UPDATE ON WHEELS
Fri, 01/22/2010 - 00:17 — admin
RIM TECHNOLOGY Q & A
Rims are an integral aspect of car styling but are they just superficial or are there real technological advantages besides a pretty face? Torque takes an in depth look at wheels or rims or alloys or mag wheels ……..
Q. What are the advantages of alloy rims?
For many it’s about looking good and by far, the rims influence the appearance of the car the most with the least effort. The search for better performance is an oft-heard reason but performance has a lot more to do with the tread compound than the rims per se. One very important reason why WRC cars use large 18-inch wheels is to accommodate equally massive brakes and rotors. This is a little known fact that the need for bigger wheels was really to accommodate the larger brakes modern performance cars need nowadays. The alloy rims themselves do have a high degree of heat conduction which can be useful in draining heat from the rotors. Strength, rigidity and lower weight are other virtues.
Q. And how are rims made?
Mainly by casting but there are other ways like pressurised casting and forging. Gravity casting is the favourite method as gravity is free and the procedure is both quite simple and inexpensive. Moreover far more intricate patterns can be made by this method.
Pressure casting is a development of gravity casting and uses pressure to assist in obtaining a denser casting than is allowed by gravity casting. The problem is porosity which occurs in aluminium alloys. Porosity is the minute bubbles caused by air being trapped or oxidation occurring during the smelting process. Using first grade aluminium alloys help lower porosity but manufacturers that reuse or use salvaged aluminium alloys have s higher degree of porosity. High pressure applied during casting reduces the size of the porosities and consequently a higher density and stronger rim can be made.
Forging or squeeze process as it is sometimes called is a process where a hot almost molten casting or semi-solid aluminium billet is subjected to a precision squeeze while it is cooling down. Squeeze may be a misleading term as the forces used can be extremely high perhaps measured in tonnes. This is an elegant process but very difficult to implement. To be able to precisely squeeze all parts of the rim equally requires a complex forging mechanism as rims tend to be multi faceted unlike a con-rod or lower A-arm which are other candidates for forging. This will explain the rather simple and usually boring designs available.
However of all the methods, forging produces rims with the highest strength and as a consequence can be made lighter and yet maintain the required strength but unfortunately is an expensive process. Forging is a form of work hardening which makes a very tough exterior to resist stresses better. Another form of work hardening used in rim manufacturing is heat-rolling. Selected parts of the rim can be rolled hardened like the flanges. This also compresses the outer skin of the alloy and it gains significant strength, seen often in Formula one rims.
Q. What is the difference between a 1-piece, 2-piece and 3-piece rim. Which is better?
Some have classified the rims according to how many pieces make the entire rim, i.e. one piece, sometimes called monobloc, 2-piece and 3 piece. The rims can actually have different parts made from different processes (see below)
One-piece wheels are by far the most common type. They can have the highest accuracy and rigidity but it is not always true. The strength and accuracy is highly dependant on the manufacturer’s quality control and sophistication of their rim making process.
Two and three piece construction made its debut in the 70’s and did not gain popularity because of its higher cost and lack of nice designs. The reason for having these types of rims was to allow certain hardening processes to be carried out of separate parts of the rim. For example, the hub and spokes could be forged easily as it does not have a complex shape and the rim itself could be heat rolled to gain strength yet remain lightweight. Also a single generic rim could be mated with hub and spokes of different designs and offsets. So the theory goes but because of the high costs of manufacture, it never became as popular as the one piece designs. They are still used in racing and on some speciality cars.
Q. What exactly are alloy wheels made of?
Well as its name implies, it is some sort of alloy, most commonly aluminium alloys. Pure aluminium is too soft to be used so it is mixed with different metals to develop the desired end-characteristics. Some may just facilitate easier casting, some may reduce oxidation but mostly it improves the final strength of the casting with a minimal cost penalty.
Magnesium was once popular, remember the famous Minilites? Well apart from the possibility they combust, they were very prone to corrosion and developing hairline cracks due to its brittle nature. Sure they were very light but their longevity was a serious issue.
Unsprung mass is an overused, little understood term. So what is it and what does it do? Well simply put, the dividing point is the spring, all that is above the spring, body, engine etc is sprung mass and all that is below that right down to the contact patch is unsprung mass. This includes components like the Lower A-Arm, drive shafts, struts, disc brakes and rotor, wheels and tyres. These components will oscillate in tandem with the road and the spring, depending on the road surface and natural resonances of the primary spring (tyre) and secondary spring (actual spring itself). Left unchecked or undamped the oscillation can occasionally lift the tyre from full contact or cause unpleasant low frequency vibrations affecting both ride and handling adversely. The damper unit or strut is there to reduce the free oscillations and maintain constant contact. This really improves ride comfort and handling. It would follow that the lower the mass of these components results in less work for the dampers which allows for more headroom for its ride and handling duties.
However the wheels and tyres form another component which are not merely unsprung mass but rotating mass. Energy is soaked up in order to accelerate and decelerate this mass. The rotating mass consists of all that rotate namely the drive shaft, disc rotor, wheel and tyre. By making these lighter there is less energy wasted in rotating them. Because it is a rotating mass the savings here are worth much more than its static mass alone.
Using lightweight wheels and tyres may not always lower the overall weight as there is a weight penalty because of the larger rim and broader tyres from upsizing, i.e. plus one or plus two in rim diameter and tyre section from 185mm to 225mm. One hopes to gain performance by upsizing but only with careful selection does one realise the complete benefit of lightweight wheels.
Q. What do I look for in terms of offset?
Offset is the measurement in mm of the difference between the rim’s imaginary centreline and the mounting plate/face of the rim. Most current cars especially front wheel drive models have a positive offset. This means the mounting surface is located away from the centreline and toward the outer side of the rim. The offset is usually stamped to the back of one of the spokes, for example, ET +45 which means a positive offset of 45mm.
Negative offset means a deeply dished wheel with the mounting surface found toward the inside of the rim. This used to be the norm for rear wheel driven cars though it no longer holds true as most have now adopted negative offset for stability under braking and acceleration.
Unfortunately due to stocking costs and difficulty ordering for every specific model of car, agents here and indeed worldwide have resorted to a one-offset-fits-all strategy. So it is highly unlikely you will be able to get the exact offset as per the original specification of the car’s manufacturer.
Choosing offset is something only the cognisant do because they know the value of trying not to deviate beyond 10mm from the stock offset. Often the original rims will be extremely tolerant of out of round and out of balance tyres and rims but once you venture to a different offset, problems with high speed balance occurs. Manufacturing tolerances of the rims and tyres must be top notch to minimise the effects of high speed shimmy. Using a rim with too much deviation form stock offset results in all sorts of geometry and steering changes apart from loading the wheel bearings wrongly.
Q. What effect does the width of the rim have?
There is always a small recommended range that will work with a particular tyre width. The theoretical ideal is supposed to be a rim that allows the tyre to reach its nominal width but that today is nonsense as manufacturers have deviated from what’s embossed on the sidewall of what’s the actual width, i.e. it may say 215mm but it measures 225mm. It’s best to follow the recommendations of the tyre service provider or ask him to refer to the tyre handbook.
By right one should select rim dimensions to suit the tyre characteristics but in practice it’s the other way around. Essentially choosing a wider rim width straightens the sidewalls and if allowed to extremes extends the sidewalls. This has the effect of sharpening handling and making ride harsher. Over doing it also makes the rims prone to kerb damage as the rim protector will be ineffective. Part of the tyres stability does come from the sidewall and the rim’s width directly affects this but the tread belting actually provides most of the stability. By selecting narrower widths compresses the sidewalls forcing them to appear bloated. The accentuated curvature of the sidewall allows flex and gives a better ride.
Pitch Circle Diameter. This is the description of the bolt pattern usually with four or five bolts distributed evenly on a specific circle. There are 3 and 6 bolt designs but they are uncommon. There is no one standard so the circle diameters can vary from 100mm to 140mm with up to ]7 different fixed diameters. Usually designated as 4x100 or 4-100 which means four bolts on a circle of 100mm in diameter and 5-100 means five bolts on a circle of 100mm.
Original rims, steel or alloys have a centre bore that fits perfectly over the hub of the axle without any free play. This centres the rims or makes them hub-centric leaving the bolts to merely clamp the rim to the axle plate. Without an accurate centre bore the bolts assume the duty of centring the rims, making them lug-centric and there can be a lot of inaccuracies as there is some tolerance between the bolts and the rim holes. Aftermarket rims usually have a specially machined set of centring sleeves or rings that can be fitted to the centre bore to facilitate precise mounting.
Q. Why use torque wrench instead of pneumatic hand held which is faster and more convenient?
A torque wrench is a special tightening tool that either has a preset torque or user-settable torque limit. Typical values are usually around 100-120 Nm. Using the manual type wenches, one can over torque the bolt and treads, causing stripping. The pneumatic type wrenches are of course convenient and used in F-1 but there is a danger of cross threading if the operator is not careful. Cross threading happens when the bolt does not fit perfectly into the proper grooves and with the pneumatic wrench, there is no feedback until the bolt has cut a new set of threads over the original ones. These pneumatic wrenches also have a torque limiter but may be less accurate than a torque wrench which you can control directly unlike the pneumatic type which operates in a blink of an eye. An experienced operator will manually ensure the bolts fit smoothly before using the pneumatic wrench.
Q. Are there compatibility issues between rims and tyres?
Well, the industry is pretty much sorted out by now. Have you ever wondered just why a tyre designation is 225/45 WR 17? Why is it that the 225 is in mm and the 17 is in inches? Well today it is a common standard to ensure backward compatibility but there is still a push for metric sizes mainly from the European manufacturers. However the legacy is just too great. Specialised wheels may be seen on special models but getting replacements is a headache.
There are new developments to the rim flange shapes that possibly can cause compatibility issues. The auto industry is gradually trying to eliminate the spare wheel for obvious reasons. This means the development and introduction of run flat tyres. There have been many proposals and some are already on cars but the costs and incompatibility issues force the issue of backward compatibility with the current crop of tyres and wheels. Gradually certain features are being phased into tyre today’s designs that will allow the use of the new generation rims. These rims have accentuated tread bead seats to keep the tyre locked in place even with a deflated tyre. It may not be truly run flat but more like run-on tyres, allowing a limited driving range without losing control.
Q. What then are the functions of rims?
One function of the rim is to hold and keep air locked in. Air has a very important function as it gives the tyre a load carrying capacity. Without air the tyre cannot function properly. The more air generally the higher its load carrying capacity.
The problem with going to ultra low profile tyres is a reduction in the enclosed air volume and so its load carrying index falls if one keeps the overall height constant. This is one reason for the tyre pressures to be raised from that of normal tyres. Interestingly Porsche once equipped some high-end models with special alloy wheels featuring hollow spokes. This had two purposes, to allow monitoring of air pressure and to increase the enclosed volume. Such a complex, expensive solution could only be used in expensive cars. The use of wider, larger tyres with improved load index ratings have largely solved this problem for sports car manufacturers.
In racing or rallying, the rims have another special feature. The tread bead area is grooved and sometimes has a crosshatched knurled appearance. This allows the tread bead of the tyre to be securely fastened to the rim. With the kind of force generated by the brakes and sheer power of these race machines, the tyres can actually shift position because of the enormous forces causing the whole assembly to go out of balance which is a huge problem at high speeds.
Q. I heard that air can be lost through the alloy rims. Is this true?
For some rims it is true. The reason is a high level of porosity. This constantly lets minute quantities of air out. To counter this rims are usually epoxy bonded or powder coated especially in the bead area and rim flanges. A large scratch here can lead to air loss. With better alloys or forging, these problems are far less. Most of the loss is due to the valve assembly and the tyre itself.
Recently Nitrogen has been made commercially available and the benefits are less pressure change due to heat, less diffusion and since it is inert, the tyre compound is not affected by oxidation.