Suspension travel

Rob09msport

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Oct 28, 2017
1,178
Monroe CT
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Corporal
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Jun 4, 2018
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https://bmw.spoolstreet.com/threads/need-help-with-alignment-on-a-335xi-w-kw-v3-m3-rca-megan-racing-adjustable-rear-camber-arm.3744/unread
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Now that I have my suspension set as high as it can go my springs have 0 pre load but are seated at full droop.
Another important question is how much travel from static height is recommended on our platform? I have front m3 arms I don't think that alters much besides caster and camber though.
So at full droop your spring is seated on both the upper and lower perch but not "preloaded" any? If so that is good. (springs being seated on both perches at full droop is actually a DOT/TUV requirement for OEMs)

How much recommended travel depends on the design of the suspension kit more than the chassis platform. The problem is there is no way to answer the question of how much travel you need. There are too many factors at play.

Person X could tell you they never bottom their 6" spring. Person Y could tell you they bottom their 6" spring all the time. It depends on damper valving, spring design, bump stop design, the stroke position of the damper at ride height, what kinds of bumps you are hitting on the road, weight of the car, list goes on.

I will say a general rule of thumb that I like to use is 1.5" to 2" of bump travel on the street. Again, it depends on so many factors. For example, if you live in a city or where roads are bad you would want 2"+ travel. If you live where roads are nice and not torn up you likely could get away with 1.5" of travel. If you have adjustable performance dampers set to "stiff" you could get away with 1.5" of travel. If you have softer dampers you might want to allow for 2"+ of travel.

In general try to fit as tall of a spring as possible. If you can't, set it up to that a bump stop is actuated before the spring binds. You can trim and stack bump stops to accomplish this.

I'll toss out some random things to note...

Obviously you never want the spring to coil bind or the damper to bottom out (assuming no bump stops). That means your wheel rate goes infinite and you loose traction instantly.

A cylindrical "race" style linear spring is not linear from free height to coil bind. Its linear from free height to roughly 80% of its travel then the rate starts to ramp up quickly the last 20% or so. (differs from spring to spring of course)

The BMW MacPherson strut has close to a 1:1 wheel rate vs spring rate (depends on KPI). Because of this a cylindrical "race" spring in the 250 lb/in to 500 lb/in spring rate range is difficult to setup. They are soft enough to allow lots of suspension movement over bumps but they don't have a lot of travel to coil bind. This is why you see a lot of aftermarket suppliers moving to custom wound barrel springs or tapered springs. Basically they allow a lot of spring travel.

The stock suspension actually relies on the bump stop as part of the dynamics of the suspension. The OEM bump stop has multiple rates (slowly progressive for lack of a better term). It is very soft for the first inch or even two inches, then the force ramps up quickly to prevent bottoming the shock. This way the manufacturer can run a soft spring for ride comfort then use the bump stop for roll resistance. They use tall bump stops so they are actuated as soon as the suspension compresses.

An aftermarket suspension kit typically does not rely on the bump stop as part of the dynamics. They typically use a very small and very stiff bump stop. They are usually progressive as well but much more stiff than a stock bump stop (quickly progressive) and are not used in the same way. They are mostly to prevent an instantaneous spring rate and to prevent damper damage but not to assist in roll resistance.
 

Rob09msport

Captain
Oct 28, 2017
1,178
Monroe CT
So if I change to a 7 inch spring I should get more linear response for the full stroke of my shock? I know standard eibach springs have a useable travel of 3.3 inches 3.8 inch to bind I dont really see how gc could improve on that much with their version of the 6 inch spring. Last 2 things should I try to see if I can go to a 2.25 inch spring if I am changing anyway or no real benefit and in your experience do swift springs really have a more compliant ride. I dont really understand how they could if they are linear springs but supposedly they claim they respond quicker. I would be willing to pay you to go over my setup and tell me if it would be beneficial to change my springs I at this point feel it cant hurt but also have tendency to waste money on unneeded things ,I know you aren't close but I could measure and or give all specs.
 

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Jun 4, 2018
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So if I change to a 7 inch spring I should get more linear response for the full stroke of my shock? I know standard eibach springs have a useable travel of 3.3 inches 3.8 inch to bind I dont really see how gc could improve on that much with their version of the 6 inch spring. Last 2 things should I try to see if I can go to a 2.25 inch spring if I am changing anyway or no real benefit and in your experience do swift springs really have a more compliant ride. I dont really understand how they could if they are linear springs but supposedly they claim they respond quicker. I would be willing to pay you to go over my setup and tell me if it would be beneficial to change my springs I at this point feel it cant hurt but also have tendency to waste money on unneeded things ,I know you aren't close but I could measure and or give all specs.
Yes a 7" spring will give you more linear response through the full stroke of the shock.

I can't imagine the GC spring improving much on the travel, but that is speculation on my part. There is only so much you can gain. Do they post up specs of their version to compare?

2.25" vs 2.5".... This depends but I always try to stick with the larger diameter spring. 2.25" springs tend to bow in the middle when compressed and can rub the threaded collar. Not good for many reasons obviously. 2.25" springs don't gain you any travel, in fact, if you look at the Eibach race spring chart is has less travel to bind than the 2.5"of the same specs. A 2.25" spring weighs less than a 2.5" spring because the coil diameter can be smaller to achieve the same rate. However, add up the weight of the wheel, tire, wheel bearing, brakes, upright, control arms, shocks, etc and your at 80-100+ pounds. A spring that is 8 oz lighter isn't going to get you anything. We aren't dealing with a $500k purpose built racecar. The one reason to go with a 2.25" spring could be if you need clearance for the tire but I prefer to get the clearance in other ways like wheel offset or wheel spacers. There is enough negative scrub built into the suspension to do this with no detriment.

I don't have much experience with swift. I always use Hyperco. I will say I have seen the swift springs deform and have never seen Hyperco springs deform. That was enough reason for me to keep using Hyperco. I can't for the life of me figure out how one spring can be more compliant than another with the same rate. Sure you can use different materials and winding techniques and gain in areas like longevity and weight, but force is force. That's like saying my 10 lb weight weighs less than your 10 lb weight. Maybe, just maybe, it has longer linear rate travel than another brands and that's why people say its more compliant. One day I will have to measure to see.

Post here or PM me pics and measurements, I don't mind reviewing.
 

Rob09msport

Captain
Oct 28, 2017
1,178
Monroe CT
You verified what I thought but i ask cause their is alot I don't understand. I really hope that people read this and it brings you business cause you have been nothing but helpful and haven't tried selling me anything and best of all you have made sure to give advice without bad mouthing another vendor.
 
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You verified what I thought but i ask cause their is alot I don't understand. I really hope that people read this and it brings you business cause you have been nothing but helpful and haven't tried selling me anything and best of all you have made sure to give advice without bad mouthing another vendor even when they may have given an improper setup.
I appreciate the kind words, I'm glad I can help.
 

Rob09msport

Captain
Oct 28, 2017
1,178
Monroe CT
Jay pointed out I may not be able to use a 7 inch spring with my rim and tire combo I will check clearance but this is pic at static loaded height I realized that 300 pounds is prob not being supported by spring so I am prob closer to 1 and half inch compression at static height correct?
 

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Rob09msport

Captain
Oct 28, 2017
1,178
Monroe CT
Also where are you located I would love to have you set up my car with further mods gc is way too far for me I would travel a few states though
 

Rob09msport

Captain
Oct 28, 2017
1,178
Monroe CT
It looks like I am going to 7 inch spring as per Barry my suspension is borderline that is static height. My question is with 200 tw tires what would you guys rec I have 440f 700r currently and 90 perc time it's great but i havent been to a track yet i think its pretty nuetral currently but if I should change my front spring rate now is the time I don't know if I should lower a little to match my wheel rates better or keep same since I daily 200tw and if anything will eventually go r comps when I gain more experience with this chassis.
 

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