M42club.com - Home of the BMW E30/E36 318i/iS
DISCUSSION => Suspension => Topic started by: romkasponka on October 29, 2006, 07:21:48 AM
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I would like to mount spring on shock absorber. So do anybody have expirience with it? Does the mounting place can carry with bigger load?
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The main problem I see is the upper shock mount. It would need reinforcement as the sheet metal sometimes fatigues with just the shocks and solid mounts. If there was clearance and the upper mount was made strong enough, it would work. Funny but now we run stiffer springs on the rear because of the spring moving less than the wheel. With the coilover to the rear of the wheel the spring rates would then need to be correspondingly less. Thus for an actual wheel rate of 150# now requires a 250# spring. With coilovers as described above, you would need aproximately a 125# spring.
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I have seen this done with solid upper mounts and tower reinforcement.
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In addition to what has been posted above - it's alleged that some cars really don't have the whole rear shock tower collapse due to fatigue. Most cars with coil overs are running a cage and the tie-in point obviously helps them. I know of someone who just built a system with the stock location sans reinforcements. We'll see how long it holds up. He works with race cars and claims never to have seen a collapsed rear shock tower...
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VW MK2/3 rear coilovers were dangerously close to bolting into my car :D
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I have seen E36's with cracked/broken towers just from using bilstien sports.
But we all know that E36's suck :p
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Thus for an actual wheel rate of 150# now requires a 250# spring. With coilovers as described above, you would need aproximately a 125# spring.
What suspension frequency do you have?
What do you think about 155 front and 170 cpm rear?
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What suspension frequency do you have?
What do you think about 155 front and 170 cpm rear?
It's a very hard question to answer. I can't say for sure but I get the feeling that there a few folks who think that the rear is actually sprung stiffer than the front on an E30. The springs themselves are stiffer but the efective rate is less due to the location of the spring. There is further complication in that the actual wheel rate is difficult to calculate. I've heard of rates of .65 to .5 for the factor - i.e. 1" of wheel travel is .6" of spring compression. Sway bars also play a factor in spring rates in roll.
I'm spent a lot of time with purpose built competition cars that were very easy to change springs and (adjustable) bars. It's common for them to change springs, bars, and revalve shocks from one track to another. I've also worked with drivers that lied to get you to change the car to the way they wanted it because that's how they liked it.
On an E30 it's quite a bit of work to change the setup and also costs money. I'm now on my third setup and just beginning to get it to where I like it.
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I think it is better to messure effective rate.
I know how to calculate springs for my car, but it is better to know setup which works :)
I thinking about 450 front and 450-500 lbs/in rear with spring on shock if wheel rate in rear will be ~0,9 (frequency would be 155 front and 161 rear).
If rear spring in stock location lets say wr 0,67 it shoul be from 600-700 lbs/in to get the same fequency.
Could you say what spring you have now?
I think there is mistake http://e30m3performance.com/tech_articles/susp-tech/eff_rate/eff_rate2.htm (http://e30m3performance.com/tech_articles/susp-tech/eff_rate/eff_rate2.htm), macpherson struts have wheel rate 1.
Anti roll bars 25 and 21-22 I think would be good.
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Spring rates:
Stock M3 -> 140 -> 250
Dinan Sport -> 172 -> 300
H&R Race -> 315 -> 570
H&R Sport -> 185 -> 380
Eibach Pro-kit -> 102 -> 277
Eibach Race -> 160 -> 445
Ireland Stage 3 -> 315 -> 570
Tokico Kit - >165 - > 265
Adjusted for wheel rate per http://e30m3performance.com/tech_art.../eff_rate2.htm: (front X .94, rear X .67)
Stock M3 -> 132 -> 160
Dinan Sport -> 162 -> 201
H&R Race -> 296 -> 382
H&R Sport -> 168 -> 255
Eibach Pro-kit -> 96 -> 186
Eibach Race -> 160 -> 445
Ireland Stage 3 -> 296 -> 382
Tokico Kit - >155 - > 178
So it appears that the rear is actually sprung stiffer that the front. This says "way loose" looser than the car actually is.
It appears that sway bars play a more important role in balancing the car (stiffening the front more than the rear in roll) than they are usually given credit for and may explain why really stiff setups disconnect the rear bar entirely - to soften the rear in roll in relation to the front.
The average ratio is about (front spring rate X 1.8 = rear spring rate). Right in the ballpark for everything except the Eibachs. Wheel rates are 1.2 to 1.25.
Pick your poison!
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in my setup the roll should be about under 40mm I think. I do not know mass center height, do you know?
in front with 25mm antiroll bar front spring rate become ~685lb/in, so I think it is quite stiff ;)
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There is a formula in several of the chassis books for calculting COG height. One of Carroll Smith's books has one. Unfortunately it requires weighing the car with four wheel scales, jacking up the car to a 45 degree angle, and weighing again.
How did you determine the front spring rate?
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Natural Frequency [CPM - cycles per minute] =
187.8 times {square root of (wheel rate [lb/in] divided by sprung weight [lb])}
Wheel Rate = Coil Rate [lb/in] divided by {suspension leverage raised to the second power}
(Low leverages - MacPherson struts - mean that wheel rates are approximately the same as coil rates.)
Since the tire is also a spring acting in series with the coil, you can take it into consideration by calculating:
Reciprocal value of combined spring rate [lb/in] =
reciprocal value of coil rate [lb/in] + reciprocal value of tire rate [lb/in]
Tire Rate = say 1,500 lb/in
Suspension Frequency values:
60-80 for comfortable road cars
80-100 for firmer and sport suspension
100-175 for non-ground effect racers
Sky is the limit for Formula 1 with minimum suspension movement
In order to avoid uncontrollable pitching, the front frequency is 10 to 20 CPM lower than the rear frequency.
Progressive springs are highly desirable for traction and road holding. Progressive springs will result in a softer suspension at full droop (e.g. on landing after a jump or a bump) and much stiffer suspension at full compression in order the delay (not to completely eliminate) grounding to higher G levels.
When designing the suspension, care has to be taken to divide the useful stroke of the shock absorber correctly between the bump and rebound. Adjustable spring perches do not automatically solve this. For instance, a higher preload on the spring will result in more bump and less rebound.
Special notes:
1. All of the above will be more difficult to do for progressive springs since you do not have a single value of rate like for linear springs and it will prove to be impossible to do it correctly unless you obtain the deflection diagram for each progressive coil from the manufacturer.
2. Typically, in order to get a progressive rate coil with a sufficient crush length, you stack a linear coil on the top a progressive coil. Resulting combined coil rate is calculated as follows:
Reciprocal value of the combined coil rate = the sum of the reciprocal values of the two coil rates
The correctly designed progressive coil will work through the whole useful stroke of your shock absorber.
3. Beware of progressive "helper" coils which are fully crushed at the ride height or sooner! As you can see, they result in a linear rate suspension for the most part of the suspension movement!
Regarding the setup of the shocks:
For rally suspension, the ratio between the rebound and bump resistance is usually 2.5 to 1. For racecars operating on smooth surfaces perhaps 1 to 1. For really bad rally surfaces the ratio can be as high as 5 to 1.
Some rally suspensions, in order to compensate for insufficient stiffness of the spring at full bump (typical deficiency of linear springs) use higher bump values (i.e. rebound to bump 1.5 to 1), since the resistance of the shock in bump acts as an additional spring.
The shocks are typically rated in Newtons at the shock shaft speed of 0.52 meter/second. [Pounds of force x 4.448 = Newtons]
Of course, there is a range of values for rebound and bump for corresponding shaft speeds. Unfortunately, this is rarely volunteered by manufacturers but can be obtained easily on a shock dyno.
Regarding a 2,300 lb car:
You have not given the weight distribution.
I am not familiar with two wheel drive cars. It seems that 250 lb/in spring rate in front will work. However, the 110 lb/in in the back may be way too low (unless your weight distribution is 70/30 or worse) and will give you a much lower frequency in the back than in the front (highly undesirable) and severe bottoming out problems in the back.
(Yes, I know that these rates were published this year by a certain magazine in connection with a front wheel drive car that will remain nameless.)
Of-the-shelf racing shock absorbers, such as Bilstein 300/120 and 270/100 [deka Newtons stamped on the body of the shock] should work well for you and if you find in the future that you need different valving, Bilstein will revalve them for you at $55 a piece. This would require extensive testing - look for the guidelines in the literature listed below.
Literature:
Race and Rally Car Source Book, Allan Staniforth, Haynes, 1993
Car Suspension and Handling, Donald Bastow and Geoffrey Howard, SAE, 1993
Performance Handling – How To Make Your Car Handle, Techniques for the 1990s, Don Alexander, Motorbooks International, 1991
How To Make Your Car Handle, Fred Puhn, HPBooks, 1981
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Are there any isses having both the spring and shock exerting all that force on the one small bolt-point on the trailing arm hub? The top definitely could use reinforcement IMO, but what about the bottom?
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I think there should be no problem.