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Adding a damper per suspension corner


CBraden
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Let's say you built a car that has no damper options available that would control a much firmer spring and you want to run a much firmer spring. Let's also say you like to fabricate things, and could feasibly add a damper to each corner of the suspension (meaning there would now be two dampers per corner, with one spring). Lastly, let's say it is EC so points do not matter, and this question is purely technical.

 

What are the main considerations you would weigh to decide if you would pursue this path, and if so, what would you look for in that second damper (since you are fabricating mounts for it, you can easily use a wide range of dampers and can choose by damping characteristics and stroke)?

 

If you need specifics, let's say your original front dampers are good to about 350lb wheel rate springs, and you want to run 700lb front wheel rate, and springs are readily available to do this, so the only question here is how to control that spring rate... (also please assume you can't just replace original damper with one from another car because it is a strut and it serves more than one function).

 

Thanks for your expertise in advance.

Edited by CBraden
typo that confused a main point. Edited before any replies...
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I'd start with trying to find an appropriate strut insert that was 2X and able to handle 700 lb springs, and see if I could fit that inside the stock strut housing. 

 

For mounting an extra shock, it would be best to get the lower end of the extra shock out near the lower ball joint (most important) and try to keep the angle as close to vertical as possible.  Least loss due to motion ratio and angle. Poor design of the mounting can half the effectiveness pretty quickly. Don't forget that you have to square the motion ratio then use the reciprocal to get your actual wheel damping rate.

 

Another possibility is to see if you can tie into the front sway bar ends or with a link off the side of the existing strut. The direct mount/link to the strut is the most effective.

main-qimg-cd64c9bdc0162f6f43c2945112f716d8See the source image

Edited by mender
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The front is a strut. It does not have a separate housing (and I would not be that comfortable with significant cutting off and re-welding to strut tubes to adapt one from another application - I could do it, but before I would choose that option, I would go another way... like another damper). The strut housing bolts directly to the knuckle.

 

Mender - thanks for the input, the mounting location and math is helpful. Can you help me understand why you are bringing up the sway bar mounting? Is it related to another damper? Sorry - I am not making the connection. Are you saying mount the lower of the additional damper in same location as the sway bar end link anchor?

 

Thanks

On 1/11/2020 at 5:18 PM, morganf said:

Should work that's what the 2x rule says r9ght? 😜

There was a post on grm once where someone did this on a GM minivan with strut originally and they said it worked great.

I searched GRM, but apparently not well enough...  Thanks

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I'm not sure you understand how a cartridge in a strut works.  You literally hack off the end of the strut and bilsteim makes inserts that go into your existing strut.  It's not hacked together or anything that is how they are designed.

 

Google bilstein strut insert.

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4 minutes ago, CBraden said:

Are you saying mount the lower of the additional damper in same location as the sway bar end link anchor?

Yes. Or mount the lower end to the end of the sway bar. Either way, the shock will tie into a reinforced connection to the lower control arm. Part of the problem is hooking into the suspension without interfering with the steering or brakes. Depends how much room you have around the strut/spindle.

 

If I know what suspension you're working with I can be more specific. :)

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3 minutes ago, wvumtnbkr said:

I'm not sure you understand how a cartridge in a strut works.  You literally hack off the end of the strut and bilsteim makes inserts that go into your existing strut.  It's not hacked together or anything that is how they are designed.

 

Google bilstein strut insert.

 

I kinda do, and one option is to hack a receiver tube from an old strut and use inserts. e36 struts... they look like this:

V364058.jpg

When I looked into ID of these, and OD of inserts... options were very limited (and nothing that was for 700lb springs).

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10 minutes ago, mender said:

Yes. Or mount the lower end to the end of the sway bar. Either way, the shock will tie into a reinforced connection to the lower control arm. Part of the problem is hooking into the suspension without interfering with the steering or brakes. Depends how much room you have around the strut/spindle.

 

If I know what suspension you're working with I can be more specific. :)

 

Well, for the sake of this discussion, it looks very much like an e36:

E30_front_suspension_diagram.jpg

The lower control arms are L shaped.

 

Also, assume I am stubborn as all hell and generally build things not everyone thinks is a good idea (the whole car matches this fairly well), then learn my lesson only after failing a few times. So while I know I can buy a solution to this problem, I am looking at possible hacks to get there... I have a pretty good idea where I would put the upper and lower end of the second damper in the front - just inboard of the strut lower end on the control arm (this would actually be in between the sway bar end link and the strut). It would be a pita, but it looks do-able.

 

 

 

 

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43 minutes ago, CBraden said:

 

Well, for the sake of this discussion, it looks very much like an e36:

E30_front_suspension_diagram.jpg

The lower control arms are L shaped.

 

Also, assume I am stubborn as all hell and generally build things not everyone thinks is a good idea (the whole car matches this fairly well), then learn my lesson only after failing a few times. So while I know I can buy a solution to this problem, I am looking at possible hacks to get there... I have a pretty good idea where I would put the upper and lower end of the second damper in the front - just inboard of the strut lower end on the control arm (this would actually be in between the sway bar end link and the strut). It would be a pita, but it looks do-able.

Doesn't have to make sense to anyone else, journeys are an end in themselves. ;)

 

Your location should be strong enough, and I assume you can tie into the strut tower structure for the upper. 

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19 minutes ago, Huggy said:

Your building using e36 front suspension?  If so, there are definitely options for stiff springs...

 

Understood. I like to look at and understand the pros and cons of all options before I do stuff...  all options in my head includes many more things than most people consider normal...

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Just now, mender said:

Doesn't have to make sense to anyone else, journeys are an end in themselves. ;)

 

Your location should be strong enough, and I assume you can tie into the strut tower structure for the upper. 

 

Cheers to that. I spend _way_ more time engineering and building the car than racing it.

 

I built the structure up there (this stuff is all grafted onto an Opel, so nothing stock is used anywhere), and I can modify it as needed...

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33 minutes ago, CBraden said:

 

Cheers to that. I spend _way_ more time engineering and building the car than racing it.

 

I built the structure up there (this stuff is all grafted onto an Opel, so nothing stock is used anywhere), and I can modify it as needed...

Many years ago I had what was left of an Opel Manta that was going to be the starting point for an interesting project. Fortunately, I got distracted. :)

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16 hours ago, mender said:

Many years ago I had what was left of an Opel Manta that was going to be the starting point for an interesting project. Fortunately, I got distracted. :)

 

You dodged a bullet on that one...

 

If you could use circle track stuff (with the ability to select damping in compression and rebound) as a damper, would you have any reservations about using them on a road course car? I like the ability to get basically any stroke and damping I want... but I know nothing about how they would hold up or perform over very long races on road courses (which I would guess is less demanding than any dirt asphalt application, but that is only a guess). 

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I use the AK series on the front of my Fiero. No problems, calmed things down and no noticable change in performance in about 40 hours of road course racing. :)

 

I was having issues with the front of my Fiero, under heavy braking the front end flopped (actually has pro-dive from the factory - not a sports car!) and the tires were tucking up into the wheelwell enough to rub through the inner plastic fender liner and onto the steel frame rail. This also caused a significant increase in negative camber in the last inch of suspension travel, making it very easy to go just a little past the camber knee and suddenly washing out  the front end. This was back in the cut stock spring days so the car was quite soft.

 

I put in some anti-dive and changed to the AK shocks (4040F, pretty mild because of the light front end) and was very pleased with the result. The front end was predictable, the limit to braking was from the tires and not the geometry, and no more frame rail rubbing to wear a line in the tires!

 

If you go to about 3:35 in this clip and watch out the passenger's window of the camera car you can see how controlled the front end is under braking as I go up the inside for the pass:

 

Edited by mender
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7 hours ago, mender said:

I use the AK series on the front of my Fiero. No problems, calmed things down and no noticable change in performance in about 40 hours of road course racing. :)

 

I was having issues with the front of my Fiero, under heavy braking the front end flopped (actually has pro-dive from the factory - not a sports car!) and the tires were tucking up into the wheelwell enough to rub through the inner plastic fender liner and onto the steel frame rail. This also caused a significant increase in negative camber in the last inch of suspension travel, making it very easy to go just a little past the camber knee and suddenly washing out  the front end. This was back in the cut stock spring days so the car was quite soft.

 

I put in some anti-dive and changed to the AK shocks (4040F, pretty mild because of the light front end) and was very pleased with the result. The front end was predictable, the limit to braking was from the tires and not the geometry, and no more frame rail rubbing to wear a line in the tires!

 

If you go to about 3:35 in this clip and watch out the passenger's window of the camera car you can see how controlled the front end is under braking as I go up the inside for the pass:

 

 

Looks really planted, and seemed drama free even in the emergency "Probe avoidance" braking. I just never understood why the AK stuff is way cheaper than a similar looking shock for a BMW... and gives you the benefit of picking your damping. Assumed it was different somehow inside somehow that drove costs.

 

Did you up spring rates when you went to the AK's and put in anti-dive?

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11 hours ago, CBraden said:

Looks really planted, and seemed drama free even in the emergency "Probe avoidance" braking. I just never understood why the AK stuff is way cheaper than a similar looking shock for a BMW... and gives you the benefit of picking your damping. Assumed it was different somehow inside somehow that drove costs.

 

Did you up spring rates when you went to the AK's and put in anti-dive?

Pretty sure the price is because of volume, not internals but I could be wrong. The ability to select and play with rates for cheap was the main reason I went to them.

 

That was the soft spring set-up; wheel rate was quite low as I only have about 1000 lbs to support up there. The next time out, the Fiero will have a set of adjustable Konis to complement the new spring rates, mainly because I have the Konis sitting on the shelf! Aiming for about 2.1 hertz ride frequency: 

https://rqriley.com/automobile-ride-handling-and-suspension-design-and-implications-for-low-mass-vehicles/

 

You can use the formula:

sus-equa.gif

NF = Natural Frequency in Cycles Per Minute (divided by 60=Hz).

SD = Static Deflection in Inches.

 

to help decide how much suspension travel you want then converting that to wheel and then spring rates. The Fiero after my mods has decent suspension geometry through the mid range but not great at the extremes. Most cars are like that so limiting the travel limits the issues. One of many ways to set spring rates.

 

The Probe avoidance was actually well below my emergency braking, I saw the situation developing and was giving him lots of room while flashing my brake lights to get the camera car's attention. The more abrupt braking (still moderate) was at the end of the short straight after doing the crossover when we were all trying to get around the other Fiero. I purposely braked early to see if I could get the #24 car in between us but that didn't work. 

 

Those were the first 5 or so laps on a cold track that I hadn't driven for a couple of years and with a new suspension set-up so I was being quite cautious into the corners. The rear tires take a while to heat up and the car was a little twitchy when trail braking until the tires warmed up. You can see that in several places. The car really plants the rears under throttle so I can get on it very early in the corners.  

Edited by mender
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So with the AKs and low spring rates, was it overdamped?  If you had to pick underdamped or overdamped, which would you pick and why? My lack of experience drives me to want to test these things to see the trade-offs. I have never driven anything overdamped, I am guessing on washboard stuff it sucks, but am thinking on smooth stuff it could be good.

 

Would you ever swap out the Koni/stiffer spring for the AKs and softer springs (say on a bumpy track) after you make that switch?

 

I've been aiming for a higher ride frequency (~3.3) in the front. It seems the stiffer I make the car, the higher the ultimate limits, but the steeper the dropoff if you go over (less forgiving). It seems there is a lot of guys who know what they are doing who say the frequency should be higher in the rear, but I find it easier to drive with a higher frequency in the front.

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With the temperatures we're getting right now, any driving I do is overdamped! Looking at -36 tomorrow morning. 

 

I'd say the front was a little overdamped under compression, you can see that the front smoothly drops to its compressed height under braking. Fairly quick but not immediate. That was on purpose, to keep the front end from flopping like it did before the mods. I could probably go a bit lighter on the compression numbers. I don't usually run the curbs with the Fiero so I wasn't too concerned but if I did it would be better with less. The rebound damping seems to be about right, the car doesn't pump down over bumps. I was wanting to keep it up off the bump stops and in the mid range of the suspension travel.

 

When you go up on the spring rates you usually reduce the compression damping unless you're tuning for a specific result. The spring resists compression but of course has a fair bit of stored energy when it does compress, so more rebound damping is needed to control the release and not overshoot/oscillate. That's why most single adjustable shocks adjust rebound and leave the compression damping mostly unchanged and slightly under OEM.  

 

With the range that I'm going for, my car should be fine on a moderately bumpy track so not likely to change back. 

 

You're getting up there with 3.3 in the front, that's usually for cars with a fair bit of aero:

https://www.drtuned.com/tech-ramblings/2017/10/2/spring-rates-suspension-frequencies

 

Worth reading.

 

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Very helpful.

 

The limit of rebound damping is what was driving the original question - we are looking at racing Harris Hill in March. The current setup hits the bump stops pretty hard on the big bumps in the straight there (see setup notes below), and the rebound is rapid enough to know I am already beyond what these struts/shocks can control. See 2:21 in video (sorry for crappy quality):

then again at 3:50, and every minute and a half through the video.

 

 I am at decision time to either add travel (and the car will lose grip in sweepers when CG goes up - I know this from testing) or to find a way to find a damper solution for even stiffer springs  - or potentially more bump stop length. So far I have just cheated to get around huge issues with lack of rebound by using shorter springs with a couple inches of 25lb tender spring (the damper keeps damping the motion after the too stiff main spring has reached it's full length) in the front. This is obviously far from optimal, but probably just from luck, the car is stable and really predictable setup as it is - and we are always in the top part of the field for pace. The rear currently has a full length spring, but may end up going to shorter stiffer spring and a tender there as well as the rear rebounds too fast to the top of the travel. Or I could be sensible and add travel and make peace with losing some grip. But clearly, I am looking for ways to avoid that...

 

Setup notes: My car has all the aero I can bolt to it within the rules (except a flat bottom, I have cooling issues already), and it is also low enough to have pretty limited wheel travel before I make sparks. It didn't start out that low or with that much aero, but the car is faster the way it is (testing proves this). An unfortunate consequence of the height/limited wheel travel is that I need to run stiffer springs than I feel are a good answer for bump compliance and bearing life (and obviously than the current dampers can control), and spring rate goes up even higher once compressed into the bump stops. You can see the car bounce off the bigger bumps in the video above.

 

 

 

 

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That's a pretty bumpy track! Some questions that you've probably answered before, but humour me. :)

 

1. I can hear four skips at 0:49. Much easier to tell when driving the car, but it looks like the car hits the new pavement seam then bounces a few times on the bump stops. Does that sound right? Are you getting any suspension movement during that time?

2. I assume your car is lowered. Are your lower control arms pointing up toward the wheel? In other words, did you fix your camber curve when you lowered the car?

3. Do you lose grip from the raised C of G or from something else? Geometry? Splitter height? 

4. What would hit the track first if you shortened the bump stops?

Edited by mender
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17 hours ago, mender said:

That's a pretty bumpy track! Some questions that you've probably answered before, but humour me. :)

 

1. I can hear four skips at 0:49. Much easier to tell when driving the car, but it looks like the car hits the new pavement seam then bounces a few times on the bump stops. Does that sound right? Are you getting any suspension movement during that time?

>>Yes, it absolutely skips/slips. The suspension is moving in the front, but obviously not enough to keep the tires on the ground (very short springs in the front, with only probably 2.5 inches of usable travel from ride height - with 1.5 inch compression of main spring at ride height). I don't remember for certain, but I think there are two heaves (the first two skips audibly), on that line (the first is after the pavement seam a bit). It pushes during that skipping, the rear is totally settled.

 

Quote

2. I assume your car is lowered. Are your lower control arms pointing up toward the wheel? In other words, did you fix your camber curve when you lowered the car?

>>The car gains about a degree of negative camber per inch of compression in the front, slightly more in the rear. It was setup for this ride height when I swapped all the suspension to this car.

3. Do you lose grip from the raised C of G or from something else? Geometry? Splitter height? 

>>I assume it is CG, but don't know how to prove or disprove it. I can get the geometry basically the same in the front with higher ride heights, but the rear loses some static camber as ride height goes up (I can cut up/add adjust-ability there if I need to but since I have not raced it higher, I haven't yet - but the rear is not where I feel loss of grip when car is raised... only the front).

4. What would hit the track first if you shortened the bump stops?

>>The suspension will bottom out on all four corners without the car touching the ground if done in a controlled setting (e.g. in my garage by removing springs and letting car sit on bottomed out dampers). The lowest part of the car is an exhaust hanger, and then the bellhousing lip hangs down over an inch past the oil pan, it is the only scary part down low. Drivers have found ways to bend that lip of the bell back a bit in offs... There is a 1" steel sq tube down each side as a makeshift skid bar if either side hit instead of the middle.

Thanks

 

*edit to add this video where similar bump induced push can be seen mid corner @ 51 seconds in, then each minute and a half-ish at same corner.

 

Edited by CBraden
add another video example
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