How to choose Spring Rate
10th March 2005
This is a long story please bare with me. It is accumulated over two years of modifying suspension experience with theoretical education.
The car under study is Toyota Corolla GL 1982 code name KE70, 4 door sedan (KE70R-EEMNS). It received a new heart (engine) and muscles (drive-train) successfully on December 2003. The car was so much faster than before (not faster than other cars) that a proper suspension is required as well as braking and tyres for grip.
The first upgrade was a major success for the front, come to think back. The rear absorber was a old technology and does not rebound and due to that the overall suspension is not good. No grip when the road is slightly uneven or minimal bumpy. However driving in the track was brilliant as the road is nearly all flat.
Summary of first upgrade suspected 8 kg/mm front and 6.6 kg/mm rear with around 14.6 cm front absorber stroke and 15 cm rear stroke. Due to lack of experience the suspension was abandon and sold after acquiring the second set of upgrade.
This second set front is coilover suspected 8 kg/mm with extremely strong absorbers and 6.5 kg/mm rear. Their combination is so strong that for daily road use that the occupants always bounce off the seats and the driver CANNOT maintain the accelerator to the metal when needed. The roads are not that bumpy but little unevenness is enough to push / lift the foot off the accelerator above 120 / 130.
The front is so strong that the car does not sink much. Due to this the previous owners reduce the bottom coil seat so that the car will sink until desirably level. However there is allot of free play and a helper spring was used to hold the main coil in place. Contrary to allot of people's believe that helper spring assist. Most helper spring are so soft that it can be compressed completely by human hand. So this means that the front top motion is fully absorber alone and without the main coil. Whenever the car falls back down from a jump the impact is very very hard.
Both suspension was abandon and sold after acquiring a proper solution form TiTAN Suspension.
At first TiTAN Suspension gave me 500 psi / 3 inches front coil and only 100 psi / 3 inches rear. Needless to say the rear spring rate is not high enough and a quick test drive confirms that. The rear coil was immediately modified by cutting a few rounds to become 200 psi / 3 inches. It had allot of numbers of round to allow the cut. A quick test drive still reveal it is not matching to the front. Too soft. TiTAN Suspension then re-valve and re-pressurized the absorber's character to assist and everything turns fine. But not perfect.
I drove the 500 front coupled with 200 rear for nearly two months and keep feeling it's not right and visited TiTAN Suspension every week to ask them to provide me the 300 psi / 3 inches plus modify rear coilover setup. TiTAN Suspension did not want to do it because the space was very tight. Still they gave in to my persistent request and successfully made their first rear coilover for a car that is not suppose to have rear coilover and double inverted installation.
Using 720 psi / 3 inches as reference to 8 kg/mm the: - see details Hot to measure spring rate
500 psi / 3 inches = 5.5 kg/mm
200 psi / 3 inches = 2.2 kg/mm
300 psi / 3 inches = 3.3 kg/mm
5.5 divide by 2.2 = 2.5 ratio
After getting the 3.3 kg/mm for the rear the front rear ratio becomes 5.5 / 3.3 = 1.66 ratio I managed to test drive the car in Sepang International Circuit (F1) as well as Street ShootOut course and found the car severely "understeer".
The front coil was later reduce from 5.5 kg/mm to 400 psi / 3 inches = 4.4 kg/mm and the new ratio becomes 4.4 / 3.3 = 1.33 ratio This time the car turned from "understeer" to slightly "oversteer".
After much testing and most notably noticing rear passengers bounce off the seats when passing through thin metal strips on the bridges suggests that the rear spring rate is too high. So the rear is reduced to 260 psi / 3 inches = 2.88 kg/mm
During this time the front coil was measured and found to be 360 psi / 3 inches = 4 kg/mm and not 400 as informed by TiTAN Suspension.
This means all the while I was driving on the ratio of 4 / 3.3 = 1.2 ratio instead of 1.33
With this new finding it is summarized that with ratios of : -
1.66 ratio = understeer 15% - may be 20% - may be 10% - too long ago, cannot remember.
1.20 ratio = oversteer 10%
So, with the data above I presume / make educated guesses that if I can get the ratio between 1.66 and 1.20 the car may behave neutral. With the newly reduce rear spring rate the new front to rear ratio becomes 4 / 2.88 = 1.38 ratio
And true enough .............................. After some test drive the rear becomes much less bumpy, in fact observing rear passengers shows no bounce off the seat condition anymore. The rear of the car also feels much more in contact with the ground and not bouncing off the road. Especially during turning, it does not jump lane anymore.
1.66 ratio = understeer 15% - may be 20%
1.38 ratio = neutral
1.20 ratio = oversteer 10%
So, without improving my driving skills I can go through a corner faster than before and safer than before.
Now it is unknown at this stage if a pure track car with higher spring rates will enjoy the same ratio as above.
Some are recommended at 8 kg/mm front with 6 kg/mm rear which have 1.33 ratio, which looks pretty neutral
Some are recommended at 6kg/mm front with 4 kg/mm rear which have 1.5 ratio, this would tend to be understeer I think.
Come to think of it my 1st set was suspect 8 front 6.6 rear = 1.21 ratio it didn't oversteer at that time. Well I didn't think so anyway, I was also not experience enough to make this kind of judgment back then.
Come to think of another thing, my 2nd set was indeed easier for me to handle in the track. Perhaps my skills have improve somewhat 1% :)
Another issue which brings about the "understeer" or "oversteer" is front to rear angle. Some likes to setup their cars flat. I like to setup the front slightly lower than the rear. Driving experience have always felt faster acceleration and fuel economy due to slightly better aerodynamics.
This indirectly puts weight towards the front and makes the front tyres more gripper than the rear. This will also affect the "understeer" or "oversteer" behavior. Not just the spring rate.
Regardless, fellow KE70 owners / drivers as well as AE86 players. I hope this article is able to serve you and do feel free to drop a email for discussion.
Actually if someone were to use the hydralic press machine and reads of xx psi / yy inches and that all coils are measured that way, there is no need to convert them to kg/mm. A direction division will give the ratio value.
Disclaimer - this figures / ratio MAY NOT be suitable for other vehicles. However this technique of records of comparing previous and later setup is useful for all vehicles for searching for the ultimate front rear spring matching.
These spring rates are chosen to accommodate Malaysia Public Road conditions. Will update race track applicability in the future.
The spring rate is not the only properties affect the car. Other items like weight front versus rear, bottom stabilizer bars, chassis strengthening, tyres grip compound, perhaps aerodynamics all affect each other.
I've got a simple car, using simple engine transplant, not overkill 2JZ to over weight the front and basic gearbox and axle from AE86 with standard bottom stabilizer bar front and rear, hence the above ratio technique is easily applicable.
Cars with only front upgrade of certain components may not be able to enjoy the above analysis easily. Examples cars with front upgrade bottom stabilizer bar but without suspension upgrade and rear without any stabilizer at all with upgraded suspension. This non-linear upgraded vehicles are not consistent and may not work.
Make sure to upgrade both front and rear at the same time and make sure of same ratio, example bottom stabilizer bars. Don't just upgrade the front or just the rear. Or that front enlarge allot while rear enlarge a little.