We will first go over the theory and practise, then speak about the options for the Lazer ZX range.
The theory
Universal Joints (UJ’s in short, and also called Cardan Joints) are an upgrade you see a lot on RC cars. You also see the term ‘CV’ (Constant Velocity) joints being thrown around, in rather inaccurate ways. A short explanation with the good and the bad of each type we see.
Dogbones
Dogbones are in fact Constant Velocity joints. They are however a rudimentary way of implementing the Constant Velocity principle because they have moving and dragging parts inside. There are many drawbacks to dogbones: friction and thus wear (a pin sliding in a slot), limited functional angle due to the cup length and dogbone angle being a compromise,… which all cause (audible) chatter at sharper angles. Due to the tolerances needed they generally also introduce some slop in the drive train and differential functionality.
But dogbones are cheap to produce, cheap in case something bends or breaks, and they offer some solutions to problems that would require way too intricate parts at our typical scale (like variable length shaft when working the suspension).
Universal Joints
Universal Joints are NOT Constant Velocity. When one input is constant in speed, the other will fluctuate. This fluctuation will become larger with the angle. Like with the dogbone setup, you could combine two UJ’s so that, if done correctly, one cancels out the notching of the other. UJ’s are often found at the front of RC cars because they can take a larger angle and still work without much friction and without locking up – which is handy at the front because that’s where suspension movement AND steering get combined.
Most UJ’s found in RC cars are dogbone on the car side and UJ at the wheel side. This is easy to produce and the dogbone end is able to slide in and out the drive cup, as opposed to the fixed position UJ. Some cars have double cardan shafts, but they will use a sliding part so that the length adapts to eg. suspension compression where the shaft has to become a bit shorter.
The joint’s center pivot itself should align with the steering knuckle king pins so that the steering angle will not additionally affect the distance to the inner drive cup..
This all means that UJ’s will cause a fluctuation in speed at the wheel, although the dogbone part and UJ part do cancel each other out a bit. In most cars you will not notice this too much, as this is being soaked up by the differentials (this one doesn’t apply on locked ‘spool’ setup), belt drive, rubber of the tires,… The benefits in RC are: larger angle of operation, less friction, no loose dogbone that can fall out and get lost.
Constant Velocity Drive
The ‘CV’ or ‘CVD’ (Constant Velocity Drive) joints you see in RC are NOT true Constant Velocity. They are just a different approach to the universal joint. True ‘constant velocity’ joints like Tracta, Tripod or Rzeppa are way too intricate designs to fit in small scale RC cars.
Like UJ’s, the joint’s center pivot should align with the steering knuckle king pins.
The RC CV joints however do offer the benefit of being able to function over much larger angles than dogbones, less friction than dogbones, and like UJ’s they don’t have a loose connection that can fall out. I’m personally not convinced about rebuilding them. In my experience the ball, cup and slot wear out as quickly as with dogbones.
Vibration / notching / phasing
One thing I see mentioned a lot online is the dogbone pins being located on the same plane as opposed to being perpendicular to each other, and the same thing being mentioned about CV joints and the pins on the dogbone end.
For dogbones, it should be noted that in theory they will suffer less from sliding out of the cups when the pins are perpendicular to each other, but this will introduce some phasing issues.
For checking the construction of CV’s, it should be noted that the pin in a CV joint serves the opposite purpose than the pin in a dogbone. In a CV joint, the pin acts as a pivot while in a dogbone the pin acts to transfer the drive and it does NOT act as a pivot. The virtual pivot is 90° to the dogbone’s pin. Bear this in mind: correctly phased CV’s will have pins at the CV and dogbone end that sit perpendicular to each other.
Racing
What the speed variance / phasing mentioned above does mostly, is make the car work itself loose slightly. So, CV’s and UJ’s have a bit less corner grip as compared to dogbones. Depending on your car, you may like this.
Dogbones also feel a bit more agressive, more punchy than the other types. This is probably mainly due to the little slop, which allows the motor to just slightly build up a bit of inertia before it engages. Or: UJ’s or CV’s will have a smoother feel, which may be your preference.
Choosing between UJ’s and CV’s is again something to think about. UJ’s will fee very loose and adaptive to the track, while CV’s have kind of a self-centering effect when they are rotating, which makes them more actively self-center. They offer the effect of actively levelling out the suspension a bit. Racers often say the CV type joint make the car square up a bit faster, which adds to the traction. Choosing between those may also be a result of what track you are running. UJ’s being more freely you may want to use them on slower and lower grip tracks with bumps and corners. Your car will feel a bit more compliant. The rotational friction effect of CV’s may on the other hand help you to stiffen the car under load, making them more responsive to your input which is what you may want on higher speed grippy tracks with corners where you will actively accelerate and a quite even surface.
Most if not all RC top racers run CV type joints up front and dogbones at the rear these days.
Other types
Losi have their LCD and Xray have their ECS shafts , which combine 2 CV joints, making it a Constant Velocity joint using 2 cardans. They should increase grip, cornering speed and smoothness.
More theory vs. practise?
- Check out this video on UJ phasing.
- Check out this video on RC dogbone vs. CV joint and the speed variance
Universal Joints for the Kyosho Lazer ZX
The Lazer ZX had dogbones as standard, as did the ZX-Sport. The ZX-R came with dogbones rear and W-5061 (62.5mm) universal joints at the front. The ZX-R mk2 and ZX-RR came with W-5063 (65.5mm) universal joints at the front.
Alternatives to the W-5061
W-5061 parts are hard to find, but so are some alternatives listed below.
JC Racing in the UK has replicated the W-5061. [link to their webshop] These universals also allow for a 1.5mm drive pin, but newer hexes that would fit these are not yet available.
The LA-245 CV style universals have the same dogbone-to-joint length (62.5 mm), but the hex (which needs to be the pin-type style) sits further off the joint, meaning you will need about 1 – 1.5 mm shimming behind the hex or behind the inner bearing (or a combination). Shimming behind the inner bearing is not recommended as the center of the joint is best to be aligned with the steering pivot king pins.
The UM-510 CV style universals are again the same dogbone length (62.5 mm), but will need about 2 – 3 mm shimming behind a pin-type hex. This can be a bit much, but on the other hand you can get +3mm OT-19 hexes from Factory Works so the modification in width is not that big of a problem and you might actually want a wider wheelbase.
The W-5062 universal joints are suited for the Raider, but the axle part is longer between the bearing face and the tapered part because the distance through the knuckle is longer . You could use this if you want a wider stance, but you will have to shim it to take out the play.
Alternatives to the W-5063
The LA-303 universal joints have the correct dogbone-to-joint length (65.5 mm) and were used on the later Lazer ZX-5. They don’t need shimming. You will have to use with pin-type mounting as the drive shafts are not tapered.
The LA-232 CV style universals have the correct dogbone-to-joint length (65.5 mm) and were used on the later Lazer ZX-5. They don’t need shimming. You will have to use with pin-type mounting as the drive shafts are again not tapered. The 3racing ZX5-19/HD is the same.
The UM-522(B) CV style comes with the same dogbone length (65.5 mm). They will need about 1 – 1.5 mm shimming behind a pin-type hex.
Combining or modifying parts
The axles of the Kyosho universal joints can generally be recognised by markings on the shaft. For example:
LA-303 have no line around the shaft, they are the correct length.
LA-245 have 2 lines around the shaft, they are slightly long
UM-510 have 3 lines around the shaft, they are even longer
An alternative to shimming the axles is to (have a machine shop) drill a new pin hole through the axle perpendicular to the existing one, closer to the inside of the car to correct the bearing to hex distance. Or you could machine out the pin slot in a hex so that the pin sits deeper.
The “02” extension axle parts LA-232 (LA-232-02) or from LA-302 or LA-231 can sometimes be found separately to combine with the “01” extension dogbone part of eg. UM-510 (UM-510-01) / UM-522 / LA-245 depending on whether you need 62.5 or 65.5 dogbone length).
You could get a machine shop like Manobet produce a modified suspension pivot bracket set where the suspension pivot shafts sit a few mms wider, to accommodate larger dogbone sizes. You will also need to adjust suspension steering and upper rods.
What doesn’t fit
LA-302 (front of the ZX-5, dogbone is 68mm which is too long)
LA-231 (same reason as LA-302: dogbone is 68mm long)
