Helper Springs vs Tender Springs

Hmm interesting I never had that happen on my setup but it could of without me knowing it. I have 8" tall springs I think yours are shorter by an inch, maybe that is the cause.

I think it'd be a bad idea to have helper springs with an actual spring rate as that would make the car do funny things, similar to a progressive spring but worse as your jumping immediately from 100 to 500 lb/in. Do you think on very large bumps the springs become unseated? The ride quality has been good except on the really large abrupt bumps (never dips/holes) and I've theorized that it is the springs becoming unseated/drooping, like when you jack up the car and then the chassis falling back down on the springs. Don't know if 0 rate tender springs would help this though.

Maybe look at what other coilovers systems do and steal from them =)

I also found this while searching for a few mins, seemed decent
http://www.miataturbo.net/showthread.php?t=57543

It seems XIDA uses the dual rate springs in their miata coilover setup, might be worth looking into how they do it. Here is a thread about them:
http://www.miataturbo.net/showthread.php?t=32004


I also read in a miata thread:

YellowSR said:

The other thing you can do is safety wire the spring to the upper spring perch.

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My springs are also 8", front and rear. Regardless, spring length really only determines where you set your perches, and how much bump travel you have before the springs coil bind. The weight of the front of the car will compress each spring by about 1" with 500lb springs, regardless of the springs free length.

I'm sure that on some bumps, or maybe just aggressive cornering, the springs are coming un-seated, as I had one come un-seated at a track day last month. Noticed a weird noise from the front end, and when I pulled into the pit I found the spring wasn't properly aligned with the upper perch and was rubbing on the strut housing. I also notice a lot of clatter that I assume is from the springs on large slow transitions, such as entering a driveway. I'm pretty sure that standard zero rate helper springs would solve these issues by keeping a small amount of tension on the springs at all times.

That XIDA setup is similar to what I am thinking. If the tender is fully compressed at ride height it won't really be dual rate. It will be progressive in rebound after a certain point, but not in bump, which I think is what you want. On the other hand, you see a lot of very fast cars picking up one corner, which I think is essentially what you are doing when you run out of "sprung" droop travel. Maybe by the time you do that you are at a point where that corner wasn't contributing much anyway.

So I have been doing a fair amound more reading on this subject, and am becoming convinced that I do in fact want to run a stiffer "tender" spring vs a zero rate helper spring. I found rennlist.com (Porsche forum) to have quite a bit of good info on the subject.

Consider this: http://forums.rennlist.com/rennforu...-forum/234721-double-spring-suspension-3.html

[email protected] on Rennlist.com said:

Try this 1d analogy. Much easier to draw, but here goes... on each side there is a vertical, imovable wall. in between the walls you have a block mass in the center and a spring (horizontal) on each side of the block. So you have a block suspended between 2 walls by the 2 springs. The springs have a free length of 6" and a rate of 200 lbs/in. With no additional load added (assuming no friction) the springs are compressed to 4" in length, so each is exerting 400lbs on the block. How much force will it take to move to block toward one wall 1"? Now one spring will be compressed only 1" and will put 200lbs on the block. The other spring is now compressed 3" and will put 600lbs on opposite side of the block. So it will require 400lbs of force to move the block 1". Balance forces so that there is 600 lbs on each side of the block. The loaded spring is 200lbs/in, but it takes 400lbs/in to compress it. That is because the spring that is becoming uncompressed is contributing to the movement resistance. Now think of the same example, but you move the block 2". The inside spring becomes loose, and the outside spring is compressed 4" and will exert 800lbs on the block. So, the movement resistance is still 400lbs/in. Now move the block another 1" in the same direction, for 3" total movement, 5" total compression. This requires 1000lbs total force. It takes only 200 additional lbs to move the block 1". The movement resistance is only 200lbs/in. This is a 50% loss in movement resistance once the uncompressed spring becomes completely unloaded. I did this all linear, but it will work for rotation/roll exactly the same. When the inside spring becomes comletely unloaded, the roll resistance from the springs gets cut in half.

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This thread was also pretty interesting, although not exactly what I am wanting to do: http://forums.rennlist.com/rennforu...sign-your-own-dual-spring-rate-coilovers.html

The earlier thread by the same guy, talks about his use of actual helper springs vs the dual spring setup using two regular springs: http://forums.rennlist.com/rennforu...forum/465757-tender-springs-oh-so-tender.html

So heres the point I am at now. I have 500 lb main springs that (at least for now) I want to keep. I am beginning to think that Eibach tender springs are my best option, they have the most rate options and also the longest tenders, which I will need if I am to close the entire gap and slightly pre-load my springs.

My rate options are 150, 200, 250, and 300. All offer 2.25" of travel, and will close under loads of 338lb, 450lb, 563lb, and 675lb, respectivly. I think I want the tenders to be fully closed at normal ride height, so that rules out the 300lb tenders for the front (I estimate 570lb on each front spring, and 780 on each rear).

Now, the way springs in series work is your spring rate is K=(k1*k2)/(k1+k2). So for example, the 150lb tenders with my 500lb main springs would give me an effective rate of 115lb untill the tender springs are closed. On the other end of the spectrum, the 300lb tenders will give you a rate of 187lb untill they close. After the tenders close you of course end up at the 500lb rate of the main springs.

Now, here is my dilema. By going with a lighter tender spring you get a lower spring rate while the tender is active, but its also active for a smaller portion of your droop travel. I estimate that with the 150lb tenders you would have about 1/2" of droop travel (remember, they block at a load of 338lb, which is a little more than half the weight of a front corner) before engaging the tender springs, whereas currently I have about 1" of droop travel, after which I have nothing (zero spring rate). On the other hand, if i use 250lb tender springs (which block at 563lb), I will have virtually zero droop travel before engaging the lower rate, but that lower rate will be 166lb. So what do you think, is it better to have more travel at the higher rate before switching to a much lower rate, or to have less travel at the high rate and then switch to a slightly higher low rate. Remember these are all droop travel, or in cornering the travel on the less loaded inside wheel. Bump travel / the more loaded outside wheel will be at the 500lb rate all the time.

Hey Alex,

Did you pull the trigger on your tender springs yet? I've run a few different setups (none on the MR2 as I'm just getting started with mine). On an EG civic I did use several different stiffer tender springs to try something like you are. I actually liked the result quite a bit. For one set I chose a spring rate that would barely go solid under static loads, but that had enough travel to keep the spring seated all the time. I also did a setup that gave me about 1" of wheel travel before the tender spring locked out. The latter setup made for a much nicer street ride, and still behaved great for auto-x. But I could certainly tell the crossover point, and it took some adjustment to get used to. It didn't help or hurt any times. However, at higher road course speeds, the transition may have been too upsetting to the chassis. Never did track that car.

On my MKIV Supra I had a set of double adjustable coilovers that I used zero rate tender springs to keep the springs seated. As expected, they didn't do anything out of the ordinary, except keep the springs from rubbing on the bodies.
-Dan

Mulholland said:

<snip>
Can't you get taller springs at the same rate you have now? I.e. one inch longer, same rate, with tender springs just in case? Surely that one inch would be enough to avoid your spring sliding loose under full droop?

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Not quite, but that's a really common misunderstanding. If the same ride height and spring rate is used, the length of the primary spring is almost irrelevant. A longer spring would have the same problem the 8" spring does now. Using Alex's numbers, he's got an 8" long 500 lb/in spring, and aproximately 580 lb on the front corner. This means his spring will compress ~1 1/8" (assuming there is no preload, which there wouldn't be if the spring is losing contact with the seat). To get the ride height he wants, this must represent a height where there is more than 1 1/8" of droop left in the front strut when unloaded.

If a longer spring were used (say 10"), this doesn't change the situation at all. If it's a 500 lb/in spring, it will still compress 1 1/8". And if the ride height is the same, there will still be more than 1 1/8" of droop, and the spring will still lose contact with the seat when unloaded.

Tender springs are really common, and necessary when spring rates start getting high. There are other ways to eliminate the problem, such as eliminating droop travel. If you have coilovers that are seperately adjustable for both preload and ride height, this is an easy change to make (though not necessarily desirable). But if spring perch location is the only method of changing ride height, the easiest way to eliminate a stiff spring from coming unseated during droop is with something like a tender spring.

Alex, what are you using for roll bars?
-Dan

I agree that having the transition point before static ride height is probably the right idea. What you can do is create a spring force vs. displacement curve. Start with the known point (580 lb on the front corner), and then look at how the spring force changes as the strut extends.

Further complicating things, you mention all the tender springs block out at 2.5" of travel. But if you were to get longer or shorter springs, they will have a different distance before blocking out. Based on the numbers you're using, I'm guessing you are looking at the ERS linear tender springs. You may also want to explore if there are any other springs available. For instance, a 150 lb/in spring that was 6" long may also be an interesting choice (I haven't run the numbers on this yet, so this is just a guess at this point). This would have a ton of preload on it, and as such even when the strut is at full extension there would be a lot of force pushing the wheel down.

Also, for clarification, when you mention you have ~2.5" of droop, is that 2.5" after the 500 lb/in spring is unseated, or is that 2.5" of travel from the static ride height?
-Dan

Aahh, I figured there probably weren't roll bars on the car. But even with roll bars you can run in to similar problems going over hills on a course. I think both Spokane County Raceway and Oregon Raceway Park can be particularly bad for unseating springs with or without swaybars

That's a great point about the regular springs not intended to be blocked out. I thought that some of the other companies (Hyperco, H&R, etc.) also had linear tender springs of varying length. Although I can't seem to find their catalogs at the moment.

I would *love* to design a set of 2817 Koni's for the MR2. However, last time I checked those were still *way* out of my price range. I'm still completely undecided about what I want to do for suspension on my car. I need to get a drivetrain in the car first....

If you already have an Excel model started, it may be interesting to plot the results. I would start at static ride height and see how spring force changes as it extends another 4". This way you can see some key points like the transition and spring force as you get closer to full droop. I remember doing all this with my civic project and I thought it was really helpful to visualize that.
-Dan

cord4530 said:

Not quite, but that's a really common misunderstanding. If the same ride height and spring rate is used, the length of the primary spring is almost irrelevant. A longer spring would have the same problem the 8" spring does now. Using Alex's numbers, he's got an 8" long 500 lb/in spring, and aproximately 580 lb on the front corner. This means his spring will compress ~1 1/8" (assuming there is no preload, which there wouldn't be if the spring is losing contact with the seat). To get the ride height he wants, this must represent a height where there is more than 1 1/8" of droop left in the front strut when unloaded.

If a longer spring were used (say 10"), this doesn't change the situation at all. If it's a 500 lb/in spring, it will still compress 1 1/8". And if the ride height is the same, there will still be more than 1 1/8" of droop, and the spring will still lose contact with the seat when unloaded.

Tender springs are really common, and necessary when spring rates start getting high. There are other ways to eliminate the problem, such as eliminating droop travel. If you have coilovers that are seperately adjustable for both preload and ride height, this is an easy change to make (though not necessarily desirable). But if spring perch location is the only method of changing ride height, the easiest way to eliminate a stiff spring from coming unseated during droop is with something like a tender spring.

Alex, what are you using for roll bars?
-Dan

Click to expand...

Clearly there was a slight misunderstanding to what I said... I said rather than getting helper springs with some effective spring rate, why not get longer primary coils at the current rate with tender springs...

I didn't say don't get any tender/helper springs. And to that effect, if he is considering getting helper springs with an effective rate, why not just get longer primaries running at the same ride height, but with a higher rate, as that's what he would end up with having helper springs?

It just seems to me like the worst possible solution between the few options to run the existing spring that isn't working properly, with a helper spring that will change the rates of the springs anyways.

And sway bars would probably help a LOT with droop travel, and again, I go back to the issue of you "chasing that last few % suspension wise"... I reiterate, you want to avoid dropping to 0 on any corner.... which right now you're dropping springs off their perches lol, so even if sway bars reduced mechanical grip slightly, still a better option than losing your spring entirely... you'll be faster on the track with sways than in the pit with a temporarily downed car lol

Mulholland said:

Clearly there was a slight misunderstanding to what I said... I said rather than getting helper springs with some effective spring rate, why not get longer primary coils at the current rate with tender springs...

Click to expand...

The point we are trying to make is that changing the length of the primary spring will have zero effect on my issue, so why bother? It doesn't matter if I run helpers or tenders, the length of the primary spring is irrelevant.

Mulholland said:

I didn't say don't get any tender/helper springs. And to that effect, if he is considering getting helper springs with an effective rate, why not just get longer primaries running at the same ride height, but with a higher rate, as that's what he would end up with having helper springs?

Click to expand...

Tender springs won't raise the rate of the main springs. At ride height the tenders will be fully compressed and the only rate that is functioning will be the main rate. Under droop the tenders will decompress and the rate will drop off. At no point will the rate be higher than what I have now.

Mulholland said:

It just seems to me like the worst possible solution between the few options to run the existing spring that isn't working properly, with a helper spring that will change the rates of the springs anyways.

Click to expand...

My goal is to change the rates. I want a lower rate on rebound so that I can have more sprung travel, while keeping my current rate at ride height and under compression.

Mulholland said:

And sway bars would probably help a LOT with droop travel, and again, I go back to the issue of you "chasing that last few % suspension wise"... I reiterate, you want to avoid dropping to 0 on any corner.... which right now you're dropping springs off their perches lol, so even if sway bars reduced mechanical grip slightly, still a better option than losing your spring entirely... you'll be faster on the track with sways than in the pit with a temporarily downed car lol

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Obviously my goal is to keep the springs on the perches and avoid dropping to zero rate. Sway bars will keep the springs on the perches, yes (at least under roll, but not under pure droop), but will actually make the rate drop off worse as they transfer load from one side to the other. Ever notice that cars with stiff sway bars tend to pick up wheels? Yes, a sway bar may be a better option that having the spring fall of the perch, but its not a better option than a 20 dollar helper spring (the minimum I need to solve the spring falling off problem). They certainly aren't the ONLY option.

Share my experiences with significant helper/tender springs. I didn't like them. They acted just like progressive springs, which is probably why there aren't many companies making significant rates. Things that you may or may not have considered:

1. You are trying to "save" grip on a tire that has no appreciable vertical load, and hence, has no appreciable lateral grip to save.
2. Changing the effective rate in droop enough to change #1 means that you are making significant changes to the spring forces vs roll and pitch
3. This leads to the car having more body roll. Roll is comprised not just of the outside compressing, but also the inside extending. If you figure out what the equivelant single spring rate is based on roll stiffness you would find that most of your travel issues would be solved by going to equivelantly soft springs.
4. For gods sake only do it on one end of your car. This better be the front. Otherwise your handling can change somewhat fiercely since weight transfer can be even more seriously affected by fore/aft weight transfer.
5. Your damping will be all honked. There is no dampening that will be any good for your 115lb/in spring rate on the inside that even remotely works well with your 500lb/in outer rates (similare problem to big anti-roll bars).

Off hand the best solution sounds like shortening your struts, but failing that I would suggest that you only do one end.

O