Ideal Coilover Spring Rate- PLEASE HELP!!

[RU2SLOW?]

Here's another 1G coilover question for everyone.... I've been looking at the D2 coilovers that Martin is selling right now and he has told me that he can get any spring rate that I desire for the setup, which is awesome. But....I'm really nowhere near a suspension expert and I was wondering what the ideal spring rate would be to order for my needs- this is where I was hoping some of you suspension pros could help out. I have a 91 TSi FWD with GC coilovers from Nopi and KYB AGXs all around. I HATE this setup! The suspension bangs very harshly on even moderate bumps and turns on my wipers on even minor bumps/holes in the road. Essentially the ride is unbearable. My car is predominantly a street car that I plan to race at the drag strip about once or twice a year. I would like it to handle very well in corners- well enough that I could even compete on some casual road course events- but still retain the ability to take a tolerable cruise, and stiffen up my rear suspension for straight-line launches when need-be. This is what my car will be used for and please keep in mind that I don't want a luxury ride, but I am looking for something that doesn't feel like I'm getting kidney punched every 2 seconds whenever I drive. I lower my car a good amount also- 2.5"-3" incase you need to know that as well. Thanks for any help you all can offer- I appreciate any comments and suggestions.

 

[RU2SLOW?]

So I've done some research on other car's forums (240sx, Sentra, Camry) and from what I can gather 7kg in the front and 5kg in the rear seems to be the most common spring rates used for coilovers on aggressively tuned FWD "street" vehicles. However, these rates are nowhere near car specific, and I'm looking for some kind of answers and/or suggestions based on DSMs (1G FWDs) in particular. I also want to be sure that these rates will be an improvement in ride quality/comfort from my current setup as I previously stated...

Also, if anyone can help me by telling me how to convert the single digit kg (7 kg)numbers that people speak of in terms of spring rates to the lbs. figures that are all in the hundreds (450 lbs) I would appreciate it.

 

[RamenPride]

I lower my car a good amount also- 2.5"-3" incase you need to know that as well.

that's most likely your problem right there. with that much of a drop you've essentially eliminated almost all of the suspension travel.

 

[RU2SLOW?]

Yes, I understand that lowering the car that much will eliminate a good deal of suspension travel. However, I also have been made to understand that coilover kits do exist that allow for considerable suspension travel to still exist even at lower ride heights like the range I mentioned (JIC is one setup). I also have been made to understand that components such as GCs and AGXs leave a GREAT deal to be desired when it comes to adjustability and maintaining a level of ride safety (having the car bounce all over and dart across lanes because of bumps in the road) and comfort. That's essentially what I'm looking to improve on- hence my question about ideal spring rates for my desired setup with my desired ride height in mind.

 

[RU2SLOW?]

So after doing some more research I found the formula for converting spring rates listed in kg to their equivalent rates in lbs. I found it on a suspension website by doing a search in Yahoo, so I'm not exactly sure why the formula is what it is, but I cross referenced it as much as I could and the numbers seem to make sense. In this example (just to make things a little easier) I'll use an 8 kg spring rate-

8kg x 2.22 (lbs in a kg) = 17.76 lbs in a mm ---THEN---
17.76 lbs in a mm x 25.4 mm in an in. = 451.104 lbs in an in. ---THEREFORE---
an 8kg spring = a 451 lb spring

***keep in mind that these conversions don't always come out to be exact so you may have to round a bit to get the exact spring rate in lbs that's available on the market***

So now that I can compare spring rates of the various kits out there by converting kg to lbs and vice versa, things become a tiny bit easier, but I'd still LOVE to get some suggestions on what an estimated ideal spring rate combination would be for what I'm looking to do with my car.

PS- If anyone reads this post and doesn't agree with this formula or knows of a better means of conversion, please let us know- I think a bunch of people on here may be able to benefit from knowing the proper way to do this if my suggestion isn't it.

 

[Toshiro]

I'm not much of a suspension expert either, and I have a 2G FWD, but I think your main problems are the amount of lowering you have, and the shocks you're using, which IIRC are close to stock-length and not very good for really low applications. Another reason I say this is that I run pretty stiff spring rates on my GST (500lb front / 450lb rear) and Koni Yellows, with my car lowered about 1.5 inches, and my ride is pretty smooth and comfortable, at least by my standards.

 

[RU2SLOW?]

Thanks for the advice and the info- I always appreciate suggestions based on personal experience. I know what you're saying and I understand that there is no "perfect" soltuion for what I want- especially given the limited amount of suspension travel that I'm allowing myself- but I'm still looking for any kind of suggestions on spring rates that would be up to this task with the D2 coilover setup.

 

[RU2SLOW?]

As I go through and research and find more things out for myself, I'm just going to add to this thread in an attempt to either spark up some thought with other members or help someone else out who may be able to learn from my situation.

After looking over different recommended spring rates in older threads for various setups on different DSMs, I used the conversion formula I posted above and I think I have a guess-timate of what spring rate may be best- 8 kg in the front, 6 kg in the rear, which would translate to roughly a 450 lb spring in the front and 340 lb in the rear. The problem with this estimate though is that with a lack of understanding with really technical and intricate suspension concepts my idea is still just a guess. Again, please feel free to shed any light on this subject.

 

[MNGSX]

Good because quite a few people who auto-x very competatively said I should run 450/350 so I do.

 

[RU2SLOW?]

Thanks for the reply MNGSX- that's pretty reassuring. If you don't mind me asking, how much is your car lowered?

With the more research that I do the more confident I'm becoming with my above spring rates being a really good place to start for performance- I think upping the rear spring rates may allow the car to get loose a little too easy for my liking, and if I decide that I want my setup to be that aggressive later a rear sway bar should do the trick. However, I'm still concerned with whether or not these spring rates will be stiff enough to keep things from bottoming out, since I plan on lowering the car so much...I guess this has become my major concern.

 

[MNGSX]

About 2.5" but I have raised p-ball mounts and hats

 

[NDgsx]

Comparing 1G and 2G spring rates is useless, they have a different mechanical advantage and so for the same spring rate the wheel (which is all we care about) will se a different rate. In laymans terms 2Gs will require a higher spring rate than 1Gs for the same suspension stiffness.

 

[RU2SLOW?]

Comparing 1G and 2G spring rates is useless, they have a different mechanical advantage and so for the same spring rate the wheel (which is all we care about) will se a different rate. In laymans terms 2Gs will require a higher spring rate than 1Gs for the same suspension stiffness.

Point well taken. At the same time, the spring rates that I came up with seemed to be consistent throughout 1Gs and 2Gs when I searched through past threads- 450-500 lbs on the front springs and 350-400 on the rears. I understand that there are huge differences in the suspension makeups of the 1Gs and 2Gs also, but the numbers I mentioned also matched up with what people have listed for the JIC 1G FWD setup (8kg/6kg) which I've read almost all good things about. I know that there's an exact science to this (measuring unsprung weight and so on), but in general terms am I piecing information together improperly or wrongly assuming something?

 

[MNGSX]

Maybe springs only come in 50lb increments so.. If one cars optimum is 380 and the other 410 everybody runs 400lb. Just a guess. Honestly I use what the track guys told me to and it works.. Thats about as far into auto-x as I get.

 

[NDgsx]

or wrongly assuming something?

Yes, you're assuming that what the majority of people out there are running the ideal setup

 

[DG-FNR]

OK, here's how to work out the ideal spring rate for a given setup. Mods, please make this post sticky or something so that I don't have to do this again.

What you need to do is work out the natural frequency of the suspension.

In order to do that, you need to know:

1) The corner weights of the car as raced. For a street car, you want to weigh the car twice - once with the car empty (low fuel and driver) and once heavy (full fuel, driver, passenger, and the max cargo load you ever expect to see)

To do this, disconnect the sway bars, and then weigh the car on a proper set of race scales. If you don't have race scales, contact your local SCCA region - they can put you in contact with someone who does.

2) The unsprung weight front and rear (one front corner and one rear corner) To do this, remove the shock/spring assembly from one corner front and rear, reattach the wheel, and with the sway bars disconnected and the chassis supported on jacks stands, stick the scale under the wheel. A bathroom scale can suffice here if you're in a pinch.

Weigh the shock/spriung assembly too. The unsprung weight is that corner + 1/2(shock/spring)

3) The motion ratio on one front and one rear corner VERY IMPORTANT! You CANNOT skip this step!!

The wheel has a mechanical advantage on the spring and shock. If you imagine the suspension as a wheelbarrow, the wheel is on the handles, the spring is in the bucket, and the pivot point is the wheelbarrow's wheel. The actual spring force felt by the wheel is LESS than the amount of force the spring is producing, and what we care about is the spring force (ie, the rate) at the WHEEL, not at the SPRING.

What you need to do is measure the amount of spring travel per inch of wheel travel. You do this by getting a pair of dial indicators (this needs to be accurate to at least 0.01") and seeing how much the distance between the spring perches changes for each change in wheel travel. I like to check it every 0.1" over about a 3" range, starting at full bump and working down.

Measuring this can be a real pain in the ass, but it's absolutely necessary.

Once you have this information, you can calculate the natural frequency for any given spring rate. What's more, you can also estimate the amount of wheel travel the car will see with any given spring rate.

(The formulas for doing the calculation are in Miliken or in Staniforth's Competition Car Suspension - see http://autocross.dsm.org/books.html)

For a street car, a natural frequency of about 1.8Hz in front and 2.0Hz in rear is about right (you want the rear a little higher to keep the car flat when encountering bumps - bumps hit the front first and then the rear, so the rear has to react a little bit faster) Select springs accordingly.

Now that you have your ideal spring rate, you can estimate wheel travel as well. Adding the rear corners to each front corner gives you the maximum possible front load (you're doing a stoppie) Adding the left sides to the right sides gives you the maximum possible right load (the car is up on 2 wheels) The delta corner loads over the wheel rate (the spring rate at the wheel) tells you the maximum dynamic wheel travel - make sure that once you've finished lowering the car that you have this much travel BEFORE hitting the bump stops - and note that this is a MINIMUM. Fudging a little bit high is not a bad idea.

Simple!

DG
Learn: http://autocross.dsm.org/books.html

 

[RU2SLOW?]

Wow

Now that I'm on information overload I just wanted to thank you guys for contributing to this thread- especially DG-FNR & NDgsx (for the PMs). I guess there is no general answer like I thought there might be. I was hoping that I could get a baseline idea of what rates would hypothetically suit my needs best- the fact that suspension is a science in its own probably should have tipped me off differently though.

I wonder then how so many coilover companies out there can offer OTS coilover sets with different available spring rates and be able to recommend which rates will suit different people's needs best without completely throwing off the suspension geometry of all of these people's cars? Would I be wrong in guessing that up to 3/4 of the people out there running coilover kits most likely don't even have the proper setup for their cars?

Guess it's time to start doing some hands-on research into my own car so I can figure out what the hell spring rates I need.

 

[NDgsx]

Would I be wrong in guessing that up to 3/4 of the people out there running coilover kits most likely don't even have the proper setup for their cars?

A LOT of modified cars are setup ass backwards. Getting the true ideal setup takes a great amount of researched, follow by a lot of R&D. For your average street car (like yours) you can get a decent setup, as far as the spring rate for ride quality, without a ton of work.
If you need it I have the unsprung weight for 1Gs written down somewhere, I can look it up for you. I do not have the rear unsprung weight for FWDs though.

 

[prostreetdsmx1]

sorry i didn't chime in on this one earlier but i just stumbled across it. I did speak to D2 quite extensively about this and they are sticking to their guns as far as what they set the standard rates at. They said that after all the testing that have done the standard rates were set considering lowered heights, daily use and comfort. According to them their standard settings are what they recommend.

 

[aleccolin]

So after doing some more research I found the formula for converting spring rates listed in kg to their equivalent rates in lbs. I found it on a suspension website by doing a search in Yahoo, so I'm not exactly sure why the formula is what it is, but I cross referenced it as much as I could and the numbers seem to make sense. In this example (just to make things a little easier) I'll use an 8 kg spring rate-

8kg x 2.22 (lbs in a kg) = 17.76 lbs in a mm ---THEN---
17.76 lbs in a mm x 25.4 mm in an in. = 451.104 lbs in an in. ---THEREFORE---
an 8kg spring = a 451 lb spring

***keep in mind that these conversions don't always come out to be exact so you may have to round a bit to get the exact spring rate in lbs that's available on the market***

So now that I can compare spring rates of the various kits out there by converting kg to lbs and vice versa, things become a tiny bit easier, but I'd still LOVE to get some suggestions on what an estimated ideal spring rate combination would be for what I'm looking to do with my car.

PS- If anyone reads this post and doesn't agree with this formula or knows of a better means of conversion, please let us know- I think a bunch of people on here may be able to benefit from knowing the proper way to do this if my suggestion isn't it.

This is close, but not exact. The spring rate is just that, a RATE, given in kg/mm. Converting it to imperial yeilds lb/in. The conversion goes like this:

kg/mm x 25.4mm/in x 2.20462 lb/kg = lb/in

As it works out, it's a simple factor of multiplication of 55.997348, or basically take the metric rate (kg/mm) and multiply by 56 to get the rate in lb/in. Conversely, divide the lb/in rate by 56 to get the rate in kg/mm.

*Note, if the rate is given in N/mm, first divide the force in newtons by 9.81 to get a number in kilograms/mm first.