REAR END ADJUSTMENT
By Mike Unger
The rear end of a kart is not only where the power is put down for acceleration, but it is also where the majority of the weight of a kart is, the most movingparts, and one of the more overlooked areas on the chassis. In this article I will try to explain why you need to get your rear end working to get the best performance from your kart.
The first thing we need to talk about is how the rear end of the kart works. Most karts are setup with 55-57% rear weight bias that means the rear is doing more than half of the work to keep you and the kart on the track. The front of the kart has the primary duty to set the direction of the kart and lift the inside rear wheel so the turn can take place. After that is accomplished, the rear end does the rest. Weight transfer, how easily that inside rear lifts, acceleration, braking, and chassis stiffness are all primarily a function of how well the rear end works with the front. When you approach a corner you turn the steering wheel into the corner causing the front of the kart to turn in and through the caster built into the front end it lifts the inside rear wheel causing it to slip. After that point the stiffness of the rear end determines how much grip is transferred to the outside rear tires and how fast it is transferred. Getting that all correct has to do with the stiffness of the rear end components and the track width.
So how do you control stiffness of the rear end? Well there is a number of ways and I will break them down part by part from biggest to smallest effect. Keep in mind I am only talking about the rear end of the kart. That means, axles, bearings, hubs, torsion bars, seat struts, seats (yes, the stiffness of the seat will affect the handling of the kart), how the axle is clamped, rear bumper, and track width. You see while the rear of the kart looks simple enough there is a lot of stuff to work with.
The biggest change you can make to the rear of a kart that will affect the handling is the stiffness of the axle. For those of you who have been to a big race with factory teams you know what I mean. All of the chassis manufactures have several different stiffness of axles available for any number of track conditions. The stiffness of the axle is usually controlled by wall thickness. The thicker the wall of the axle the stiffer the axle, the thinner the more flexible the axle. The rear axle stiffness is one of the most basic and important things to need to get right before you start making any other changes. (Here’s a link to an article on axles, and here’s another one) When conditions are cold or the track is slippery use a stiffer axle. This will help transfer the weight to that outside tire the quickest and most direct way helping to push that tire into the track. For conditions when the track is hot and sticky you need a softer axle. The softer axle will flex more and will more slowly and less efficiently transfer that weight. That will make the tire able to slide a little more on the hot sticky track.
Next up is rear hubs. Hubs come in a huge variety of sizes and shapes and they work very much like the axle does. They manage the weight transfer to the tire contact patch through the stiffness of the axle. The hubs work like a stiffener for the axle. A long hub will overlap more axle therefore making the whole axle system stiffer. A shorter hub has the opposite effect. It clamps around less of the axle allowing more flex. Use short axles for sticky high grip situations when you are experiencing understeer. Use long hubs on cold slippery days when your kart is too loose.
Ok now, let’s talk rear track. When I talked about the effect of hubs and axles I was only talking about using those parts without changing the rear track width. (Here’s a great article on adjusting track width) That is if you are running with a 55in rear track with short hubs and the kart is loose you put long hubs on and keep the track at 55in to get more grip in the rear. But of course changing the rear track by itself can also have a big effect. Widening the rear track does 2 things. First it increases the lever arm to the outside rear tire decreasing the amount of weight transferred to it and therefore decreasing grip. It also will uncover more of the rear axle and allow the axle to flex more also reducing the rate of weight transfer. So changing the rear track can be one of the easiest and quickest ways to tune the rear of your kart. For slippery days when you are loose, narrow the rear track. For days when you have understeer and it is hot and sticky, widen the rear track.
Rear torsion bars are next. Most karts today have adjustable and removable rear torsion bars. These are usually a flattened piece of tubing that is clamped between the rear frame rails. The bar adds stiffness to the rear frame of the kart increasing the rate of weight transfer and increasing the efficiency of that weight transfer. Add the rear bar when the kart is loose and when the track is slippery. When the kart is tight (understeer) and the track is sticky remove the rear bar. Many torsion bars are also flattened to allow you to rotate the flat part to get a different amount of stiffness. With the torsion bar parallel to the ground it is most soft (lower grip) with it rotated 90 degrees to the ground it is stiffer (more grip).
From here the tuning parts become much less effective as the axle, hubs, rear track and torsion bars tend to be the biggest influence on tuning the rear of the kart. But that doesn’t mean that the remaining things should be ignored as in combination they can be a big influence.
The inner race on the rear axle bearings are kind of like rear hubs. They clamp around the axle stiffening the axle in that local area. If you flip that bearing inward so the inner race is between the frame rails you will decrease the rear stiffness of the kart and therefore decrease rear grip. If the inner bearing race in outboard the opposite happens more stiffness and more grip.
Rear seat struts. More seat struts, more rear grip generally speaking. I say that because it also depends on the angle the strut is mounted to the seat and where on the rear bearing it is attached. Most karts that I have driven tend to work best with at least 1 seat strut on each side of the seat. Because the seat struts work with the stiffness of the seat that is in your kart, consult your chassis manufacturer about seat struts and location. It can be different for different karts.
Seats can be a big effect to the rear of the kart but since they tend to be expensive and not so easy to replace it is often overlooked as an effect on the rear end. A seat bolted between the frame rails, triangulated with seat struts definitely becomes part of the chassis system. A very stiff seat will increase the stiffness of the chassis where a softer one will decrease the stiffness. That being said seat stiffness is a fairly sensitive thing to most chassis. Because seats are acting on such a large portion of the kart I recommend changing seats only as a last resort when absolutely nothing else seems to work. Contact your chassis manufacturer for advice but generally speaking stiffer seats will give you more grip and a softer one will be less grip.
The lower bumper bar can also have some influence on your kart. Though you can’t remove it because of the rules you can usually tighten or loosen the bolts holding it in place to change the stiffness. A loose fitting bumper bar will decrease the stiffness and give you less grip and one that is clamped down tightly will give you more grip.
OK, those are the basics to the rear end and that is a lot of information to work with. Now let me explain how I would go about tuning a kart with all of the tools above. Let’s assume that it is a hot sticky summer day and there is a lot of rubber on the track, the kart is tight (understeer) and can’t seem to break free of the track. Given the number of adjustments you have at your disposal this is how I would attack the problem.
1.Remove the rear torsion bar if installed. This is by far the easiest change to make at the track. If this doesn’t work go to step 2.
2.Widen the rear track in 1cm increments until the problem improves or until you reach the maximum width allowed by the rules or the maximum you can go per your axle length... If that doesn’t fix the problem go to step 3.
3.Change to shorter rear hubs and keep the rear track width what it is from step 2. Again in step 2 if you ended up at 52 in. in the rear, put shorter hubs on and set the rear track at 52 in. If that doesn’t work move to step 4.
4.Loosen the rear horizontal bumper bar if it is not already. Don’t remove it and make sure you use nylon lock nuts so you can run it loose without it falling off as it is needed for the rules. If it is already loose go to step 5.
5.Remove a rear seat strut on each side of the seat. Be careful with this one though. While it does decrease rear stiffness it also will allow the seat to flex more which might feel strange to the driver that is sitting in the seat. If the driver is not comfortable with the feedback he is getting, put it back and move on to step 6.
6.If all of that has been done and you are still struggling with freeing up the kart now is the time to switch to a softer axle and start over again. Changing the rear axle can be a pain but you do want to win right? Make the change and start from the top of the list again.
That is my recommendation on how to go about tuning your rear end on a hot sticky day. On a day when you are searching for grip I would recommend the same order of events just instead of short hubs go with long ones, instead of removing the torsion bar add it and so on. Finally if all else fails consult your chassis manufacturer as they will know best how your particular chassis should work the best and how to help make it work. Make sure you tell them what kind of seat you have in the kart as that will have an influence.
So now you know how the rear end works, and why it works. You now know there is a lot more back there to work with than at first glance. You also have a basic order on how to attack the problem. So what are you waiting for? It’s the karting season! Get out there and get your rear end working for you!