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Should new Bilstein sports feel this still.

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  • stevesingo
    I have been researching suspension options for some time now and I will share my thought/findings.

    The stock E30 M3 Spring rates are 100lb/in Front and 300 lb/in Rear based on what an ex user here discovered after having had a set tested. It would seem that the rear is much stiffer than the front, but
    The springs are not mounted on the same plane as the centre of the wheel. In the case of the front, as the spring is inboard of the wheel for every 1” of wheel travel we only get 0.926” of spring compression due to the relative positions of the pivot points and the spring. At the rear, as the spring is mounted much further inboard towards the chassis mounting point, for every 1” of rear wheel travel we only compress the spring 0.67” The motion ratio also effects the force acting on the wheel required to compress the spring. This is calculated by using the following formula;

    MR*MR*Spring rate=Wheel Rate

    Where MR=Motion ratio

    Front- 0.926*0.926*100=85.75lb/in
    Rear- 0.67 * 0.67*300 = 134.67lb/in

    When you take in to account the motion ratio of the suspension system, the difference is not so great, a ratio of 1.57:1 Rear:Front.

    Note: For those with coil over rear springs the motion ratio is 1.06; this will be important when it comes to dampers.

    So, as stock the E30 M3 has rear wheel rate which are 57% stiffer than the front.

    Something which effects the ride quality is the natural frequency of the suspension system. This is related to the sprung mass (Axel mass-un sprung mass) and the spring rate.

    sqrt(wr/((m*0.5-um)/1000))/2/Pi= Frequency

    sr=spring rate (N/mm)
    m=weight on axle (kg)
    um=unsprung mass for one corner (kg)
    Lb/in / 5.7=Nmm

    If we take our stock springs and some mass figures I pulled off the net:

    Front axle weight 660Kg
    Rear axle weight 630
    Front un sprung mass 41.3
    Rear un sprung mass 43.5
    Front WR in Nmm 85.75/5.7=15Nmm
    Rear WR in Nmm 134.67/5.7=23.6Nmm

    To save me the long hand it comes out at

    Front 1.19Hz
    Rear 1.53Hz

    The ratio between the two is 1.28:1 Rear:Front.

    There is a good reason for the rear being of a higher frequency than the front. If the front axle hits the bump first (hopefully J), in order for the car to remain flat, the rear needs to catch up, so therefore it is normal for a road car to have a higher frequency.

    I would suggest for a road car, we shouldn’t stray too far from these ratios. For a track/race car, what is best for lap time will dictate.

    As it turns out, keeping the unsprung and sprung mass the same, the ratio front to rear and the ratio of frequency front to rear are interdependent; increase the spring rate by the same front and rear, the frequency increases by the same proportion.

    My practical experience from using H&R Sport springs, it is worth mentioning at this stage that the part numbers for H&R sport springs differ in the US from those in Europe.

    The USA H& web site lists the following for the E30 325i

    50404-55 OE Sport lowers 1" Fr & Rr
    50406 Sport lowers 1.3" Fr & Rr

    For the E30 M3

    50404-55 OE Sport lowers 0.25" Fr
    50406 Sport lowers 1" Fr & 0.75" Rr

    Same parts! I don't know what the weight distribution is Fr/Rr on the 325i, but with an engine that I guess is heavier and more of that engine in front of the front axle line, it would not be too much of a stretch to believe there would be a requirement for a stiffer front spring.

    In Europe we have

    29663-1 for 4 cyl models lowering 35mm or 1 3/8”
    29664-1 for 6 cyl models lowering 35mm or 1 3/8”

    Luckily, in Germany all aftermarket parts need to be approved with the TUV and have a certificate which is available for download.

    The above springs share the same TUV certificate, on which certificate it states spring dimensions.

    The rear springs are the same 204mm long 6.5 turns of 13.5mm wire. The front springs are 272mm long, 6 turns, but differ in the wire diameter, the 29663-1 being 12mm and the 29664-2 being 12.5mm. Presumably this is to account for the heavier 6 cyl engine.

    Why in the US the same springs are cold for both I don’t know.

    I had a set of 29664-1 fitted to my car and found the front ride height to be too high and the front ride to be very stiff and the rear to be very soft. I removed the springs and had them tested.

    Fronts are 110lb/in-205 lb/in rising rate, hitting 205 at 5in compression

    The installed length was approximately 145mm of 5.7in and the free length is 245 mm or 10in. If we take 5.7 off 10 we get 4.3in compression when installed, so nearly 205lb/in when installed.

    Rears are 259lb/in linear-softer than stock which explains a lot.

    If we plug this in to the spread sheet we get wheel rates of:

    Front 176lb/in

    Rear 116 lb/in

    Ratio of 0.66:1 Rear:Front- substantially softer at the rear than the stock 1.57:1 Rear:Front

    Natural Frequency ratio of 0.86:1 Rear:Front again very different to the stock of 1.28:1 Rear:Front

    This would explain why the rear felt so soft on both road and track.

    I suppose the question in relation to the OP is, what is the wire diameter, number of turns and free length of the 50406 springs and are they the same part under a different number to the 29664-1 sold in Europe?

    Myself, I wasn't happy with the off the shelf springs available, so went for some custom wound springs of 155lb/in front and 435lb/in rear which gives wheel rates ratio of 1.47:1 rear:front and frequency ratio of 1.24:1 rear: front. The ride is much flatter than the H&Rs. I still believe the front reacts much more than the rear when hittin high speed compressions so I am going to look in to the dampers next.

    I’ll come on to damper rates later.

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  • stevesingo
    I had a set of H&R Sport Springs tested. Much stiffer front than stock, and softer rear than stock. I believe these springs were disigned for the 6cyl cars, and sold for the M3.

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  • autox320
    Well just reworked the struts on another 91 318is which is very similar sharing the same as the M3. I've my old M3 springs on this car which work great btw. Anyways popped off the front struts cause I never trimmed the stops on it. It's new maybe 500 miles before parking and we are building an engine for it (go M42 pwr). The bilstein bumpstop had the first segment crushed in pieces inside where it was slamming the stops. Trimmed off the two knuckles like I've done on the M3 using HR race springs. It's simply too long a stop for application.

    This should be part of the instructions for use of bilstein sports with short springs IMO. I've been trimming the stops since learning that's what specE30 does. They run HR race for the class. It works and track proven.

    Just another note in here but the HR sport springs are barely stiffer than the stock M3 spring. Only real higher rate stock spring is the HR race. Group N's would be cool if can find any but need custom bilsteins to handle the rates. I use a GC true coil over kit on a E21 that the rates above 400 lb springs will eat a bilstein sport strut without revalving. With the .63? rear ratio of the stock e30 spring location just keeps within limit.

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    The Bilsteins are fine. I still blow through all the travel (that makes for a harsh ride) over potholes and railroad crossings. It was worse with the Konis. I guess another 150# of spring rate front and rear (from the H&R race springs) would be ideal. My suspension geometry is pretty darn close to correct and I still don't have enough spring rate to keep the car off the bump stops over many road imperfections. (Its a lot better than it was however.)


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  • Kevin///m3
    Happy this discussion is going on. I'm in the process of planning a pretty comprehensive restoration of my M3, and I don't want to cut the struts.

    Likely going with H&R Race springs and want a street friendly shock. Peoples comments on the Bilsteins are scaring me a bit. Wonder if Konis are more street compliant?

    Car could possibly see an HPDE day but will be 99% street.

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  • autox320
    If running bilstein sports, you should trim the internal stop to keep travel especially in the front. If have the insert strut in hand it's a simple process. The bottom silver plate can be tapped off with a srewdriver and hammer. The shaft will stick out and reveal the stop on it. Trim up to two knuckles of the three off with a razor knife. Stick the cap back on and tap the bottom.

    I run HR race and bilstein sports. Vorshlag front plates. No spring pads up front, and stock ones in the rear. No rear bumpstops (never bottoms in the back). BW spec e30 bars. Completely streetable to me and sees track time. Sure there are other suspensions, but IMO only gain about %10 improvement over a HR race setup. The only real setups with more than a %10 advantage are if like mentioned, caged out, and true coil over beyond a moton like a MCS triple comes to mind.

    I like the GC kit myself and ran it on previous cars, but the HR race is plenty without any setup hassles really. It's one of the areas people question me all the time at the track "what coil-overs you running". They are surprised to find off the shelf HR race on there.

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  • stevesingo
    Bilstien B8 dampers

    Springs 150lb/in Front and 450lb/in rear

    Front - 9 5/8" long 100lb/in for the 1st inch, 150 there after.

    Rear - 7 1/8" long 265lb/in for the 1st in and 450 there after.

    Uprated sway bars 24mm front 18mm rear

    Leave a comment:

  • ttw18
    You have a legitimate concern about irreversible modifications. Unmodified struts are hard to come by as they are unique to the M3. A new set is expensive, if they are even available.

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  • Konig
    I spoke with Ground Control today trying to understand which of their offerings best suits my application. I am comparing their Street Complete Coilover Kit, PN 2020.71 ( to their Complete Coilover Suspension, PN 2020.76 (

    As mentioned in an earlier post, I'm looking for slight lowering (will be going to 17s) and a sporty street ride. I am hesitant to modify the stock strut housings, because I don't like doing anything irreversible to my cars. I narrowed it down to three options:

    1. Street suspension, no modification to housings
    2. Complete suspension, no modification to housings
    3. Complete suspension, shortened housings

    I'm looking for advice on this from persons with experience with some/any of these kits. For street application, is it worth stepping up to custom-valved Konis? Is bottoming out a legitimate concern if I do not shorten the strut housings? Are camber plates beneficial on the street? Should I just suck it up and modify the strut housings?

    Much appreciated...

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    commented on 's reply
    Ok, based on this, we are talking about two different ride heights. I'm talking about body height, body height will change with an RCS. Think of it as a triangle, the bottom is the A arm, and it can pivot. Assume we are looking at the left side of the car head on. I'll say the right point of the triangle is the body, the two points being the inner pivot (lets call it "A"), and the top being the strut mount in the body (call that "B"). The outer end of the control arm on the bottom is "C" Now, the left side of the triangle is the strut. If you shorten that arm, while leaving points A and B fixed (since they are the body), C will have to move upward for it to attach to the strut. Assuming this takes place on both sides, the ride height of the body will be lowered because the distance between B and C has been shortened, and the distance between C and A is fixed. Obviously, going the other way, making the left side of the triangle longer, the base of the triangle will drop because the distance between B and C is now longer. Since the wheels keep everything off the ground, the longer arm between B and C now cause the control arm to slope upward toward point A. Point C is now lower than point A, so the body must rise since points A and B are fixed. Anything added between B and C will raise the ride height of the car. Adding taller wheels/tires will also, but lets not worry about that since it makes no change to the geometry of the points A,B, and C. The distance between B and C dictate both roll center and ride height. The RCS still sits above the lower control arm and above the pivot there. The pivot point at A doesn't matter, it just allows things not to break since the distance between B and C is variable (the strut) Anything that mounts between B and C will affect ride height and roll center because that is the one side of the triangle that changes. A and C are a single piece that does not change distance, A and B are the same, they do not change either. Only the distance between B and C changes therefore camber/caster plate stack height, spring height, spring hat and rubber spacer height, and RCS all affect ride height. This is what I ran into with my car. I changed from the stock strut mounts to camber/caster plates that had 1.5" less stack height, and went to struts (Konis) that were again, shorter than the Bilsteins, (so even topped out, they were shorter than the bilsteins under load. When they were loaded, the car dropped more) and I added H&R race springs that again were shorter than what they replaced. The next thing I know, the car is bottoming out the suspension over the smallest imperfection, even though the spring rate is much higher and its so low that I ripped the splitter off driving through a small puddle. The only part of the equation that changed was the distance between B and C.

    So, what did I do to fix it? I started with spacers that fit on top of the camber plates (had to weld longer mounting studs into the plates to make them work) That effectively made the stack height the same as the stock mounts. I found a set of Bilstein sports that were longer than the Konis, that also gave me a little compression damping (its harder to compress the bilsteins than it is the Konis, even with them set full hard) This again lengthened the distance between B and C forcing C to drop from above A to equal to A under load. I then added the 13mm RCS again making the distance between B and C longer, dropping C relative to A. Now, the control arm between A and C slopes downward toward point C thereby raising point A relative to the ground. This is how RCS spacers work. They make the distance between B and C longer there by raising the car and the roll center.

    This is why cranking the preload on a set of coilovers to the moon fails to raise the car. All you are doing is compressing the spring because the distance between B and C can't become longer if the strut is already topped out. Its as long as you can get it. Yes, you can lower the car by screwing the adjuster down because the strut will simply compress in its travel since the adjuster has been moved down the body. Gravity has shortened the distance between B and C because the spring just sits lower on the housing. B is still a fixed point. It still weighs X pounds. All you have done is lessened the amount of available suspension travel because the strut is already sitting partly compressed, and raised C relative to point A.

    Does this make sense?


  • ttw18
    I'm with AlpineRunner, the RCS doesn't effect ride height. As he said, it lowers the control arm connecting point, increasing the angle of the arm. It doesn't add to "stack height" as the spacer is below the hub. The distance between ground and hub is fixed and controlled by your wheel and tire combo.

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    commented on 's reply
    Well, after I added the 13mm spacers on my car, the splitter went from not clearing parking curbs to clearing them. I made no other change. Now I can pull into a parking spot and the splitter just clears the top rather than hitting. Either way, the length of the strut assembly increases by adding them (effectively). The angle of the control arm has changed relative to the ground, if its level with the ground, the car will sit lower, if it slopes outward toward the wheel, the ride height will rise because the geometry has changed (or drop if the strut length gets shorter because the control arm will now slope inward toward the oil pan) The height of the wheel stays the same, but the body will rise or fall based on the length of the strut since the control arm length does not change, it just pivots either up or down.


  • Konig
    Grinder , here is the response I received from GC this morning. My question first:

    I'm looking for a sport-street suspension. While the full coilover suspension (SKU 2020.76) is appealing I want to keep everything reversible, so modifying the strut housings is not an attractive option. Is the coilover kit (2020.71) a good fit for my application? I see that there is ride height adjustment for the rear, is there any for the front? How much of a drop should I expect? Will my application require camber plates to achieve correct alignment?


    Thank you for contacting us. The suspension kits that you referenced both have front and rear ride height adjustment. Kit #2020.76 is able to be installed into your stock strut housings. Shortening your strut housings is optional. When installed into standard length housings, both kits have the ability to lower up to -2.2". Our camber plates are designed to add negative camber, unfortunately if you need to add positive camber our plates will not work. Please feel free to contact me if you have any additional questions.

    For reference, this is 2020.71:

    and 2020.76:

    Two questions I have:

    1. Did he mis-type in his response, and mean to say 2020.71 instead of 2020.76?

    2. Does 2020.71 actually offer front ride height adjustment?

    Leave a comment:

  • AlpineRunner
    Will, I still disagree. What effects ride height is the distance between the wheel hub and the top of the strut. That is the ground point of the system. The roll center spacer is below that point thus has no effect on ride height. If the strut tube would shoot to the ground below the spindle it would have no effect on ride height. Picture unbolting those 4 steering knuckle bolts to remove the roll center spacer while the car is on the ground. What would happen? Nothing (assuming you didn't give it a push to the side). I could be wrong but I'm pretty convinced in my head.
    I agree with all your points about the effect of the roll center and lowering it too much. You want to minimize "roll couple/moment" and when you lower a car, that gets larger. Hopefully your springs counter it, or you are able to raise the roll center.

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    commented on 's reply
    Yes roll center spacer DO have an effect on ride height. The distance between the top of the camber plate and the control arm is what controls ride height and ground clearance. If you have one that measures (call it) 30 inches and another that measures 28 inches point to point, the 28 inch one will have a lower ride height (not the actual distance, just using the numbers for example). Its a set distance. If you need X distance from the strut mounting point to the control arm, raising or lowering that number will either raise or lower the front of the car. If you have one assembly that is 30 inches, it will cause the control arm to sit at X degrees to the ground. If you have one that is 28 inches long, the control arm will slope upward toward the wheel at Y degrees to the ground. The goal is to have the control arm sloping just slightly upward toward the oil pan/inside of the car. That will ensure your roll center is above ground. Yes, lowering the CoG is a good idea, but not below the point where the roll center does below ground level. Once RC goes below ground level, things go to hell very quickly.