Go Back   Team-BHP > Under the Hood > Modifications & Accessories > Tyre & Alloy wheel Section


Reply
 
Thread Tools Search this Thread
Old 10th September 2015, 09:35   #1
BHPian
 
PatchyBoy's Avatar
 
Join Date: Nov 2007
Location: Earth
Posts: 822
Thanked: 1,189 Times
Default PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

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.

PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality-img_20150905_1546541.jpg

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

Name:  wheelbolts14mmtaperdome.jpg
Views: 12617
Size:  18.3 KB

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

Name:  techWheels5.jpg
Views: 11929
Size:  39.3 KB

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.

Name:  rim.jpg
Views: 12925
Size:  44.7 KB

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.

Name:  Bolt1.jpg
Views: 11920
Size:  18.8 KB

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

Name:  Bolt2.jpg
Views: 11886
Size:  16.7 KB

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 -
Name:  Variation_bolt_face.jpg
Views: 13198
Size:  34.9 KB

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.

Name:  1326393084_2_dp.jpg
Views: 12198
Size:  24.2 KB

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.

Thanks for reading.

Last edited by PatchyBoy : 10th September 2015 at 15:11.
PatchyBoy is offline   (73) Thanks Reply With Quote
Old 11th September 2015, 10:29   #2
GTO
Team-BHP Support
 
GTO's Avatar
 
Join Date: Feb 2004
Location: Bombay
Posts: 47,735
Thanked: 89,308 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

Thread moved out from the Assembly Line (The "Assembly Line" Forum section) to the Tyre & Alloy wheel Section. Thanks for sharing!

Superb detail there, Patchyboy. Much appreciated.

I'm still not convinced though and am unwilling to give it a try, mainly because of the safety inherent to the tyre & wheel setup. I see wobble bolts as a fix for something that isn't broken. As fine as they may be, why wouldn't you rather just buy wheels of the exact PCD as your car? Over the years, I've successfully bought rims with the exact PCD even for niche cars like my Jeep.

Exacting PCD is the only way I'm going to upgrade the rims of my cars.

Last edited by GTO : 12th September 2015 at 11:53. Reason: Adding reply after reading thread in detail
GTO is offline   (1) Thanks Reply With Quote
Old 11th September 2015, 14:56   #3
BHPian
 
FarPatel's Avatar
 
Join Date: Oct 2012
Location: Mumbai
Posts: 170
Thanked: 159 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

PatchyBoy, that was very informative and very well illustrated. I had never heard of PCD variation bolts earlier so I appreciate your post here. Do you know if they are readily available at after-market stores?
FarPatel is offline   Reply With Quote
Old 11th September 2015, 15:17   #4
BHPian
 
PatchyBoy's Avatar
 
Join Date: Nov 2007
Location: Earth
Posts: 822
Thanked: 1,189 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

Quote:
Originally Posted by FarPatel View Post
PatchyBoy, that was very informative and very well illustrated. I had never heard of PCD variation bolts earlier so I appreciate your post here. Do you know if they are readily available at after-market stores?
Thank you. Initially, all I could find was negative remarks from people who have never used it, world over. Most frequent post across forums seemed to be "Haven't heard one fail, but I will not use it". That is when I decided to get deep into it and find out for myself, rather that trying to find convincing arguments to support what I believed to be right. This post is the outcome. I was decided that if the vendor cannot provide PCD Variation bolts, I am not going to buy the alloys. Fortunately, the OZ alloy vendor had them in ready stock.

I am not sure about ready availability elsewhere. Most alloy dealers in Bangalore were neither aware of spigot rings, nor PCD variation bolts, so I doubt easy availability.
PatchyBoy is offline   (2) Thanks Reply With Quote
Old 11th September 2015, 16:03   #5
Distinguished - BHPian
 
BlackPearl's Avatar
 
Join Date: Aug 2005
Location: Calcutta
Posts: 2,311
Thanked: 4,387 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

@PatachyBoy, this is a very informative thread and this level of detail was not available before in the PCD related discussions. Thanks a lot. It will help people in taking informed decisions rather than rejecting PCD variation bolts straightaway.
BlackPearl is offline   Reply With Quote
Old 12th September 2015, 11:09   #6
BHPian
 
noo.b's Avatar
 
Join Date: Mar 2015
Location: Gurgaon
Posts: 58
Thanked: 57 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

@PatchyBoy - Thank you for such an informative thread, loved the work you put in and how you explained it so simply. Great Job
noo.b is offline   Reply With Quote
Old 12th September 2015, 18:38   #7
BHPian
 
PatchyBoy's Avatar
 
Join Date: Nov 2007
Location: Earth
Posts: 822
Thanked: 1,189 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

Quote:
Originally Posted by GTO View Post
Superb detail there, Patchyboy. Much appreciated.

I'm still not convinced though and am unwilling to give it a try, mainly because of the safety inherent to the tyre & wheel setup. I see wobble bolts as a fix for something that isn't broken. As fine as they may be, why wouldn't you rather just buy wheels of the exact PCD as your car? Over the years, I've successfully bought rims with the exact PCD even for niche cars like my Jeep.

Exacting PCD is the only way I'm going to upgrade the rims of my cars.
Thanks. I wouldn't use these bolts either, if I could find rims of the correct PCD for my car, that looked good to me, was of good quality and in my budget. I mean, if I could find something like that, I wouldn't need to use these bolts, right?

The intention of sharing this here was to demonstrate that these bolts are not unsafe and is an option available for those who are looking for such a solution. I know of at least one friend who was sold 100 pcd rims claiming to be 98 pcd and fitted using OE bolts. These are a much safer solution.
PatchyBoy is offline   Reply With Quote
Old 13th September 2015, 11:34   #8
BHPian
 
Join Date: Jan 2010
Location: Hyderabad
Posts: 106
Thanked: 99 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

Thanks for the informative post.
Default face for bolts is flat, in general. The conical/ball faces are for taking up minor misalignments or is it for resolving perpendicular forces into components at an angle to reduce shear stresses?
I used semi-spherical washers for machinery foundation bolts to take up minor misalignments - now they are no more used. Now the specs say the base shall be within tolerances when levelled. That's it.

Anyway this method is "jugaad" work, even though safe to use. There is still a factor of the forces being uneven on the washer, as the bolts are not concentric to the conical washer.
SKavuri is offline   Reply With Quote
Old 13th September 2015, 18:36   #9
BHPian
 
Join Date: Nov 2011
Location: Gurgaon
Posts: 186
Thanked: 159 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

Quote:
Originally Posted by PatchyBoy View Post
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.
Great post PatchyBoy, and thanks for sharing!

To clarify, does this mean one should get wobble bolts when one is going for aftermarket alloys, even if they are in the correct PCD? Or is a normal bolt in the correct size (M12 or M14) will do, because the conical / ball shape will take care of the play of the wider bolt hole?
bosporus is offline   Reply With Quote
Old 14th September 2015, 08:33   #10
BHPian
 
PatchyBoy's Avatar
 
Join Date: Nov 2007
Location: Earth
Posts: 822
Thanked: 1,189 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

Quote:
Originally Posted by SKavuri View Post
Thanks for the informative post.
There is still a factor of the forces being uneven on the washer, as the bolts are not concentric to the conical washer.
Thank you. Yes, that is there. But the clamping force is far greater than any possible shear force and this small variance is well within acceptable limits.

Quote:
Originally Posted by bosporus View Post
Great post PatchyBoy, and thanks for sharing!

To clarify, does this mean one should get wobble bolts when one is going for aftermarket alloys, even if they are in the correct PCD? Or is a normal bolt in the correct size (M12 or M14) will do, because the conical / ball shape will take care of the play of the wider bolt hole?
Thanks. No. If the aftermarket wheel is the same PCD as OE, then you will not need these PCD variation bolts. You will only need spigot rings to fill the gap between the OE hub diameter and the aftermarket alloy's center bore. That said, using a PCD variation bolt in this circumstance will not have any adverse effect and is perfectly safe.
PatchyBoy is offline   (1) Thanks Reply With Quote
Old 14th September 2015, 12:27   #11
BHPian
 
Join Date: Aug 2006
Location: chennai
Posts: 308
Thanked: 110 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

I have seen people file out slightly on the alloy wheel bolt holes to match the 98 PCD holes and use stock bolts with hub spacer rings. This is also juggard but safer as it uses strong OE bolt. Also Hub rings need to be Mild steel than aluminium/plastic spacers
vijaycool is offline   Reply With Quote
Old 14th September 2015, 22:23   #12
BHPian
 
slicvic's Avatar
 
Join Date: May 2007
Location: KA 19
Posts: 752
Thanked: 360 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

Excellent thread . I finally understand how these work. was skeptical when choosing alloys for the Punto.

MODS please remove the watermark from the last image. the illustration is partially hidden
slicvic is offline   Reply With Quote
Old 15th September 2015, 16:44   #13
Team-BHP Support
 
Jaggu's Avatar
 
Join Date: May 2004
Location: Bangalore
Posts: 17,752
Thanked: 7,471 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

Quote:
Originally Posted by slicvic View Post
MODS please remove the watermark from the last image. the illustration is partially hidden
Mod note: Use Report post to notify mod team. The watermark is an automatic insert and cannot be disabled or edited by Mod team.

Now on the topic of PCD bolts, yes it can be a work around for minor corrections. But i somehow would not be comfortable doing the same in my vehicles, especially if i do lot of highway or expressway runs. This is a worry i would rather not have.

Very informative post though. Thanks for sharing.
Jaggu is offline   (1) Thanks Reply With Quote
Old 16th September 2015, 08:43   #14
BHPian
 
PatchyBoy's Avatar
 
Join Date: Nov 2007
Location: Earth
Posts: 822
Thanked: 1,189 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

Quote:
Originally Posted by vijaycool View Post
I have seen people file out slightly on the alloy wheel bolt holes to match the 98 PCD holes and use stock bolts with hub spacer rings. This is also juggard but safer as it uses strong OE bolt. Also Hub rings need to be Mild steel than aluminium/plastic spacers
Filing the holes is very dangerous as it can never be as accurate as CNC drilling. Also, it is not the holes that matter. It is the surface area that the bolt actually contacts which is important, as this determines the clamping force applied to the alloy. All readymade spigot rings are either fiber or aluminium. I haven't seen one in mild steel yet.

Quote:
Originally Posted by slicvic View Post
Excellent thread . I finally understand how these work. was skeptical when choosing alloys for the Punto.
Thank you

Quote:
Originally Posted by Jaggu View Post
Now on the topic of PCD bolts, yes it can be a work around for minor corrections. But i somehow would not be comfortable doing the same in my vehicles, especially if i do lot of highway or expressway runs. This is a worry i would rather not have.

Very informative post though. Thanks for sharing.
Thank you. This was the most common observation I found. People not being comfortable using this, even though they do not have any scientific reasons to explain why. That said, finding a correct PCD wheel should be the best solution. This would be the second best option for minor correction.
PatchyBoy is offline   (1) Thanks Reply With Quote
Old 8th October 2015, 18:35   #15
Team-BHP Support
 
Rehaan's Avatar
 
Join Date: Feb 2004
Location: Bombay
Posts: 22,373
Thanked: 22,588 Times
Default Re: PCD Variation Bolts (aka Wobble Bolts) - Myth versus Reality

Excellent & very informative thread PatchyBoy!

Quote:
Originally Posted by PatchyBoy View Post
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.
Didn't know this! In fact, I don't think I've ever seen anyone take hub dia into account when fitting alloys (unless it's a non-fit!).

However, later on you say :
Quote:
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.
Which implies that the center hub itself isn't making solid contact and transferring shear load to the alloy? Even if it later shifts to make contact, it will only be doing so at 1 point/angle -- quite useless for a rotating wheel. Could you please explain.

Quote:
Originally Posted by PatchyBoy View Post
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.
Question: If M12 & M14 are the bolt sizes, why do aftermarket manufacturers make their bolt holes 15mm, and not 14mm?

Last edited by Rehaan : 8th October 2015 at 18:37.
Rehaan is offline   (1) Thanks Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Key decision points for Audi A4 versus Mercedes C220 versus BMW 320D ashishkashyap Luxury, Imports & Niche 40 30th April 2017 19:35
SUVs & the Bullying effect: Myth or Reality? smartcat Street Experiences 140 31st March 2014 00:02
SX4 ZXi GC - Myth or Reality ? rkbharat Technical Stuff 14 15th May 2009 15:33


All times are GMT +5.5. The time now is 05:37.

Copyright 2000 - 2017, Team-BHP.com
Proudly powered by E2E Networks