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Help slightly stiffening the rear suspension

 
Old 07-08-2019, 01:19 PM
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dr bob
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Carl --

Do you offer just the spring fittings for the more standard hyper-coil sized springs? That would allow the OP to buy the critical parts from you but save some on import duties by getting the springs locally.
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Old 07-08-2019, 05:01 PM
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I have not done this in the past. The challenges to selling just the spring perches without selling the springs is this:

Springs are wound from spring wire, and every manufacturer has their own combination of wire diameter and coil count to make up a certain spring rate. Although referred as "2.5" springs" - that refers to the I.D. of the spring, and the O.D. of the spring varies depending on what gauge wire the manufacturer uses to wind them. I know my Hypercoils, and I know my 928 spring perches fit Hypercoil springs up to 850 lbs/inch (1000 lbs/inch require some modification to accept the large wire gauge). The point is; if I just sell the spring perches, but not the springs, I cannot be sure whether the spring from XYZ spring manufacturer will fit within my spring perch, but I can be sure that if they don't, I will be blamed for it. Hence, I am much better off selling them together.

Still, because so many of my customers are International, we do have a spring and spring perch only kit on our website. This is everything but the shocks, so you can avoid the cost of shipping shocks that I know you can acquire within your country. Have you looked at that?

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Old 07-08-2019, 05:56 PM
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Originally Posted by dr bob View Post
Carl --

Do you offer just the spring fittings for the more standard hyper-coil sized springs? That would allow the OP to buy the critical parts from you but save some on import duties by getting the springs locally.
Would of course be nice, but would honestly feel OK purchasing the springs as well from Carl and paying the import duties when I in return get some qualified help in determining suitable spring rates.

@Carl, as per my question in the thread, I already have the bilsteins, so would highly prefer to stick with these. The front feels fine as it is, so I guess a similar to stock spring is available in hyper coil. For the rear, I need something that is a little stiffer, what is the recommended range for spring rate for the rear that the Bilsteins can handle? Thanks 😀
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Old 07-08-2019, 06:45 PM
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The 928 seems balanced best when the rear springs are about .65 the spring rates of the front. Hence the 600 front/400 rear spring kit which is generally accepted as the sweet spot for the 928. Works great and is very well balanced for high-performance street or HPDE days.

For those not tracking the car, or those on poor roads or wishing for a more touring car feel, I recommend the 500 lb front springs. Again, using .65 as my front-to-rear ratio, that calls for a rear spring at about 325 lbs. I like 350.

I find the Bilstein's control the springs well for our spring kits up to 600 lbs/inch, but the next step, the 800 lb/inch springs, over-powers them and the Koni's perform better.

Other considerations:
1) curb height of the car. If you want to lower your 928, then by all means you want a higher spring rate because your suspension has less to travel before the car pans. So: lowered car = higher spring rate to protect the engine from damage. Protection Plates are also essential on a lowered car. We are the originator of the Protection Plate and our new design is even better.
2) weight of the car. The owner with a fully equipped GTS (heavy) thinks the 600/400 spring rates are not that stiff, while the fella with a gutted 928 S track car thinks they are very strong. Same springs, lighter car.
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Old 07-09-2019, 11:26 AM
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Originally Posted by Carl Fausett View Post
The 928 seems balanced best when the rear springs are about .65 the spring rates of the front. Hence the 600 front/400 rear spring kit which is generally accepted as the sweet spot for the 928. Works great and is very well balanced for high-performance street or HPDE days.

For those not tracking the car, or those on poor roads or wishing for a more touring car feel, I recommend the 500 lb front springs. Again, using .65 as my front-to-rear ratio, that calls for a rear spring at about 325 lbs. I like 350.

I find the Bilstein's control the springs well for our spring kits up to 600 lbs/inch, but the next step, the 800 lb/inch springs, over-powers them and the Koni's perform better.

Other considerations:
1) curb height of the car. If you want to lower your 928, then by all means you want a higher spring rate because your suspension has less to travel before the car pans. So: lowered car = higher spring rate to protect the engine from damage. Protection Plates are also essential on a lowered car. We are the originator of the Protection Plate and our new design is even better.
2) weight of the car. The owner with a fully equipped GTS (heavy) thinks the 600/400 spring rates are not that stiff, while the fella with a gutted 928 S track car thinks they are very strong. Same springs, lighter car.
Thanks for the info, Carl! Much appreciated - I guess I know what is on my "next to buy" list now.. Are the 500/350 closer to stock?
Since the car with bilsteins and original springs feels a little too soft in the rear compared to the front (perhaps due to it being widebody, with larger track width in the rear), would a higher ratio front to rear be advised? E.g. 0.75 instead of the 0.65??
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Old 07-09-2019, 12:15 PM
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The 500/350 setup is closer to stock than the 600/400's, but still stiffer than stock. How old are your Bilsteins? Are they OEM installation Bilsteins? If yes, I would wonder if they haven't gone bad - they'd be 30 years old now.
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Old 07-09-2019, 06:31 PM
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Originally Posted by Carl Fausett View Post
The 500/350 setup is closer to stock than the 600/400's, but still stiffer than stock. How old are your Bilsteins? Are they OEM installation Bilsteins? If yes, I would wonder if they haven't gone bad - they'd be 30 years old now.
They are only a couple of years old - and with the amount of KM/miles I put on the car, they are practically like new...
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Old 07-09-2019, 08:00 PM
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Hmmm. What makes you think the back end is "soft"? Is the car sagging back there (that would be springs). Does the car feel spongy? If you open the hatch, and press down on the hatch receiver up and down a few times and really get the car bouncing and let go... how many oscillations do you get before it stops?
Or does your opinion come from a little fishy oversteer in corners now that you have installed those wide rear tires?
Did this "soft" rear end feeling come about after you went to the wider rear tires?
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Old 07-09-2019, 10:11 PM
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Just a(nother) casual reminder that stiff isn't necessarily faster. A good rear anti-roll bar functionally shares some corner loading with the inside spring, so you can have less body roll without using stiffer springs. It's tempting to try and use the rear to minimize roll in the front, but seldom a good idea, especially if you don't have great differential slip management. Use just enough spring to keep the car off the bump stops in the highest-load corners. Anything more is a waste. Lots of folks drive too-stiffly sprung cars and complain that it doesn't handle well. Might be bumps where there aren't bumps, traction "issues" related to tires bouncing. Stiffer rear is tempting to "hold the tire down", but it's often better to "let" the tire follow the road better. After all, that's really what we want.
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Old 07-10-2019, 10:35 AM
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Opinions will differ. I've never driven a 928 with Eibachs, but I have driven my previous Jetta and 240SX with them. Personally, I like linear springs better. For stuff that is currently available, Carl's Hypercoil kit is probably the way to go.

I was lucky enough to find a set of used (but never installed) Weltmeister springs about 5 years ago that I've been really happy with. They are a linear rate but you need the rear upper and lower spring plates from an 83 or earlier because the outer diameter matches the diameter of the earlier springs.

https://rennlist.com/forums/928-foru...n-a-88-s4.html

Also - more than one way to skin a cat. There are some old threads on the forum that reference guys using an extra set of front springs on the rear. I've never tried it, but I know I read it years ago when I was looking at all the options for stiffer springs. As I recall the front springs are around 215 lbs per inch and the rears were supposedly around 120-150 depending on what thread you read. I think the Weltmeister rear springs were 270 lbs per inch, FWIW.

If you want to experiment with cutting a coil, there was a big discussion on that back in 2014, but I don't know how many people actually tried it. My only issue with cutting a coil is that's a one-way operation.

https://rennlist.com/forums/928-foru...ing-rates.html
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Old 07-10-2019, 06:29 PM
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Originally Posted by Carl Fausett View Post
Hmmm. What makes you think the back end is "soft"? Is the car sagging back there (that would be springs). Does the car feel spongy? If you open the hatch, and press down on the hatch receiver up and down a few times and really get the car bouncing and let go... how many oscillations do you get before it stops?
Or does your opinion come from a little fishy oversteer in corners now that you have installed those wide rear tires?
Did this "soft" rear end feeling come about after you went to the wider rear tires?
The rear does not feel spongy, and the oversteer is quite predictable (and controllable). The softness is best experienced when going over speed bumps, the rear feels to "fall" much more than the front. Also on hard acceleration, the car "sits back" a bit more than I'd like it to.. To be honest, the softness has been there since I got it. After buying the car, I spent 4 years overhauling it. After that, I have now had it on the roads for 5 years. Ride wasn't perfect after the overhaul, so upgraded the original shocks to Bilstein. Generally improved a lot, but I have noticed the slight imbalance between front and rear. In the end, people might disagree with me that the rear feels too soft compared to the front, but in my own opinion it feels a little softer. I think for now, my best option is to go for the hyper coil kit, which allows me to (more easily) play around with the stiffness of the rear. Thanks...

Dr Bob, thanks for the reminder. I fully agree. I am not trying to make the car faster and I am quite happy with the way it corners as it is. I am simply trying to make the springs a bit more balanced.
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Old 07-10-2019, 06:35 PM
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Originally Posted by z driver 88t View Post
Opinions will differ. I've never driven a 928 with Eibachs, but I have driven my previous Jetta and 240SX with them. Personally, I like linear springs better. For stuff that is currently available, Carl's Hypercoil kit is probably the way to go.

I was lucky enough to find a set of used (but never installed) Weltmeister springs about 5 years ago that I've been really happy with. They are a linear rate but you need the rear upper and lower spring plates from an 83 or earlier because the outer diameter matches the diameter of the earlier springs.

https://rennlist.com/forums/928-foru...n-a-88-s4.html

Also - more than one way to skin a cat. There are some old threads on the forum that reference guys using an extra set of front springs on the rear. I've never tried it, but I know I read it years ago when I was looking at all the options for stiffer springs. As I recall the front springs are around 215 lbs per inch and the rears were supposedly around 120-150 depending on what thread you read. I think the Weltmeister rear springs were 270 lbs per inch, FWIW.

If you want to experiment with cutting a coil, there was a big discussion on that back in 2014, but I don't know how many people actually tried it. My only issue with cutting a coil is that's a one-way operation.

https://rennlist.com/forums/928-foru...ing-rates.html
Thanks for the input, Chris, I have been looking for Weltmeister springs, but it seems they are rare to find these days. Didn't know about the spring perches, but I do have some spring perches from pre 83 already. However, I still think I will go for the Hyper Coil set when I can fit it in the budget. I also more or less scraped the idea of cutting a coil, unless someone chimes in saying that it works perfectly, 😀
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Old 07-10-2019, 07:25 PM
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Originally Posted by cschou View Post
I do mostly street driving on country roads and in the city, with an occasional longer trip. Once or twice a year I do a track-day. Generally, the setup is, in my opinion, really well matched for street driving, and still gives plenty of comfort. The front is spot on how I want it, great control, feels tight, but far from harsh. The rear however feels a little too soft. The Ott-droplinks helped a lot with the body roll, but going over a speed bump, the rear "sags" a little too much for my taste. Especially compared to the front, it feels soft. For what it is worth, the car is a widebody with increased track width - might that play a role?
Do feel the rear is too soft only for your occasional track day?

If that's the case, I'd just leave it alone and adjust your driving style accordingly on the track. That way you won't upset the balance of the car for how you drive it the other 99% of the time.

Last edited by SwayBar; 07-10-2019 at 07:48 PM.
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Old 07-10-2019, 07:49 PM
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"Feels soft"....not exactly very informative.

Does it understeer or oversteer? Under what conditions? Does it change from understeer to oversteer (or visa versa) as the speeds change?
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Old 07-11-2019, 06:41 PM
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Originally Posted by GregBBRD View Post
"Feels soft"....not exactly very informative.

Does it understeer or oversteer? Under what conditions? Does it change from understeer to oversteer (or visa versa) as the speeds change?
I only rarely track the car, and it has been more than a year since I last had it on the track. I know it isn't very informative or scientific, but my notion of "feels soft" is based on street driving and personal feel. Regarding understeer vs oversteer, (with limit track time), it tends to slightly understeer with neutral throttle, but easily converts into oversteer as soon as moderate throttle is applied.
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