Axle removal and replacement
Introduction
From time to time it may become necessary to replace the rear axle on a kart - this could be to fit a harder or softer axle to suit the track conditions, or possibly the axle is damaged and needs to be replaced with one of the same or different stiffness. The procedures to carry this out are outlined below, and are similar for all axle diameters and kart sizes with a few minor additional steps being required for a Rotax DD2 gearbox driven kart.
Identification of components
For those unfamiliar with karting terminology or kart maintenance, let’s go through the names of some of the components that we’ll be taking about in the axle removal and replacement procedure.
Rear wheel hub. There are three important features on a rear hub viz. the keyway groove, the cap screw, and the slot. The keyway slot plus the key that locates in the grove and a corresponding slot in the axle prevent rotation between the hub and the axle. This arrangement still allows axial movement along the axle, and to prevent this, either one or two cap screws are fitted into the side of the hub itself. When tightening the cap screw/s, the slot/s are compressed and friction then prevents any axial movement of the hub along the axle.
Keys. These keys, made of hardened steel, come in two different styles viz. a ‘taller’ one that fits into a plain groove in the axle, and one that isn’t as tall but has locating pins on its underside. This type fits into a shallower groove on the axle that has holes to line up with the pins on the key. The keys are available in a variety of lengths to suit the application e.g. shorter ones for a sprocket carrier and brake carrier compared to those that would be used on say a wheel hub.
Bearing insert. A typical rear axle bearing insert has three grub screws that fix the axle within the insert in an axial direction. To prevent the axle sliding sideways within the carrier due to the large forces applied during heavy cornering, the grub screws are usually tightened to the extent that they will cause local deformation (with resulting burrs) on the surface of the axle. To prevent the grub screws from loosening whilst racing, it is best to apply a drop of Loctite to the threads of each one when fitting them.
Sprocket and/or brake rotor carriers. Both the carriers for the sprocket and the brake rotor (otherwise referred to as a brake disc) have the same three identifying features as a rear hub (cap screw, keyway and slot). Both the hubs and the carriers have the slot that allows them to be firmly clamped onto the axle. It is worth noting that sometimes due to machining tolerances, some of these components don’t easily slide onto the axle. When this is the case, one can aid the process by tapping the tip of a flat screwdiver into the slot after loosening off the cap screw/s.
DD2 overload clutch. An overload clutch is fitted onto the rear axle of a DD2 kart and transmits the torque at the gearbox output shaft into the axle itself and hence onto the rear wheels. As the name implies, it is designed to slip (rotationally) on the axle during overload conditions. The overload clutch housing slips over the machined hexagon on the end of the gearbox output shaft and is clamped onto the axle using the six cap screws shown in the picture below. Additionally, to hold everything in situ, a clamp ring is butted up to the LH end of the assembly and is firmly held in place on the axle with the ring’s cap screw.
Axle removal on chain driven kart
The steps outlined below are numbered because some of them will also be used when removing or replacing the rear axle on a DD2 kart.
Step 1. Lift the kart onto its stand and then remove the rear wheels together with their hubs. To do so, you need to loosen up the cap screw in each of the hubs before sliding it off the axle. If it doesn’t slide off the axle easily, it will become necessary to open the slot in the hub with the tip of a flat screwdriver. Don’t use more force than is necessary when driving the screwdriver into the slot as you are effectively increasing the internal diameter of the hub and don’t wish to overdo this. Using the wrong sized screwdriver can also result in damage to the slot on the hub.
Step 2. Remove the keys that prevent the hubs from rotating on the axle. Sometimes they can be lifted out easily, but often they require a bit of coaxing. The easiest way to do this is with the use of a side-cutter. Lay the side-cutter along the length of the axle and firmly grip the outer surfaces of the key with the jaws as shown in the picture. Then prize the key out of the axle using the leverage provided by pushing down on the handles of the side-cutter.
Step 3. Loosen off the bolts on the engine clamp, and slide the engine slightly rearwards to put more slack into the chain. Remove the chain guard to gain free access to the sprocket carrier. Slip the chain off the engine pinion and rear sprocket and let it lie on the axle.
Step 4. Loosen the cap screw on the sprocket carrier and slide the carrier plus sprocket to one side before removing the key from the axle.
Step 5. Remove all the grub screws in both the bearing inserts being careful not to drop them as retrieving them once dropped can sometimes be like finding a needle in a haystack. You may have a third inboard bearing fitted to your kart, so don’t forget to remove the grub screws in that one as well.
Step 6. Loosen off the cap screw/s on the brake rotor carrier. Then tie down the brake pedal to the bumper bar with a cable tie as shown in the picture. Ensure the cable tie is really tight as this will effectively hold the brake rotor in place within the brake caliper housing once the axle is removed.
Step 7. The axle is now essentially no longer connected to any of the components that are still ‘fitted’ to it. The axle can be removed by tapping it out from the LHS of the kart (brake rotor side) towards the RHS (engine side). Ideally one would use an axle drift for the axle diameter used on your kart (refer picture). This drift has the same outside diameter as the axle and has a short stepped portion that fits inside the hollow axle. The major advantage of using the drift is that as the axle is driven out, the drift will enter the brake rotor carrier and will additionally hold it in place. If a drift is not available, the axle may be tapped out using a soft mallet on the end of the axle. Don’t use a steel hammer directly on the end of the axle as the end of the axle could easily be burred.
Step 8. Once the axle is moving freely (and the drift has entered the brake rotor carrier), remove the brake rotor key. The axle may show signs of burrs or indentations (refer picture) where the grub screws of the bearing inserts have bitten into the axle surface. Before completely removing the axle by hand from the engine side, if burrs are present on the axle, these need to be removed using a fine flat file. Failure to do so means you may encounter some difficulty and also cause some damage as these burrs are dragged through the sprocket carrier and the engine side bearing insert. Clean off any fine filing residue left behind with a cloth before removing the axle. Remember that the sprocket carrier and chain will fall free of the axle as the axle is removed.
Axle removal for DD2 kart
The procedure described below is applicable to a Rotax DD2 kart which doesn’t employ a chain drive linkage between the engine and the rear wheels. The other important point to note is that a DD2 does not use a bearing insert or carrier on the RHS of the kart as the axle is supported within the hollow shaft of the gearbox through which it passes. The engine/gearbox combination is best left bolted to the chassis during all operations.
Steps 1 thru’ 6. For this direct drive kart, the first 6 steps required are identical to that of a chain driven kart except that Steps 3 and 4 above are not applicable, and also that there is only one bearing insert on the LHS of the kart.
Step 7. The axle cannot be removed yet as it is still connected to the gearbox via the overload clutch described earlier. Loosen off the cap screw on the clamp ring which sits to the LHS of the overload clutch, and slide the clamp ring towards the brake rotor.
Step 8. Loosen off the 6x M5 cap screws on the overload clutch housing and ensure that they are all completely loose i.e. each cap screw and its nut can move as a unit. It is unnecessary to remove the cap screws and nuts from the housing.
Step 9. To free the overload clutch off the hex drive of the gearbox shaft, lightly drive the axle from the RHS of the kart towards the LHS until the clutch housing is free of the hex drive. Once free, the overload clutch and the clamp ring can be moved left along the axle to expose the four clutch bushings. Note that a few of them may still be hidden inside the housing but that is unimportant.
Step 10. This is identical to Steps 7 and 8 of a chain driven kart. As the axle is removed by driving it from the left (brake rotor end) to the right (engine side), instead of the sprocket carrier falling free of the axle, the overload clutch, bushings and clamp ring will fall free.
Replacement of rear axle
Axle replacement is essentially done in the reverse order of that described above for the type of kart in question. A few pointers to consider when doing so are as follows.
Ensure that all the components being replaced are cleaned properly and apply a light lubricant such as WD40 to the axle before refitment.
Always install the axle from the engine side of the kart.
On the DD2 once the axle has protruded past the hex of the gearbox, fit the four clutch bushings onto the axle, then the overload clutch housing, and finally the clamp ring before sliding the axle through the brake rotor (don’t forget the key) and the LH bearing insert.
On the DD2 kart, next slide the clutch bushings and overload clutch to the RHS and firmly engage the hex of the clutch onto that of the gearbox. Tighten up the six cap screws on the clutch. Don’t over-tighten the cap screwsas one can crack the housing of overload clutch in doing so. If a torque wrench is available for this, then tighten and torque them to 7Nm. Lastly, slide the lock ring against the clutch housing and tighten its cap screw.
For chain driven karts, remember to refit the chain as well as the sprocket carrier and its key when assembling these components onto the axle.
Before any cap screws are tightened up or grub screws fitted to the bearing inserts, the axle needs to be centralised. Using a steel rule, take measurements from each end of the axle to a fixed point on either side of the kart. This could be the outer edge of the bearing carrier, the inner bush of the bearing insert, or the outer tube of the chassis. Only once these are identical on both sides of the kart can everything be bolted firmly into place.
The final alignment of the sprocket with the pinion (discussed in another chapter), and tightening of the sprocketcarrier is only done after the axle has been centralized.
For all karts, when sliding the axle through the brake rotor which is still clamped in the brake caliper, it will require some rotation of the axle to line up the key with the keyway.
Prior to refitting the grub screws into the bearing inserts, rotate the axle with respect to the inner bush of the bearing insert so that the grub screws will line up with the depressions that are already on the axle. This obviates new depressions being made.
When refitting the grub screws, apply a drop of Loctite onto the threads of each of them. Don’t put any Loctite on the ends of the grub screws
Remember to remove the cable tie on the brake pedal once everything is in place and bolted up firmly.
The final operation is refitting the wheels and hubs. When doing so, remember to measure and align them equally on each side of the kart before tightening up the cap screw/s on the hubs. For those classes where a maximum rear track is specified, don’t exceed that value when refitting the wheels and hubs.
First time axle fitment to chassis
When assembling a kart from scratch there are a few additional things to keep in mind. In the foregoing discussion on axle refitment, it was assumed that the bearing inserts and/or their cassettes were not removed from the chassis uprights. That being the case, the inner bores of the bearings on either side of the kart should have remained in alignment with each other and the axle would line up perfectly as it is about to enter the bore of the LHS bearing when pushed from the RHS. On a DD2 kart, because the engine hasn’t been loosened off, the same rationale applies.
However, when assembling a kart ‘out of the box’ one needs to line up the bearing bores with each other otherwise some axle binding is built into the rear end of the kart. It is imperative that the bolts holing the cassettes into the uprights on either side of the kart are in the same set of holes so that the ride height on either side is identical.
Each bearing insert also has a spherical outer surface. Hence its bore is effectively free to swivel within the cassette until the cap screw/s on the cassette are finally tightened up. When building a kart from scratch it is thus best to fit each bearing insert into its cassette leaving the cassette’s cap screw/s loose. Then bolt the cassettes into their uprights. For a chain driven kart with no engine, engine sprocket carrier or brake rotor carrier in place to hinder you, slide the axle into the first of the bearing inserts and through to the next one.
The bore of the second insert may need to be jiggled a bit, and/or the axle swivelled within the first insert to allow smooth entry of the axle into the second insert. Once done, the axle should be able to rotate freely with no binding whatsoever. With the axle still in place, tighten the cassette cap screw/s and the bores of the bearing inserts will be perfectly aligned. The axle may now be removed and fitted in the correct manner as previously outlined.
The same principles apply to a DD2 kart. Remembering that there is no outboard bearing on the RHS of the kart, one firstly needs to loosely bolt the engine/gearbox combination to the chassis. Ensure the cap screw/s on the bearing insert carrier is loose so that the bearing can swivel without applying too much force. Then slide the axle through both the LHS bearing insert and gearbox hollow shaft. It doesn’t matter which side you do this from, but the LHS bearing insert may need some jiggling and/or the engine may need to move slightly so that everything lines up.
Before tightening everything up, the DD2’s axle then needs to be aligned parallel with the front axle or the kart will ‘crab’ down the track. Measure the distance between the rear axle and the front axles (or kingpin bolts) on either side of the kart and move the engine to suit until these measurements are identical. If you are using the front axles as a reference point, remember that the steering must be pointing dead ahead or your measurements will be incorrect. Once identical measurements are achieved, tighten down the engine and finally the cap screw/s of the bearing cassette. The rear axle should then be able to be easily slid out (i.e. it is in good alignment) before fitting the overload clutch and other bits as previously outlined.
Emile McGregor - MSA Technical Consultant