A vehicles appearance can be improved by lowering the car height. This can be achieved by heating or cutting the coil springs. However, this modification can have effects on the vehicle wind resistance, centre of gravity and reduce excessive body role. This modification contributes in fuel efficiency and better handling. It can also have negative impacts and could cause a big damage in the vehicle as well as being unsafe for the passengers if not done properly.
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Compression springs are the cheapest coil spring to manufacture, they are built to withstand the forces of linear compression or pushing forces that makes them as the most durable type of coil spring available. The coil springs are made of heavy gauge steel wire that is used to support the vehicle. The ends are designed to be flat because of the applied evenly force at each end of the spring, decreases the buckling tendency, or to provide exact seating requirements and solid height reduction which may be compulsory by specific uses (Kiwi Springs limited, 2008).
Coil springs usually have three types of ends which are:
Tangential end: this type of spring will fall over if someone stands on its end because the end of the coil's end remains to twist off in the space
Tangential End Coil Spring
Square end: the end of this design permits the spring to stand still when placed on the end because the end coil curves back down and touches the coil below it
. http://www.budzter.com/images/adjusterPics/DSC00479_Small.jpg
Pigtail end: this spring has the sort of square type end and contains a smaller diameter than the coil in the body of the spring
http://pitstopusa.com/images/T14599086.jpg
(EATON Detroit Springs, 2011)
Theory
Generally, the end design of the spring can tell straight whether it is safe to cut or not, which concluded that pigtail end and square end coil springs cannot be safely cut.
As mentioned briefly above, cutting or heating the coil spring in the vehicle may have some implications in terms of structural integrity, drivability or road worthy aspects.
Cutting the coil spring affects the spring rate to increase, stiffen it and making the handling and ride a bit firmer, this is caused by the spring design rule which uses the wire diameter and the number of coil to define the spring rate. The unit for spring rate is pound per inch (lb/in) which means amount of force needed to compress the coil spring by 1 inch.
For example when cutting springs by more than 2 inches of the front suspension of the 60s or 70s Chevy without following the specifications, many problems can occur, things such as ground clearance difficulties, positive camber will be gained at an increasing rate as well as suspension traveling may be less effective. It is known that cutting a spring by 1 inch will cause the ride to get lower between 1.5 and 2 inches because of the spring ratio which is 1.5:1 and 2:1 (Smith, 2008)
When having too soft spring rate the whole suspension system may be affected because the vehicle will keep on bouncing off the bump stops, while having too much spring rate can create lots of bouncing and cause the tires to lose the grip with the road.
Also spring load can affect the suspension system because of the carried weight that a coil spring supports at any moment compressed height. For instance if we use a 10 tall inch spring at free height with 200 lb/in rate, with a compressed height of 6 inches, the support will be around 800 pounds at that height (De Los Santos, 2009)
How do we calculate the spring rate?
By using the following formula we are able to find out the spring rate of the compressed spring.
K =
Where k = constant pound of load per inch
G = modulus of rigidity of spring material, pound per square inch
d = wire diameter, inches
n = number of active coils, which is the number of coils subjected to flexure (always less than the total number of coils)
D = mean coil diameter, inches = Outer diameter - Wire diameter
http://www.engineersedge.com/spring2.gif
(Engineers Edge, LLC, 2011)
Experimental method
In this project, the modification will be for lowering the car. Hence we are going to modify the 1995 Impala SS (Appendix 1).
This modification will be simply achieved by cutting the springs. This will result in reducing the free length and spring height. Since we do not have access to the real material, this modification will be done in a virtual way by using calculation.
Data: number of active coils = 6.5 inches
Wire diameter = 0.685
Outside diameter of spring = 5.4375 inches
Free length = 16.5 inches
Compressed length = 14.05 inches
By using the formula above, the conclusion of the spring rate will be 443.76 pounds/ inches (as on the appendix 1).
If we want to modify this spring, we will cut one coil that will decrease the number of coils from 6.5 inches to 5.5 inches of active coils, outside diameter and wire diameter will remain the same and free length will change from 16.5 inches to 15.5 inches and compressed length from 14.05 to 13.05 inches.
Results:
K = = 11,250,000 x 0.6854 / 8 x 5.5 x 4.75253
K = 524.44 pounds/inch
The result from the modification shows that the spring rate has been increased by 80.68 pounds/inch.
According to Law Volume Vehicle Technical Association (Inc), (2000), states that no modification for changing the height of the vehicle should be done without the spring manufacturer and the heat method should not be used during the modification (Appendix 2).
AA New Zealand allows modification of the road springs or shock absorbers if they are fitted as a direct replacement for the originals and should be fitted exactly the same way without raising or lowering the vehicle. Modifying the vehicle without a professional certifier could prevent the vehicle from getting WOF (Appendix 3).
Discussion
It is essential when modifying the coil spring to consider what reason we are modifying the spring. Other aspects that need to be considered are spring rate, spring index, total share stress and spring axial deflection. The whole suspension system might not perform as it should if proper procedures are not followed. Safety regulations should also be the main priority at all times when dealing with coil spring modification and the suspension system in general.
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Conclusion
According to the results of the experiment in this report, it can be seen that by decreasing the spring height, the spring rate increases. By cutting the spring, we change the appearance of the car to be closer to the ground. It is very important to understand the disadvantages and advantages of what we are trying to achieve. It is also necessary for the driver to understand the change the modification will bring when using the vehicle in different environment. If the user is not able to tell the effects of modification, it is recommended to see a professional.
References
De Los Santos, H. (2009). Lowering Your Car Properly. Retrieved April 14, 2011 from http://www.carcraft.com/techarticles/lowering_kits/index.html
EATON Detroit Springs, (2011). Cutting Coil Springs. Retrieved April 13, 2011 from http://www.eatonsprings.com/cuttingcoilsprings.html
Engineers Edge LLC. (2011). Compression Spring 'K' Constant Calculator. Retrieved April 14, 2011 from http://www.engineersedge.com/spring_comp_calc_k.htm
Kiwi Springs Limited, (2008). Importers of Metal Springs & Allied Products. Retrieved April 12, 2011 from http://www.kiwisprings.co.nz/coil-springs.htm
Smith, J. (2008). Chevy High Performance. Retrieved April 14, 2011 from http://www.chevyhiperformance.com/techarticles/89858_chevrolet_lowering/index.html
Appendices
Appendix 1
The default values are from the front coil springs on a 1995 Impala SS.
Number of Active Coils:
Wire Diameter:
inches
millimeters
Outside Diameter of Spring:
inches
millimeters
Free Length:
inches
millimeters
Compressed Length:
inches
millimeters
Spring Rate:
pounds/inch
newtons/meter
Pressure at compressed length:
pounds
newtons
FORMULA:
spring rate =
modulus of spring steel X wire diameter4
--------------------------------------------------------------------
8 X number of active coils X mean coil diameter3
modulus of spring steel = 11,250,000 pounds/inch2 = 78,500 newtons/millimeter2
http://www.pontiacracing.net/js_coil_spring_rate.htm
Appendix 2
Low Volume Vehicle Technical Association (Inc). (2000).
Springs
2.3(10) Springs and shock-absorbers fitted to low volume vehicles must be:
(a) of a size and rate which is appropriate for the weight and intended use of the vehicle to which they are fitted; and
(b) in the case of coil springs, of a sufficient rate so as not to fully compress upon full suspension travel.
Heavy-duty and height-changing coil springs
2.3(11) Low volume vehicles fitted with coil springs must:
(a) be fitted with shock absorbers which are able to satisfactorily control spring energy; and
(b) where a substantial increase in spring rate occurs, be reinforced as necessary in the areas of the body or chassis or sub-frame structure on which the increased loads being transmitted are likely to cause fracturing or failure.
2.3(12) Coil springs fitted to low volume vehicles that reduce the suspension travel and, as a result, the ride height of the vehicle, must be proportionately increased in stiffness rate so as to ensure against contact between the underside of the vehicle and the road surface during normal vehicle operation when fully laden.
2.3(13) Coil springs fitted to low volume vehicles must be designed in such a way that the ends of the springs, whether of a plain, plain and ground, closed, or closed and ground configuration, are shaped to match the surfaces against which they seat, both top and bottom.
2.3(14) Progressive-rate coil springs must have the closely-wound section of the coil positioned either:
(a) at the end nearest the body, chassis, or sub-frame structure;
or
(b) in accordance with the spring or vehicle manufacturer's specifications.
Coil spring containment
2.3(15) Coil springs fitted to low volume vehicles must be firmly contained within their locating seats in such a way that the springs can not rotate, move vertically, or become dislodged when the suspension travel reaches its maximum rebound, limited by either:
(a) a shock absorber of a compatible stroke length; or
(b) properly fastened retaining clamps; or
(c) properly fastened wire-rope straps of the type used in motor-sport applications, provided that the suspension mounting points are sufficiently strong to withstand the increased loadings imposed by the sttaps reaching the end of their ttavel, particularly in the case of MacPherson-sttut suspension systems.
Coil spring modifications
2.3(16) Coil springs fitted to low volume vehicles must not be modified for the purpose of changing the vehicle's ride height unless either:
(a) the spring is modified by a recognised spring manufacturer, and the configutation of the modified spring ends match the end configuration of the original spring; or
(b) the spring:
(i) fitted to the vehicle in its original configuration had plain unground ends; and
(ii) no heat is used during the modification process.
Low Volume Vehicle Technical Association (Inc). (2000). Suspension Systems. LVVTA Low Volume Vehicle Standard 195-00(00).
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