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Which rear dampers?
Posted: Fri Feb 08, 2019 9:52 pm
by chud
Does anyone have a recommendation of rear dampers to use on a lowered 912 (swb) please?
Fast road capability needed only, not track or anything extreme.
Re: Which rear dampers?
Posted: Sat Feb 09, 2019 10:34 am
by fetuhoe
There are several choices available, although the dampers fitted to the front end may play a part in the overall handling balance of the car.
The ride height will also have some influence and in general terms the only dampers that will be readily available will be of a standard length and if the car is significantly lower than standard could result in the damper making contact with the bump stops from time to time which would make if harsh.
You could buy a custom length damper but this maybe costly or you could change the bump stops for a shorter and more progressive part. Cutting down the bump stop is a common approach but this provides a very sharp transition if the car does hit the stop.
Dampers, like most 911 parts are subject to strong opinions but for most cars I would tend to favour the use of an adjustable damper so it can be tuned to the individual chassis.
Bilstein (24-001670) Dampers are popular but they aren't adjustable and tend to valved to suit a 911 which can mean they can be harsh on a 912. They cost around £140 each.
The Koni Classic (80-2088) is popular and is adjustable and costs around £150. They are, sadly, a real PITA to adjust as they need to be fully compressed to make any change and in reality this means they have to be removed before they can be adjusted. The Koni also only adjust the rebound damping with the bump setting being fixed.
Both Spax and AVO make budget dampers that are adjustable and Sachs make a very cost effective non-adjustable standard unit.
Re: Which rear dampers?
Posted: Sat Feb 09, 2019 11:11 am
by chud
Great info, thanks. The front is running coilovers from elephant racing, I’m not too sure how much lower than stock I will end up once she’s sat on the floor so this is my issue really - I’ll have a look at the tech sheets for the units you mentioned and see how the fully compressed measurements stack up against where I think my trailing arm will be sat.
Which rear dampers?
Posted: Sat Feb 09, 2019 11:30 am
by hot66
Curious as why you’re running coil overs at the front ?
Re: Which rear dampers?
Posted: Sat Feb 09, 2019 12:57 pm
by fetuhoe
The Bilsteins and the Konis have the same open and closed lengths so there is little to chose. It is more about rates and trying to achieve a good balance between front and rear.
Do you have coil overs instead of torsion bars or do you have coils in addition to torsion bars? Do you know the rate of the coil springs and the diameter of the rear torsion bars.
Interestingly the torsion bar arrangement produces a 'rising wheel rate', with the coil spring on a strut giving a falling wheel rate.
Does this also mean that that you have later type strut bodies with the later ball joints and later specification A Arms.
I would also be interested to know which steering rack you are using as if you have kept the early rack there is a difference in the mounting height between the later rack and control arm arrangement and if they are not from the same generation it will effect the bump steer curve, which on a early car will then affect turn-in behaviour due to the lack of Ackermann.
Which camber plates are fitted to the struts? If you have the later plates in an early body it will be impossible to adjust the caster to the correct value and this also impacts on turn in behaviour.
I would also be interested to know if the car has front and rear Anti-Roll Bars and their respective sizes.
Re: Which rear dampers?
Posted: Sat Feb 09, 2019 1:12 pm
by chud
hot66 wrote:Curious as why you’re running coil overs at the front ?
My car came from a swamp, so the struts it came with weren't in the best health and have actually recently been sold as donors to a DDK member. I was just interested more than anything to see how they performed against the stock torsion bar setup. I wish I could give you and answer that involved trigonometry but my reasons were as simple as that. They also raised the spindles slightly for me, and using the later struts gave me 3.5" calliper fixing for the brakes I'm using.
fetuhoe wrote:The Bilsteins and the Konis have the same open and closed lengths so there is little to chose. It is more about rates and trying to achieve a good balance between front and rear.
Do you have coil overs instead of torsion bars or do you have coils in addition to torsion bars? Do you know the rate of the coil springs and the diameter of the rear torsion bars.
They are being used instead of the torsion bars
Interestingly the torsion bar arrangement produces a 'rising wheel rate', with the coil spring on a strut giving a falling wheel rate.
Does this also mean that that you have later type strut bodies with the later ball joints and later specification A Arms.
Yes I have later bodies, later ball joints machined for early control arms and turbo track rods which have (according to Elephant Racing) been developed to eliminate bump steer when used with SWB rack.
I would also be interested to know which steering rack you are using as if you have kept the early rack there is a difference in the mounting height between the later rack and control arm arrangement and if they are not from the same generation it will effect the bump steer curve, which on a early car will then affect turn-in behaviour due to the lack of Ackermann.
Which camber plates are fitted to the struts? If you have the later plates in an early body it will be impossible to adjust the caster to the correct value and this also impacts on turn in behaviour.
Currently in debate with ER regarding these.
I would also be interested to know if the car has front and rear Anti-Roll Bars and their respective sizes.
Currently just stock 912 setup, so nothing on the rear. Yet.
Answers within the quote...
Re: Which rear dampers?
Posted: Sat Feb 09, 2019 1:53 pm
by chud
For those that haven't seen them on the build thread, here is what they look like.
Re: Which rear dampers?
Posted: Sat Feb 09, 2019 2:02 pm
by Ashley James
I’ve used Koni since the sixties when they outlasted everything else, but recently my son had some Spax made for a Hotrod. They lasted a couple of hundred miles before ceasing to damp. I had Bilsteins on my Rolls and they were good.
Re: Which rear dampers?
Posted: Sat Feb 09, 2019 2:28 pm
by chud
Ashley James wrote:I’ve used Koni since the sixties when they outlasted everything else, but recently my son had some Spax made for a Hotrod. They lasted a couple of hundred miles before ceasing to damp. I had Bilsteins on my Rolls and they were good.
Thanks, agreed - I've not had good past experience with spax stuff.
Re: Which rear dampers?
Posted: Sat Feb 09, 2019 2:42 pm
by 911hillclimber
Just my 2p...
Running as low as I think Chad will on this car I would focus on compression travel left in the damper.
You must have some fusion at the end of stroke or rather befor it bottoms metal to metal.
I ran Koni Spors for a few years and has to remove 20mm or so off the bump stop length to stop Impact.
The car was very low for hill climbs, too low to live with on the road, so jacked it back to 20mm lower than stock, then all was well.
As Chris says, Koni adjust Kent is too difficult, so I set mine mid way hard/soft and lived with it.
The adjustment dogs broke on the front, so went custom fixed damping rate Bilsteins where were great for 20 years until one rear leaked. I put the rear Koni back on and all is well.
I now believe stock damping rates are best for the road, maybe I'm too old to bother...
With the adjustability of your front I would check for bump steer once you have your static ride height sorted.
A car with bump steer is a real pain.
Re: Which rear dampers?
Posted: Sat Feb 09, 2019 6:41 pm
by Ashley James
It’s rebound damping that deteriorates hence it being adjustable on Konis to extend life.