A common problem for PX owners is a misaligned rear wheel. In this blog we examine the most likely culprit: engine mounts.
It’s not at all uncommon for the rear wheel on a classic Vespa to fall out of line: when viewed from the rear it’ll appear to be sat at angle from vertical. The problem is so common that some ‘experts’ will tell you that “They all do it” and not to worry; but they're wrong. Often this is the first visible sign that something’s amiss and the usual culprit is one or more of the engine mounts. Not only are these a potential MOT failure, they're also dangerous defects.
Eliminate other causes
In our experience a misaligned wheel will, in 9 out of 10 cases, be the result of badly maintained engine mounts. However, it’s also possible for there to be a problem at the rear axle or with the frame. Checking the frame is something we’ll discuss in the future but checking the rear wheel bearing is simply a matter of grasping the rear wheel at the 3 and 9 o'clock positions and checking for excessive play. If there’s excessive play, re-torque the nut and try again. If it stops, then bingo you’ve found the problem. If it doesn’t then the wheel bearing is failing. However, even if the bearing is at fault the chances are that the wheel’s shifting geometry will have caused excessive wear on the engine blocks so they’ll need to be checked in any event.
What parts are involved?
Although there are slight differences between the parts used for 125/150 engines and the T5/200 casings the suspension on both relies on four rubber blocks. Two are located on the swing-arm, one at the rear of the casing where the shock absorber attaches and one at the top of the shock absorber where it fastens to the frame.
There's no set lifespan for these items; much depends on how the scooter is used and stored. However, they are consumables just like oil seals, cables and brake shoes, and as such they need to be monitored.
Routine maintainance
Be honest, when was the last time you inspected the engine mounts on your scooter? We thought so, they’re one of those ‘out of sight, out of mind’ components. Unfortunately, neglecting them can lead to poor handling and, potentially, more serious problems. Fortunately, checking them is an easy matter.
One thing that’s vital is monitoring that the main swing arm bolt is correctly torqued to 60 to 75 Nm (approximately 44 to 55 lb ft). As the swing arm moves up and down this bolt is under constant rotational pressure so can come undone over time . It also depends on a spring washer with a high tensile strength. These are increasingly difficult to come by and a standard washer from a DIY store rarely has the necessary strength.
If the nut comes loose, it’s possible for the swing arm bolt to move from side to side and if left unchecked two things can happen.
Firstly, the threads will eventually reach the frame mount where they’ll ‘file’ the mounting hole into an oval shape. This can only be repaired by welding or inserting a bush and is to be avoided. Secondly, in a worst case scenario the bolt may slip out of the frame completely leading to catastrophic handling issues.
The same is true of the rear suspension bolt, which should be torqued to 9.5 to 16.5 lbf·ft (13 to 22 Nm)
Our tip to monitor these, or indeed any other torqued nut is that once you’re happy with that it’s tightened correctly draw a line across the nut and bolt/stud with a permanent marker. That way you’ll be able to make a quick visual check and see if a nut is starting to unwind itself. (The mark can be removed with brake cleaner after any future adjustment)
Checking for problems
Firstly, get down to ground level and look at your scooter from the rear. Are the rear wheel and shock absorber vertical? If the answer to either of those is ‘No’, then further examination is required.
Another tell-tell sign of a problem are ‘witness marks’ on the clutch cover and this should be checked every time the rear wheel is removed. Admittedly, this is an inexact science. Every PX motor on the planet has some form of witness mark here as space is restricted and the engine will quickly become covered in filth. We’re not bothered about the tyre scuffing off road grime, what we’re concerned about are signs of contact between the tyre and casing. This will usually manifest itself in traces of rubber being left on the clutch housing or an appearance that the metal has been polished.
Again, some will say that this has nothing to do with the engine mounts as, regardless of what angle the engine is mounted in the frame, the axle will still be at 90° from the cases. Unfortunately, this fails to take account of the scooter wanting to transmit weight vertically downwards; which results in the tyre deforming enough to brush against the casings.
Assuming that the rim is true, the tyre is of the correct profile and tyre pressures are correct, witness marks on the clutch housing can be a sign that things are starting to go wrong.
It’s vital that you don’t ignore a tyre that’s rubbing on the clutch housing; the friction involved creates heat and that can result in a blow out.
The rear shock absorber
There are two rubber components to the rear shock absorber. One located at the base, which is pressed into the engine casing, and one at the top, which acts as the mount between the shock absorber and frame.
Rear bush
The first thing to check at the base bush is that the correct fastenings are being used. Early examples used a 9mm bolt, which was changed to 10mm during the production run. Most replacement shock absorbers are drilled to 10mm, and where this is the case a 9mm bolt should NOT be used as it will introduce excess play. Likewise a 9mm bolt should NEVER be used in a 10mm bush.
Look closely at the bush, does the rubber appear to be cracked or worn and does the bolt run true? If not, as illustrated in this image below, it’s replacement time.
Top silent block
The top mount is made of a rubber block that’s sandwiched between two metal plates. One is ‘male’, and connects to the frame, the other is ‘female’ and connects to the top of the shock absorber. These plates are chemically bonded to the rubber and in addition to deterioration of the rubber compound, it’s possible for these plates to become detached. This is a particular problem with cheap, non-branded, items. If one of the plates becomes detached this will manifest itself as a sudden, and dramatic change in handling characteristics that’s often accompanied by a ‘banging’ noise from the rear of the frame.
If this happens it’s recovery time as the rear suspension has become detached from the frame.
Main engine mounts
There are different parts for 125/150 engines and the T5/200 casings, but each works in a similar way and consist of two main bushes which are pressed into the swing-arm. Because the PX’s engine weight is distributed asymmetrically the right (engine side) bush tends to wear more quickly than its counterpart. As a result many replacement bushes are ‘handed’ with a harder bush being provided for the engine side. In such cases it’s vital to follow the assembly instructions as failing to do so will lead to excess wear.
(This image is of an LML casing imported from India. To say these mounts have led a hard life is an understatement!)
The only way to fully assess the state of the swing arm bushes is to remove the engine, but an indication as to their condition can be obtained with a little effort.
To do this, first loosen but don’t remove the swing arm nut. Then remove the rear wheel and suspend the frame so that the engine is suspended on the shock and swing-arm. This can be done by supporting the rear end with wooden blocks or an axle stand (be careful not to damage the rear bumper). Now grab the exhaust stub and try to move the engine vertically. There will be some play but if this exceeds +/- 1mm it’s highly likely that the blocks are worn and need replacement. As for replacing them, we’ll come to that in another blog soon…
For more information on the parts required to replace these components, feel free to explore our website, give our sales team a call on: 01226 203377 or drop them a line at: sales@rondaleyscooters.co.uk
