Wheel offset effect on performance

Question

When it comes to upgrading wheels and tires, drivers tend to have two different goals in mind:

• Improving their vehicle's handling/performance
• Improving their vehicle's appearance

A lower positive wheel offset will typically result in the wheels being more flush to the fender, giving the vehicle a more aggressive look. A lot of people that choose a much lower offset end up modifying other components on their cars to compensate for fitment (i.e., rolling fenders, adjusting camber).

To what effect does moving away from the stock wheel offset have on the performance of the vehicle? Is it generally a wise idea to stay as close as possible to the stock offset? And does a lower offset have any benefit on the handling and driveability of the car?

Answer 1

Offset is one of a number of measurements applicable to a road wheel and it describes the distance from the centre line of the wheel which the mounting face of the hub is.

As a practical example, lets consider a 6J wheel (a wheel which is 6 inches wide). Offset is usually quoted as "ET" followed by an optional mathematical sign (+/-) and a number. A wheel with an offset of ET0 is described as a zero offset wheel. This therefore means that our 6J wheel will have an equal 3" of wheel sticking out infront of the hub and 3" of wheel sitting back over the brake disc. Offset is quoted in millimeters (mm) so for ease of conversion, a wheel with an offset of ET+25 will effectively be recessed into the arch by ~1" so will protrude 2" with 4" over the brake disc. Conversely a wheel with an offset of ET-25 (negative offset) will be pushed out closer to the wheel arch so will have 4" of wheel protruding from the hub and only 2" over the brake disc.

There are a number of reasons why you may wish to deviate from manufacturers standard offset. You may wish to fit wider wheels and encounter clearance issues with the rear face of the wheel touching the suspension leg at standard offset.

You may, as stated wish to give the car a more aggressive look so that the wheels and tyres appear to "fill" the arches better.

The effect on handling of deviation from manufacturers offset is to increase effective levels of grip whilst sacrificing a certain level of feel. More performance variants of cars employ something called "wide track". Track refers to the width between the wheels measured across an axle. For example, the Mk3 Golf VR6 used different front wishbones and ball joints to effectively push the wheels further out into the arches than the standard car.

What a wider track does is mean that cornering forces can be more effectively coped with by the outside tyres. Consider a push bike, very unstable and easy to push over. Add stabilizers to the bike and it now becomes harder to push over. Imagine increasing the width from the bike that these stabilizers extend and you will see how it becomes harder and harder to push over.

The trade-off however is that when a standard road car approaches the limits of traction it is very communicative to the driver through the steering feel that it's about to loose grip and progressively break away from it's intended line. If the driver presses on they may unsettle and ultimately loose control of the car. With a wide track car the limits are approached at a higher speed and the amount of information fed back to the driver is a lot shorter before the car breaks traction completely. Consider how many Porsche 911's are damaged on wet roads because they have such a high level of grip but it goes away very very quickly. Solutions to this immediate loss of traction may include the use of bias ply tires which always given more warning and allow the driver ample opportunity to scrub the tires well before traction is gone completely.

Related slightly to this answer is the fact that certain tuners use spacers to increase the track of the car which has the same nett effect as altering wheel offset as it pushes the wheel out by the width of the spacer from the hub.

One final note is that running a wide track through offset or spacers typically accelerates wear on semi floating wheel bearings. Full floating axles tend to favour larger negative offset.

Answer 2

The lower ride height does have a lot of advantages aside from appearance. For one, the lower a car is, the more aerodynamic it is. Another thing that's great about a lower car is that because you need stiffer springs to compensate for the smaller amount of travel they can do over bumps, your car is kept more level when you go around corners. This minimizes body roll, which means you have less forces pulling you out of line.

Answer 3

Also: Wider tire/wheel combinations - given same total diameter, etc - require more aggressive angles, particularly of Caster and/or SAI('Kingpin' for us dinosaurs!) to keep them going straight and to resist road imperfections that tend to pull at wider tires. Or, less power steering assist.

I prefer narrower tire/wheel combos because more vehicle weight is concentrated in a line PARALLEL, as opposed to perpendicular, to the direction of travel. After all, when's the last time you saw a typical sedan, mini-van, or bus rolling sideways down a street? lol!

There is more understeer associated with so-called 60-70-75-series tires than with 50-series and below, but that is actually what I prefer, as an admittedly twitchy driver. 😮 I find most of today's rides too 'easy' to steer, as in: into an ACCIDENT. I prefer more weight in turns at the wheel.

Less tire sidewall and more wheel/rim? For show as far as I'm concerned! 😉

Answer 4

Most accelerated wear on components is negligible at worst. A wider wheel with negative offset will still follow the angle of the hub, and remain flat, and only introduce wear at the highest levels of suspension compression. The ball joints will take the brunt of any abuse, not the bearings. Even though the leverage and forces somewhat increase during the highest of compression, the extra wear on tires and bearings argument is usually mostly without merit as the wheel still stays the same angle as stock through most of the compression cycle.

The reason the steering feel changes and performance on the street generally suffers in this situation is due to pushing out the location of reciprocating mass and geometry. Generally wider wheel and tire combo will be likely heavier than stock, wider than stock, add in negative offset and you have a complete change in compression dampening geometry which affects spring and shock dampening rates due to the extra leverage on occasion when the outer most portion of the wheel encounter surface anomalies. Also, any imbalance is amplified by nature. Add the higher weight to the un-sprung mass and it will change braking effectiveness, steering feel, etc. The lighter the wheels, the less of the affect.

What will take a beating is a negative offset wheel over street bumps and holes not usually seen on a race track. The deeper the lip, the more susceptible the outer rim is warpage and or cracking. This is why most reverse offset truck rims wider than 12 inches are forged. Just some food for thought.

Answer 5

Bought a decent Accord years ago that the owner had put wider wheels of negative offset at least 15mm; don't recall wheel diameter. Tires set out beyond wheel wells, and the car never handled right. Messing with offset and overall tire diameter changes the leverages exerted between the geometry of suspension pivots and the wheel; do you believe you know more about engineering than the designers? Best road handling car I owned was a 94 Legacy wagon with adjustable KYB struts, stock alloys and not modern fashion skinny sidewalls. I could push it with lots of warning, even on washboard dirt roads. My new car has 235/45 on 19x8 wheels, and I've already lost count of times I thought I'd destroyed a wheel&tire on a street pothole. "Performance" wheels on street vehicles are fashion statements that ride grossly harsh, while imparting worse actual handling for most drivers, who need feedback before tires suddenly slip in corners. Go look at the photos of the Stirling Moss car that won the 1955 Mille Miglia and rethink the skinny tire joke - 992 narrow, winding Italian open road miles in 10:07, 98.5 mph avg, up to 170. Tires look like Macy's balloons. But hey, we know all street "performance" cars are for posers, not drivers.