This blog post is the first in a two-part series on wheels. In this post we explain the common-sized wheels used on a sprintcar, typical offsets and the effect of offsets on handling.
Wheels Sizes
The typical wheel size used on the front of a sprintcar is a 15x8 on the left and right sides. On the left rear, a 15×15 (or 14) is used, with a 15" width being the most common. The right rear most normally uses a 15x17 or a 15×18, with the 17" width being the most common. The first number of this designation is the wheel diameter. The second number is the wheel width in inches.
Wheel Widths
The width of the wheel affects how much the tyre sidewall will flex. A narrower wheel width allows more sidewall flex and increases tyre traction.
The recommendation below is based on using an 18" tyre; if using a 17" tyre, decrease the wheel widths by 1". On a track with good traction, use an 18" wide right rear wheel with higher air pressure to loosen the chassis. When the track starts to get slick, use a 17" wide wheel with slightly less air pressure to tighten the car slightly. This will allow the sidewall to flex more and make the centre of the tyre contact patch crown (convex shape). This allows very low air pressure to be used without pulling the centre of the tyre up off the track.
At the left rear, a typical wheel is 15" wide. Using a 14" wide wheel will slightly make the tyre crown in the centre. This is done if you need a taller tyre to take some stagger out of the car or if you need to use a lower air pressure at the left rear.
Wheel Offsets
The offset of a wheel (also called backspacing) is measured from the wheel centre to the inner edge of the bead. Not all the way out to the outside of the rim. If that measurement is 5", the wheel has a 5" offset. If measuring to the outside of the bead, then a 5" offset measures as 6".
The most common starting offset on the front wheels is a 3" offset on the left front and a 4" offset on the right front. At the rear, a 5" offset is most commonly used on the right and a 6" on the left as a starting point.
Wheel Offset Effect on Handling
Utilising different wheel offsets can be useful in determining setup for different track conditions, so it is important to understand what is happening when you change it.
Think about where the wheel centre is in relation to the tyre. The position of your wheel centre will determine where the weight of your race car is being applied to the tyre. When using this to your advantage, you can use various wheel offsets to manipulate your tyre.
Where the centre is placed in the wheel influences how the tyre is loaded during cornering and, consequently, how loose or tight the chassis will be. For example, if you measure from the outside of the frame to the centre of the tyre and that measurement is 19", the car will be tighter with a 5" offset wheel and a 3" of spacers than with a 2" offset wheel and no spacers. This is because the 5" offset delivers the loading closer to the tyre centre, whereas the 2" offset loads the tyre closer to the inside edge.
Offsets On the Front
On the front, a 3" offset is pretty standard. The right front wheel offset can also be changed to loosen the car up or make it push a little more. For instance if the car starts with a 3" offset wheel at the right front, and then a 2" offset wheel is installed (which pushes the wheel outward 1"), this will make the car tend toward pushing, or going the other way with a 4" offset (which pushes the wheel inward 1") will make the car easier to turn.
Offsets on the Rear
On the right rear, the deeper the offset or further out the wheel centre is in relation to the tyre, the tighter the car will be to turn the corner as more sidewall deflection will be generated on the tyre due to weight being applied further outward on the tyre.
Running a deeper offset can be useful for slick track conditions when looking for more grip or "side bite" through the corner. Running a shallower offset will reduce side wall deflection, allowing the car to rotate freely, which would be more beneficial in heavy or rough track conditions.
For most dirt track sprintcar racing applications, a 6" offset is used on the left rear. However, the same principles apply to the left rear. Going to a deeper offset 15x6 to a 15x7 left rear when looking for more grip or "side bite.
When taking advantage of this, you will be able to find more consistency in the feel of your car. For example, some may adjust or reduce tyre pressure to find more grip or "side bite" when the track is slick. By changing a wheel offset instead, you will be able to run more consistent tyre pressures, and when you do decide to adjust them, it can be a less drastic adjustment.
Wheel Spacing/Tracking
Wheel spacing or tracking is a very effective chassis tuning tool. Spacing/tracking is the process of moving the wheel in or out in relation to the centerline of the race car. The rear wheels can easily be moved with axle spacers.
Can't I move/space my right rear wheel in to make the chassis tighter? Why different offsets?
Some adjust wheel spacing/tracking to try and tighten or loosen the car up for any given track condition. Many don't realise that changing wheel spacing/tracking (instead of offset) will affect your roll centre and centre of gravity. It is not wrong to change spacing; however, it can have a more radical impact on the car's handling. This can create a period of the race where the driver has to feel the car out first before they can race at 100% due to a new feel in the seat caused by a roll centre and centre of gravity change.
By leaving wheel spacing the same, these variables are no longer in play, leaving consistency in the car's feel for the driver. By simply changing a wheel offset instead, you are now manipulating the amount of grip you get out of that tyre, and you can easily make an offset decision according to track conditions.
It's important to note that adjusting wheel offsets and spacing is just one aspect of the setup in a sprintcar. Teams also consider other factors such as suspension settings, tyre pressures, wing adjustments, and more to optimize the car's performance for different tracks and racing conditions.
Teams often rely on experimentation, driver feedback, and the expertise of their engineers or crew chiefs to find the best wheel offset setup that suits the driver's style and maximizes the car's performance on a particular race day.
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