, and we have a square and straight chassis ready to go!

Next time i'll be showing the method I use to lock-down the rear axle. I hope to have this kart on the track on the 26th October, so there is a bit to do..

[tom]

Ok, time is ticking, so I got a bit done on Ms. Lara in amongst watching Bathurst over the last weekend.

Fitting the axle: Next to seat position, correct selection and fitment of the rear axle is going to have the greatest impact on the handling of your kart. Just exactly what type of axle you are looking for will depend on your physical build, type of track, and type of engine you are using. For this article, i'm setting up for KT Medium.



This is the axle, fitted to the kart. Make sure you get the best sealed bearings you can find. Also note that just because the bearing spins nice and freely on the stand, doesn't mean you will get the least rolling resistance on the track. In future articles I will talk more about bearing performance, but keep in mind that these axle bearings take a lot of lateral load, which can be compounded by axle fitment, and the lateral resistance of the bearing is a significant drag on forward progress. When you fit the bearings to the cassettes, tap the axle and cassette as you lock it down, to make sure the bearing is centred.

It's also stating the obvious, but make sure you have enough clearance either side of the brake rotor so that there is no drag.



This is the business end of axle fitment. Make sure the axle is centred to the bearings (same amount hanging out either side of the outside of the bearing face). Fit the drive side bearing in it's cassette making sure it is centred by tapping the axle and cassette as you tighten it up. What some people don't picture is that the although the majority of axle flex happens between the bearing and the hub, this flexing tries to transfer through the bearing area, and to the axle section that is between the bearings. Effectively this has the effect of the axle trying to pull the bearings closer together, which is extremely difficult to do as the chassis is supported by the cross bar under the seat, and the rear bar. What happens in reality is the axle flex puts a big lateral load on both bearings, which creates drag on the balls. (nobody wants dragging balls).

To overcome this drag, I only lock down the drive side bearing, and to this end I use axle collars, as well as the grub screws that come with the bearing. This type of bearing has ball bearings under the grub screws that are very effective in grabbing onto the axle. I also use Loctite, and a zip tie just in case! The brake side bearing I don't tighten the grub screws at all, and the axle floats through this bearing.

The handling effect of locking down your axle in this manner is that it makes it behave as though you have a softer axle fitted than if both bearings are locked.



Finally I get to introduce Lara to the KT that will hopefully power her to many victories. It looks like any other KT, hopefully all the tweaks pay off and it is scintillatingly fast! As you can see it is mounted on a vibration sleeved, slide mount. The only tricks here are to make sure the motor is exactly square on it's mount, and the mount when bolted down is exactly square with the axle. I will be fitting new fuel lines (including the tank pickup), and a Teflon lined throttle cable.



When I mount my sidepods, I like to make sure they are slightly wider than the outside of my rear tyre. If you drive side by side with someone and have contact. The side pod will slow the rotation of their rear tyre, and your rear tyre will be free. It also helps stop your kart from jumping. Try to set your pods to prevent your kart lifting when contact is made.

I think the rest of the kart put together is straightforward, but i'll try to put more tech articles on individual systems as time permits.

Also you might notice I completely skipped the seat fitment. That is the single most important part of setting up the kart, but requires a lot of information to describe. For reference, try to find the fastest person in your class that has a similar build to you and put your seat where theirs is!


More later, Cheers!

[tom]

An update on this one (over a year later). I did actually race this one a couple of times, the KT took a bit of sorting, now it is fast enough! The chassis was good, only issue I had was a dragging brake caliper, so had to disassemble, wet/dry sand, and lubricate that one. In any case the KT is now off to greener pastures, so Lara is being fitted with the latest MRC 100cc open motor.



Might actually get to race this one unless it gets sold first!