Sprint Car Torsion Bars
Sprint Car Torsion Bars are the most common type of suspension used on Sprint Cars. Torsion bars are used on all four corners of a Sprint Car and provide the link between the chassis and track surface. Made from heat treated aircraft quality alloy steel, the bars can be solid or hollow, although most teams prefer hollow bars. Hollow bars are lighter than solid bars, the hole through the center of the bar also creates two surfaces which gives the bar a faster spring rate than that of a solid bar, meaning they rebound faster.
The spring rate of a Sprint Car Torsion Bar is determined by its outside diameter, the larger the diameter, the stiffer the bar. Sprint Car Torsion Bars range from .875 inches to 1.175 inches in diameter. The most common lengths of the bars are 29-30 inches. Torsion bars have splined ends; this is where the torsion arms and torsion bar stops attach. Next to the spline on each end of the torsion bar is a bearing surface with has a diameter of 1-1/8 inch.
How do they work?
Torsion bars are fitted inside torsion bar tubes which are located at the front and rear of a Sprint Car chassis. The bars locate within bushings at each end of the tube, in the case of the front suspension, a torsion arm is fitted to one end of the bar and a torsion bar stop is fitted to the other. The torsion arm rests on the front axle while the stop locates on the chassis. The spring action is created when the axle lifts the torsion arm, twisting the torsion bar. As the axle drops, the torsion bar rebounds back to its original state. The ride height of a Sprint Car is adjusted by the torsion bars, this is done by altering the angle of the torsion arms, or by using the adjustment bolts of the torsion bar stops. Shock absorbers are used in conjunction with the torsion bars to control the rate of suspension movement.
Sprint Car Torsion Bars should be installed so that they always twist in the same direction. The left front and right rear torsion bars rotate in an anti-clockwise direction; while the right front and left rear rotate clockwise. Therefore, a bar that has been used in the left front can be used in the right rear and one that has been used in the right front can be used in the left rear. Torsion bars retain a memory of the direction they are twisting, if they are twisted in the opposite direction the spring rate will be compromised and the bar could fracture. For this reason its always a good idea to mark the torsion bars to ensure they are used in the correct corner of the car.
A basic setup for a hooky track may be something like: Right rear – 1.025, Left Rear – 1.000, Right front – 1.025, Left Front – 1.000. You may hear the term “reverse split” used when discussing Sprint Car Torsion Bars, this is when a softer bar, let’s say a .975, is used in the right rear and a stiffer 1.000 bar is used in the left rear. This can be used to keep the car flat at tracks with large amounts of banking or for when you want the car to lean over on the right rear. The reverse split can also be used on the front end of the car.
Torsion Bar Stops
The job of the torsion bar stop is to hold the torsion bar fixed in place as well as providing a way to raise and lower the car using the adjustment bolt. The stop is placed on one end of the torsion bar, while the adjustment bolt of the stop locates on a stop pad. Stops can be made out of steel, aluminum or titanium and are made in different sizes to suit the various Sprint Car chassis available.
When attaching a torsion bar stop a gap of at least 1/16 inch should be left between the torsion bar stop and the torsion tube bushing, this will ensure the stop does not bind up against the bushing. The amount of adjustment bolt between the stop pad and the stop should be no more than a 1/4 inch, any more than this could cause the bolt to deform or break, altering the handling of the car. Pinch bolts on all stops should be checked prior to every race, losing a stop usually spells disaster! Stops should be torqued to at least 30 ft/lbs. Be sure not to over torque the pinch bolts, this could damage the torsion bar or deform the splined hole of the stop.
The torsion arm bolts on to the opposite end of the torsion bar to the torsion bar stop. On the front end of the car the torsion arms rest on top of the front axle, the arms on the rear end of the car attach to the birdcages. Made from steel or aluminum, torsion arms are available in various lengths and offsets. Together with the diameter and length of the torsion bar, the length of the torsion arm determines the effective bar rate. For example a 1.000 inch torsion bar with an effective length of 23 inches used with a torsion arm with an effective length of 14 inches would produce a bar rate of 249 pounds per inch. If the torsion arm length is increased to 16 inches the bar rate would be 191 pounds per inch. I won’t bore you with the formula used to calculate the bar rates, most torsion bar manufacturers provide charts to easily calculate this. As with the stops, the pinch bolts on all arms should be checked prior to every race, losing an arm can be equally as devastating as losing a stop. Arms should also be torqued to at least 30 ft/lbs.
The bearing surfaces of sprint car torsion bars turn within bushings which are inserted into each end of the torsion bar tubes. Sprint Car Bushings are made of bronze, aluminum or nylon. Once inserted the bushings need to be reamed to ensure the torsion bars turn freely without being too loose.
Sprint Car Torsion bars should be removed from the car after every race meeting and thoroughly cleaned and checked to ensure they are in good condition. Torsion bars should be replaced after around 20 nights as they tend to lose their original effectiveness. Torsion bar testers are also available; these assist in determining if a bar has retained its original spring rate. While the bars are out of the car the bushings should also be checked for excessive wear or cracking.
Sprint Car Torsion Bars should be replaced after a big accident as its possible for a bar to be twisted past its yield point, causing a permanent twist in the torsion bar.
When installing the bars the ends of the bars should be greased, it is also a good idea to grease the centre of the bars as they tend to twist up and make contact with the torsion bar tubes. Once installed the bars should turn freely in the tubes, a torsion bar that is hard to turn could be bent and should not be used. It is also a good idea to place a small amount of grease where the torsion arm rests on the front axle to stop friction and wear.