2014 - 2021 BMW M4 Convertible
Suspension customers shopping for aftermarket springs will sometimes resort to shopping for springs by spring rate. While the rate of the spring is a measure of spring performance, it is not the only influence of vehicle performance and handling, especially when the chassis is lowered. There are many factors that come into play when it comes to suspension tuning.
When does a higher rate spring feel softer than a lower rate spring and a lower rate feel harder than higher rate spring? These differences occur when you lower the chassis and the bumpstop is trimmed or not trimmed. The bumpstop acts like a small progressive spring (see Micro Cellular Jounce Bumper). A lower rate spring that lowers the ride height of the chassis with no bumpstop trimming, making the bumpstop more active, would feel about as stiff as a higher rate lowering spring with the bumpstop trimmed, making the bumpstop less active. Also, the amount of ride height lowering also affects suspension rate.
Since there is no standard for quoting spring rates, most manufacturers just quote numbers without any regard for spring function and spring rate ramping—spring rate ramping is the difference in ride feel between springs of different shapes with the same spring rates under suspension compression. The only way to truly compare spring rates is by using working spring rate numbers.
When a manufacturer quotes a progressive spring rate such as 80#, 150#, 225#, and the stock rate is a linear 135#, the new spring looks super progressive. Visually it seems to start off softer than stock and gets progressively stiffer as needed. But what these rates don’t tell you is that the chassis is already sitting at the 170# rate at loaded height. This means the “working spring rate” is actually 170# to 225#. The lower spring rate range below 170# is the dead or inactive spring coils which do nothing but give the spring tension at full rebound. Note that this does not take into consideration the bump stop engagement and their effect on chassis ride height with the installed lowering springs. This is only one example of why using the “working spring rate” is more accurate when making comparisons. It is also important to have already driven on a different rate than stock to feel the difference before making any comparisons based on the quoted rates.
There are many other factors that influence suspension rate that include, but are not limited to: Shock dampening values, tires, bushings, and of course the most important “personal driving style”.
Keep in mind that if you’re shopping for springs based only on spring rates, then you are not taking into account a number of other variables that affect ride height, performance, and comfort.
Well, we call them bumpstops because we always have. It is what we are used to calling them. But in the age of specialized tech-talk they are “bumpstops” no more; now they are micro-cellular progressive jounce bumpers. - What? ... That was a double decafe Mocha, skinny, no whip!
Most people think of a bumpstop as a black rubber snubber hanging down from the frame above the axle to protect against metal-to-metal contact. Just like my dad’s 1974 Chevrolet Caprice station wagon. You know, the one with the big block 454 and the hide - away tailgate. The Caprice was the cruising wagon of choice when highway 5 between L.A. and San Francisco first opened and gas stations were few and far between. The song, “Head out on the Highway, look’in for adventure” keeps ringing in my head. Where was I, oh yeah, trying to avoid the dreaded bumpstop (dumpstop). The thing was, no matter where you went in that wagon; you rarely came into contact with the bumpstop.
The micro-cellular progressive jounce bumper, or simply put, the foamy bumpstop, is actually a supplemental spring. It works to make the suspension progressive, allowing a smooth transition to full compression. What most people don’t realize is that the foamy bumpstop is an active part of the suspension. Let’s say you’re sitting in your M3, 996, Accord, Golf, S4, or Civic. (Hmm I can’t decide). They all use the foamy. Ok, you’re in your brand new Integra, just drove it off the showroom floor and are sitting at the stoplight waiting for the green. The foamy is actively engaged even while waiting at the stoplight.
Car manufacturers use this brilliant piece of high-tech engineering for many reasons. By using an almost zero-weight supplemental spring, you can build the main (steel) spring with a reduced spring rate for a comfy ride. The softer spring uses less weight, and, saving weight is a number one priority of new car manufacturers. Cars have thousands of parts. A little weight saved here and there really adds up. When the manufactures save weight they also save costs, less material=less cost. So we now know why these foamy gems are used. But how do they really work?
As mentioned earlier, the foamy bumpstops are an active working part of the suspension. They work in harmony with the springs and the rest of the suspension components allowing the ride to be comfortable while cruising. When the car is in a handling situation, such as in a turn, they make the car feel firmer for better handling. Think of the foamy as a little progressive spring over the shock rod. All car manufacturer bumpstops have a specifically calculated spring rate and compressed height. (block height).
When a spring is designed the spring rate must be calculated with the rate of the progressive bumpstop in mind. Otherwise the spring will not work in harmony with the bumpstop and the frequency of the suspension will be out sync. The human body interprets a certain range of suspension frequency as comfortable or uncomfortable. Frequency in an automotive chassis is like chords in music. If two or more notes are combined in a pleasing combination you have harmony, otherwise you have uncomfortable dissonance. The same thing happens in suspension tuning. A balanced suspension is important for correct function. This is why it so critical to understand the true effect of the pro-active foamy and the spring. Everything must work together in harmony!
The compressed height is what limits the wheel travel and controls how far the wheel and tire move up into the fender well. Aftermarket suspension designers must consider that enthusiasts will be upgrading the tires and wheels. Therefore, it is vital that these designers take into consideration the needs of tire and wheel fitment. By trimming the bumpstop you allow the tire and wheel to move further into the fender. This can cause the tire to rub where it did not before. If your application does not require bumpstop trimming and you are concerned about tires rubbing, just load up your car with your buddies and drive over some dips and speed bumps. If your tires do not rub with a full load in the car, they should not rub with lowering springs installed.
There is no such thing as a bumpstop in a modern suspension. The bumpstop has been replaced by a supplemental, active foam rubber spring, which is one of the essential components in the suspension.
These terms apply to automotive suspension coil springs.
Active Coils: Coils which are free to deflect under load. Also referred to as working coils.
Approx. Lowering: The approximate amount of chassis ride height reduction. This will vary depending on optional equipment: gas shocks, age of stock springs and upper strut bearings.
Bottoming Out: The suspension reaches its maximum compressed height, not when tires rub fenders or contact the chassis.
Dead or Inactive Coils: Coils, which are in contact with adjacent coils at loaded height. Inactive coils do nothing but give the spring enough free length to stay tight in the spring perches at full rebound.
Deflection: The travel of the spring as it is compressed.
Foamy Bumpstop: Also referred to as a supplemental spring, a progressive-rate urethane jounce bumper will make the suspension progressive. Gives a smooth transition to full compression.
Free Length: The overall length of a spring in unloaded position. May also be referred to as UN-INSTALLED LENGTH.
Full Jounce, Full Compression: The suspension is completely loaded. The springs and bumpstops are totally compressed.
Full Rebound, Full Droop: The suspension is completely unloaded. Example: When the vehicle is on a lift and the tires and wheels are hanging in the air.
Linear Spring: One rate throughout the springs working range.
Loaded Height: Or INSTALLED HEIGHT, is the length of the spring compressed under the load of the vehicle weight, when the vehicle is at rest on level ground. Also may be referred to as STATIC HEIGHT.
To understand scrub, you must first know about Steering Axis Inclination (SAI). The steering axis is the line between the top pivot point of your hub and the lower ball joint of your hub. On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. On a suspension using upper and lower control arms, the pivot points are where the upright connects to the control arms. The inclination of the steering axis is measured as the angle between the steering axis and the centerline of the wheel, so if your camber is adjustable within the pivot points (i.e. Volkswagen) you can change the SAI.
Now back to the subject at hand. The scrub radius is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. If these two lines intersect at ground level, then you are said to have zero scrub. If the SAI intersects the ground at a point inside or outside of the centerline of the contact patch, you are said to have positive or negative scrub respectively.
You now know what elements make up scrub, now you may ask what it has to do with the price of tea in China? Well, the point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability. If you have not already guessed, the easiest way to change scrub is by changing your offset with either new wheels, or hub centric wheel spacers.
If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called ‘squirm’. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes ‘grabby’ causing tire wear to increase and the steering to become unstable.
Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly typically performs well with a lot of SAI and caster, a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.
As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road ‘feel’ increases, as the steering is more susceptible to road shock. Additionally, if you plan on doing some homework on, and modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero.
Well, now you know all about scrub, so the next time it comes up at the watering hole you have the confidence in knowing that your drunken compatriot is having more fun with creative banter than tuning and driving his car. That is not such a bad thing though. Order another beer and go with it. I will bet you can come up with something better.
1) WHICH ARE THE FRONT AND REAR SPRINGS?
FRONT = Springs marked: VA, or (F), or Front
REAR = Springs marked: HA, or (R), or Rear
2) WHICH END OF THE SPRING IS UP?
H&R springs fit the same way that stock springs do. The shape of the spring is most important.
If the spring is the same on both sides and fits the same either direction, then it is correct to install the spring with the closer coils “up”.
If the spring is smaller at one end and larger at the other, the orientation should match OEM factory installed springs.
3) ARE H&R SPRING PROGRESSIVE?
All H&R OE Sport Springs, Sport Springs, Super Sport Springs are progressive when possible for the application.
4) DO I NEED AN ALIGNMENT WHEN I INSTALL SPRINGS OR SUSPENSION?
Alignment should be checked after any suspension ride height modification. Follow the guidelines in the factory service manual to ensure that alignment is as close to factory specification as possible.
5) WHY DO THE H&R PART NUMBERS ON THE SPRING BOX NOT MATCH WHAT IS ON THE SPRINGS?
The part number on the box is the product part number. The part number on the springs is a component part number. If you would like to verify your product/component part numbers, please call H&R at (888) 827-8881. Please have the following information available: year, make, and model of the vehicle application, the product part number found on the box, and the individual spring component part numbers.
6) IF THE H&R SPRINGS ARE RED DOES IT MEAN THAT THEY ARE RACE SPRINGS?
No. Color does not indicate which springs you have. The part numbers printed on the springs and/or spring box indicate whether you have a Race Spring kit or otherwise. If you are not sure which springs you have please call H&R at (888) 827-8881.
7) HOW DO I KNOW THE AMOUNT THAT H&R SPRINGS LOWERED MY VEHICLE?
You should measure your ride height before and after suspension installation. Measurements should be taken from the center of the axle (center of wheel) to the lip of the fender. Refer to the “Lowering Measurements” section found on the H&R Technical Information (bright orange) sheet included with your H&R spring set.
8) DO I NEED TO TRIM MY BUMPSTOPS?
Not unless noted on the H&R Technical Information sheet (bright orange)—or accompanying inserted sheets—included with your H&R spring set.