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Why? Tender Spring rate split F/R.......

 
Old 04-02-2014, 05:58 PM
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wanna911
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Default Why? Tender Spring rate split F/R.......

Just trying to get a better understanding of handling dynamics for a dual spring setup. If the Tender springs are compressed at rest, why then, would they need to be separate rates for front and rear (example 911) in similar fashion to the main springs. What changes would one expect vs if they were the same rate?

Also for a basic race car, what scenarios other than a bump or dip are the tenders actually being used?
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Old 04-03-2014, 10:49 AM
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Van
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When you have 2 springs on top of each other, e.g. main plus tender, the spring rate is less until the tender is fully compressed, then your main spring rate takes over. The real formula is: total spring rate = (tender rate* main rate)/(tender rate+main rate). If your main is 800 lb/in, and your tender is 500 lb/in, until your tender is compressed, the rate is 308 lb/in - and once the tender is compressed, the rate is 800.

The idea is to have the tender *almost* fully compressed at static ride height, so you benefit from a softer setup while the wheel movement is small, but you have a stiff setup under full load to counteract body roll.

If your tenders are fully compressed at ride height, then your setup is wrong (for that technique), and they are just acting as helpers.

Last edited by Van; 04-03-2014 at 12:28 PM. Reason: dumb thumbs
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Old 04-03-2014, 10:57 AM
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GREAT explanation!
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Old 04-03-2014, 11:22 AM
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Originally Posted by Van View Post
The idea is to have the tender *almost* fully compressed at static ride height, so you benefit from a softer setup while the wheel movement is small, but you have a stiff setup under full load to counteract body roll.

If your tenders are fully compressed at ride height, then your setup is wrong (for that technique), and they are just asking as helpers.
How can you define the difference between "almost" and fully compressed?
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Old 04-03-2014, 12:00 PM
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Put a feeler gauge between the coil springs of the tender, put the drivers weight in the car, settle it by rolling back and forth and if you can't pull it out, it's fully compressed. If not fully compressed, push down on the corner of the car and see of you run out of clearance with the gauge...
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Old 04-03-2014, 12:04 PM
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Van,

But with the setup you described, wouldn't there be an abrupt transition that goes from 308 lb/in to 800 lb/in. Would that affect the chassis balance / driver confidence? (vs. something that is linear all the way)

So "helpers" are just there to take up the slack when the car is unloaded, while "tenders" are there actually to provide a softer ride in the initial stroke of the suspension travel?


(I am only asking, no experience in these kind of stuff)
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Old 04-03-2014, 12:33 PM
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Good discussion on tender springs

https://rennlist.com/forums/racing-a...uspension.html
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Old 04-03-2014, 12:33 PM
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Yes, you are correct - there will be an abrupt transition. The alternative is to get progressive springs - which have a changing rate as they compress.

Progressive springs will have either the wire diameter or the spacing between coils vary. Otherwise it's a "linear" spring.

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Old 04-03-2014, 12:37 PM
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there are progressive tenders as well
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Old 04-03-2014, 12:39 PM
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That is why I don't use tender springs. There is a period of time during the early turn when the tender has to fully compress, before the car takes a "set". That delay is noticeable, and when I removed them, I was able to get on the gas in early-mid turn sooner. I just have zero rate springs that help keep the main spring in position during extension.
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Old 04-03-2014, 12:45 PM
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Originally Posted by cfjan View Post

So "helpers" are just there to take up the slack when the car is unloaded, while "tenders" are there actually to provide a softer ride in the initial stroke of the suspension travel?
This is also correct. Tenders are still rated in lb/in and come in a variety of sizes.

Helpers are super light and are just there to keep the main spring against the spring perch when the extension stroke (droop) exceeds the spring's length.
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Old 04-03-2014, 12:46 PM
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Unless you are setting up for a specific track, you typically don't want the rates changing (i.e. progressive) through the stroke. Of course, that is unless you are using really high spring rates, and very short main springs are required to set the ride height accurately, and keep the shock in it's operating stroke range. In that case, you might need "tender" springs, instead of "helper" springs.

I like using very low (Zero) rate helper springs that stay compressed all the time seem, which also seem to be the most popular for our applications where lots of different track surfaces are visited.

Last edited by Mvez; 04-03-2014 at 01:13 PM.
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Old 04-03-2014, 12:54 PM
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Jeff Braun give's a very nice explanation (just more in depth than Van's) about this. https://www.facebook.com/photo.php?f...type=1&theater
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Old 04-03-2014, 02:30 PM
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GT3 cup cars (996) use approx. 350 lb. helpers both front & rear. Their respective length varies as does the main spring. Front main is 4" tall, helper is 3" tall. Rear main is 5" tall but rear helper is only 2.5" tall.

Front: Spring 100/240/60 (240 N/mm)
Helper spring 75/60/43

Rear: Spring 130/260/60 (260 N/mm)
Helper spring 60/60/50
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Old 04-04-2014, 02:18 PM
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Originally Posted by Van View Post
When you have 2 springs on top of each other, e.g. main plus tender, the spring rate is less until the tender is fully compressed, then your main spring rate takes over. The real formula is: total spring rate = (tender rate* main rate)/(tender rate+main rate). If your main is 800 lb/in, and your tender is 500 lb/in, until your tender is compressed, the rate is 308 lb/in - and once the tender is compressed, the rate is 800.

The idea is to have the tender *almost* fully compressed at static ride height, so you benefit from a softer setup while the wheel movement is small, but you have a stiff setup under full load to counteract body roll.

If your tenders are fully compressed at ride height, then your setup is wrong (for that technique), and they are just acting as helpers.

Makes sense, so using you example, in the case that the tenders are fully compressed you have 800 lb/in your tenders would only come into play over a bump or jump (so to speak) to soften the landing?
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