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Old 05-04-2011, 01:48 PM   #1
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Default Tender / Helper / High Spring Rate questions

In the process of getting my '92 ready to sell, I have removed my Bilstein coilover setup and installed a low-mileage set of NB KYB GR2's with stock NB springs and tophats. After driving it a few times, I've decided that stock Miata suspensions suck terribly (floaty with tons of body roll and brake dive, exacerbated by the ride height), but the one improvement is exactly what you'd expect -- compliance over small-to-medium road imperfections. The KYB's aren't great, but the extremely low spring rate certainly takes the edge off.

This got me thinking about how to improve the ride of my Bilstein setup. I've been investigating and searching for more information on tender springs and helper springs. There's been a fair bit of talk about tenders and helpers in various suspension threads (in particular the Xida-related threads), but nothing that explains it from the ground up. Now, as I understand it (and correct me where I've made mistakes):

Helper springs have a nearly zero-rate and the purpose is to prevent the main spring from going slack as full droop is approached (and in going slack, unseating or moving around). They have no effect on ride or handling other than to prevent the main spring from unseating and re-seating suddenly. Helper springs are fully-compressed (coil-bound) at static ride height.

Tender springs typically range from 100 to 200 lb rate and the purpose is to create a dual-rate setup in which the suspension travel includes a lower combined rate (according to the (A X B) / (A + B) = combined spring rate formula) as well as a higher spring rate once the tender spring is fully compressed (at which point the main spring rate takes effect). This results in a more compliant ride than is possible with individual high-rate springs and a greater resistance to roll than is possible with individual low-rate springs. Tender springs are not fully-compressed at static ride height.

Now, given my spring rates and the fact that, at full droop, I believe both my front and rear springs are slack by a couple inches or more, I think I have room to add either helper springs or tender springs. Helper springs would not change my ride or handling but would stop the main springs from unseating at full droop. Tender springs, if chosen correctly, would improve ride and handling by making the suspension more compliant over poor road surfaces and maintaining tire contact under more conditions.

Now, how does one select an appropriate length and rate for tender springs? Can one make a setup worse by selecting an inappropriate length and rate for the tender springs, or can one basically throw a dart and end up with something that, if not perhaps the very best choice, will still be an improvement? For example, it appears that the Xida's use 150 lb tender springs for all the main spring options, all the way from 450/300 up to 800/500. Can this be understood to mean that the appropriate tender spring rate is not closely tied to the main spring rate, or is there another factor at work with the Xida's that prevents me from applying this to my Bilstein coilovers?

Suspension gurus, let's hear it.
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Old 05-04-2011, 02:12 PM   #2
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Your definitions are correct for helper and tenders. I see it backwards a lot but your descriptions fall in line with the info on the Eibach website.

The thing about Tenders is that you need to determine where and when you want the soft equivalent rate and when you want the hard equivalent rate and how the transition will feel.

I don't know the theory but if I had to guess, I'd want about 1-1.5 inches of wheel travel on the tender before coil bind and then the rest on the main.

Select your main spring based on how you want the car to handle when you push it. For example, you might want a 700lb spring rate when you're leaning hard into a corner. Use that as your main spring.

Then select the tender spring to give you a reasonable straight line rate. for example say you want to cruise at 210 lb/in, you'd solve for the tender rate and get 300 lb/in.

Then based on the total spring travel (free height minus block height), you could calculate when it will be fully compressed and what load that would be. With the load, you can back calculate the shock travel (and therefore wheel travel) where it will occur.

Eibach also makes progressive tender springs if you want to get crazy.

http://performance-suspension.eibach...16_catalog.pdf
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Old 05-08-2011, 11:37 PM   #3
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This thread didn't really take off like I'd hoped.
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Old 05-09-2011, 11:34 AM   #4
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Honestly I doubt anyone but maybe two people on this board have even tried to calculate their own tender spring setups. I haven't. I'm just spewing my best impression based on the old eibach ERS page. At one point on their site, they had a "tender spring calculator coming soon!" and now 15 years later, it never happened.
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Old 05-09-2011, 11:48 AM   #5
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Yeah, I guess my hope was that Emilio would chime in, but I can understand that he has a very good reason to not share too much information on the research that went into the tender spring setups on the Xidas.
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Old 05-09-2011, 11:54 AM   #6
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Since the springs are linear, a rate of 400lbs. means it takes 400lbs. of force to compress that spring one inch.

With a tender spring combination, you have two springs stacked on top of each other and they are compressed simultaneously. Because both are moving at the same time, it takes less force to compress both springs one inch.

Lets say you have a 400 lb main spring and a 100 lb tender spring. That equals an 80 lb spring combo until the tender is fully compressed.

So when you have a 400 and 100 pound spring, it only takes 80 pounds to compress one inch. This is because the tender spring takes on most of the compression (3/4 of it, while the 400 lb spring only moves a little). This should give you a smooth ride over small inperfections.

Once you fully compress the tender spring the rate then crosses over to the stiffer rate of the main spring. At this point you have the agressive rate to prevent roll and absorb harsh bumps.


I'm sure you want to pick a tender spring that is long and stiff enough to not be fully compressed when sitting on the ground, so that spring is doing most the work...then only when needed on exteme bumps/corners to stiff spring comes into play.
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Old 05-09-2011, 12:24 PM   #7
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http://replay.web.archive.org/200410...ERS_wizard.htm

that's the last (and I think first!) iteration of the ERS wizard page.

I did find this:
http://www.f-o-a.com/calculator.html
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Old 05-09-2011, 12:29 PM   #8
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that second link needs the diagrams to show measure to measure from
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Old 05-09-2011, 12:40 PM   #9
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Quote:
Originally Posted by Braineack View Post
that second link needs the diagrams to show measure to measure from
It shows the measurements on the "How to use the Calculator tab" unless you were referring to something else.
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Old 05-09-2011, 01:47 PM   #10
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Quote:
Originally Posted by RavynX View Post
It shows the measurements on the "How to use the Calculator tab" unless you were referring to something else.
whoops missed those.
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Old 05-10-2011, 12:58 AM   #11
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Here's where I got mine. I paired them with a set of rear HD's with 6 inch 335# springs. The rear handles bumps much better but it doesn't feel like an overly soft setup.

http://www.sg-motorsport.com/store/p...oducts_id=1142
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Old 06-16-2011, 02:26 PM   #12
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Old 06-17-2011, 01:01 AM   #13
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Is their a reason not to use progressive or dual rates.

**** there he goes again, your all over me today jason. It seems this is basically a dual rate system anyways.

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Old 06-17-2011, 02:51 AM   #14
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Subscribing.
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