Een interessant artikel over het gebruik van zogenaamde wobble bolts.

Erwin Lagcher

Actief
16 nov 2015
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PCD Variation Bolts

What are these? When to use them? Are they safe? These were the questions that haunted me, when I considered putting 100 PCD alloys on my 98 PCD Linea T-Jet. This is an attempt to share my findings and the answers to these questions to the best of my knowledge and dispel some myths and present some realities.

For starters, the correct technical name is PCD Variation Bolt. Wobble Bolt is just the shop floor name and is kind of scary. Many people feel that these are unsafe, just because they are called "Wobble Bolts". I will be referring to them as "PCD Variation Bolts" only, if that makes people feel better.

The Science behind the Wheels

When we engage a gear and release the clutch, the engine starts to turn the transmission, which then turns the hubs and finally turns the wheels. Let us stick only to the hub - wheel power transfer. The rest is beyond the scope of this discussion. The wheels are attached to the hub by a bolted joint. Let us have a brief look at bolted joints.

A bolted joint, such as a wheel mounting system, works by tightly clamping two surfaces together. The friction of the two mated surfaces and the force created from clamping them together with bolts (Clamp Load) allows the surfaces to resist movement. The amount of friction and Clamp Load determines the level of resistance the joint has to movement.

Clamp Load is created by tightening the bolts against the mated surfaces and is normally measured in foot pounds of torque with a torque wrench. If the bolt torque specified for a joint is applied, then the resultant Clamp Load should also be within specification.

In simple terms - the rotating hub also rotates the wheel, mainly due to the friction between the mating surfaces of the hub and the wheel, which are clamped together with lug bolts or lug nuts. As long as the clamp force is greater than the shear force applied by the hub trying to turn and the wheel resisting the turn due to inertia, all is well. Therefore it is imperative, that the lug bolts/nuts are tightened to the manufacturer's specified torque settings.

Hub Centric Vs Lug Centric

Many would be very familiar with these terms. Nearly all OEM Wheels are designed to be hub-centric. The auto maker designs an OEM wheel to fit on a certain car or range of cars. The center bore of the wheel is sized to fit perfectly onto the axle of that car. This is a hub-centric connection, as the wheel is centered by its connection to the axle hub. The lug nuts hold the wheel firmly to the mounting plate, but it is the wheel-to-axle connection that actually holds the weight of the car. This is quite an important distinction, as the lug nuts are designed to handle lateral forces that push the wheel away from the mounting plate. The forces that the hub and center bore connection are designed to withstand ? the weight of the car forcing downward and impacts forcing upward ? are at right angles to the forces that the lug nuts are designed for.

Hub diameter is therefore an extremely important consideration when fitting new wheels, whether OEM or after market. If the hub diameter is smaller that the axle, the wheel will simply not fit.

Most after market wheels are therefore made with larger hub diameters to ensure that they will fit on a wide range of cars. This means that when the wheel is installed, there will most likely be a space between the axle and the hub instead of a firm contact. The wheel is therefore lug-centric, as the wheel is centered by the lugs rather than by the hub. Driving on lug-centric wheels means that any impact will apply shear force to the lug studs, forces at 90 degrees to those the studs are designed to handle. This can cause the lug studs to bend, leading to a vibration in the car as the wheel slips around on the mounting plate. To prevent this kind of thing, after market wheels will usually need hub-centric spacers, small rings of metal or plastic made with various inside and outside diameters so as to fit inside the wheel hub and then fit over the axle, making a lug-centric fitment into a hub-centric one. Here is an alloy wheel with a hub ring or spigot ring inserted.




The Bolts

Let us now get to the meat and potatoes of this discussion. Even though we just spoke about hub-centric, in reality, it is the lug bolts that correctly centre the wheel. The hub and the centre bore of the wheel just takes us close to the centre quickly. If you have ever changed a wheel on a car, you would have noticed that the wheel fitting on the hub is mostly a clearance fit and is nowhere near to the precision levels required for accurately centring the wheel on the hub. The lug bolts do this, by having a tapering mechanism (usually 60 degrees for road car application and 45 degrees for racing application) called a cone seat, or a spherical mechanism, called the ball seat. These two are not interchangeable. Using a cone seat bolt in a ball seat wheel is suicidal. There is also a flat seat application, but is not very common.

This image shows a cone seat and a ball seat bolt


1412721d1441861780-pcd-variation-bolts-aka-wobble-bolts-myth-versus-reality-wheelbolts14mmtaperdome.jpg


The cone seat is the most common, so let us look at it a bit more. Shown below is a representation of a cone seat bolt


1412719d1441861339-pcd-variation-bolts-aka-wobble-bolts-myth-versus-reality-techwheels5.jpg


When the bolt is tightened against the wheel, the cone in the bolt mates with the cone in the hole and centers itself. This contact surface is responsible for applying all the necessary clamp force to the wheel to keep it attached to the car. This is the reason why one bolt must not be fully tightened before the others are fitted and also the reason for tightening the bolts in a criss cross pattern.

Most cars use either a M12 or a M14 bolt for the wheels. Like how the after market alloy manufacturer makes alloys with a larger center bore to suit multiple cars, likewise they also make the bolt holes 15 mm to be able to accommodate both 12mm and 14mm bolts. This leaves a small gap between the bolt and the bolt hole - 1.5 mm in case of M12 and 0.5 mm in case of M14. The PCD variation bolt takes advantage of this space. Let us see how.

Here is an example of an alloy wheel of the same PCD as the hub.


1412766d1441867557-pcd-variation-bolts-aka-wobble-bolts-myth-versus-reality-rim.jpg


Note that the bolts are also aligned to the PCD and the 60 degree taper in the bolt corresponds to the 60 degree taper in the alloy bolt hole. Let us take a closer look.

This is a depiction of an OE bolt used in an OE alloy. Since the taper matches correctly, the clamping force is applied evenly.


1412805d1441870680-pcd-variation-bolts-aka-wobble-bolts-myth-versus-reality-bolt1.jpg


This is a depiction of an OE bolt used in an after market alloy of slightly larger PCD - say 100 PCD alloy on a 98 PCD hub


1412806d1441870680-pcd-variation-bolts-aka-wobble-bolts-myth-versus-reality-bolt2.jpg


This setup is inherently unsafe, as the contact area is only on one side of the taper. What can be done about this?

Enter, PCD variation bolt. What if we take the tapering portion of the bolt, make it detachable like a washer, with a 15mm hole to match the bolt hole of the alloy? IMHO, it is a very clever solution to a very silly problem. Let us have a look at what we will end up with.

We will have bolts that look like this -

1412815d1441871778-pcd-variation-bolts-aka-wobble-bolts-myth-versus-reality-variation_bolt_face.jpg


Essentially bolts with a moving taper washer, which can be set to be eccentric to the bolt centre. In other words, a washer that can wobble. That is where the "Wobble Bolt" moniker comes from. Not because they make the wheels wobble. Now let us fit this into our 100 PCD alloy and 98 PCD hub and see what happens.


1412819d1441872461-pcd-variation-bolts-aka-wobble-bolts-myth-versus-reality-1326393084_2_dp.jpg


The cone, by virtue of being moveable, aligns to the cone in the bolt hole, thus ensuring that the clamp force is evenly distributed across the circumference of the hole. Note the unequal spacing on either side of the orange tapering washer and the green bolt. THe cone washer has "wobbled" enough to fit correctly. This is actually far safer than using OE bolts with non-standard PCD alloys.

Limitations

The PCD variation bolts have some limitations though. They can only be used for small corrections. The most common applications are 98 PCD to 100 PCD and 112 PCD to 114.3 PCD. It is not a miraculous cure that will allow you to fit a 5x114.3 alloy wheel on to a 4x100 PCD hub. It only allows for minor correction - about 2.5 mm at the most.

However, extra care needs to be taken to ensure the optimal functioning of these bolts. Firstly, make sure that the bolts you are getting are TUV certified. That would guarantee quality and safety. Secondly, ensure that they are fitted to the wheels and tightened to the correct torque settings as mandated by the car manufacturer. This is critical to the safety of the car and its passengers.

After all this research, I was convinced that these are safe. So I have put 100 PCD alloys on my 98 PCD car. I also installed aluminum hub rings and the bolts I have used are OZ original fitment - TUV certified and race track approved. So, if you are in the market for an alloy and really like one, but is of a slightly different PCD, please go ahead and use these PCD variation bolts. However, if you are not the adventurous type and like to stick to the beaten path, then you will be better off sticking to the OE PCD of your car.
 

SC400nl

Stamgast
22 dec 2013
1.635
155
63
Bedankt voor dit interesante artikel , :rulez:

ik heb namelijk deze bouten op mijn achterwielen en er nooit eerder van gehoord
 
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Waarderingen: Erwin Lagcher

SC430 almelo

Heel actief
13 jun 2011
501
61
28
Ik gebruik ze bij alle vier de wielen en geen enkel probleem. Heb zelfs bij de voorwielen nog spacers erop. No problem.
 

SC430 almelo

Heel actief
13 jun 2011
501
61
28
Heb 8.5 j voor en 9.5 j achter. Steek 5?112. ET 35. Dit zijn de Lenso roadsport. Deze velgen komen veel voor op audi,volkswagen. Mijn Gs430 heb ik niet meer. Heb nu een gs300 president uit 2006.