marc abrams

I would like to discuss the grip ability of the semi trailing arm suspension system. Let me start off first that never raced, auto cross, or anything like that. I'm having a hard time wondering why Porsche used this suspension other than the VW connection. Yes the car does take the bumps very well. But the grip level seems to be less than my Daytona with a solid axle. Okay, my 944 has grand touring Kumho Ecsta LX platinum tires were my Daytona has Goodyear Eagle GT's. IMO I don't think it's the tires but more of the camber and toe change as the car rolls. The suspension is stock with a 20mm front sway bar and no rear sway bar. I have a 24mm front sway bar and 16mm rear sway bar I got off of eBay a few months ago that I have yet to install. Personally I think Porsche should have given the 944 thicker sway bars to begin with to reduce the camber and toe changes. After all it's a Porsche, not a Plymouth Valiant.
So what am I missing here?

Noahs944

I think the torsion bars are to soft personally. How old are your shocks?

924srr27l

Hi, Interesting question.

Of which I've spent approx 15+ years playing with (Handling) and 8 different transaxle cars and all 3 models (924 / 944 / 968)

The rear of this chassis (which is regarded worldwide as about as well balanced as any car) is very good.

It's always the front that loses grip first (Pushes / Understeers) unless the following are worn or HARD / perished:

- Tyres
- Shock Absorbers
- Geometry

You have no rear ARB so you have the most compliant travel (Soft) if you had over-steer I'd advise coming down to a smaller bar or taking it off, but you have no bar fitted.

The Independent Rear suspension will be better than a Solid axle so something is wrong somewhere.

The Geometry needs to be checked.

You've mentioned Tyres, Yes if they are old and hard they will not grip well ?
Also if your rear Shock absorber's are soft saggy, worn and leaking the body will not be controlled and the handling will suffer.

Do you have the same tyres on the front ?
What Wheel (Size) do you have on the front and rear .
Are the rears a Wider rim ?

How many miles has the car done? are the rear shox leaking or rusty & look old etc..

And lastly EXACTLY where and what is not gripping?

Is is off the lights? Steady state state cornering 30mph ? 60mph? higher ?

I'm assuming you have wheelspin? and / or oversteer? for your statement to say the "GRIP LEVEL IS LESS " than a Daytona.

R

V2Rocket

remember the 924 was going to be a VW.
it was designed as a "sporting GT 2+2", not purely a sports car...so it was to have good handling but also a good ride - notice that a standard 924/944 is a pretty good car to take on a road trip!

the early steel rear suspension is nearly identical to that in IRS (non-swing axle) beetles. torsion bars in general are a very european-car thing - the 914 and 911 of the 70s (out of which our cars were born) used T-bars at all corners because that is what the germans knew best, and they had to keep costs under control.

and on 1976 tires the rear end sure was "more than adequate"!

944CS

The NA 944 is too softly sprung, even with M474. If you had M474 and adjusted your Koni's to a stiffer setting, it is ok for a 143hp naturally aspirated car, but remember, this car was developed in 1980/1981 and launched in 1982. That is 35 yrs ago now.

The sway bars will help, and Koni or Bilstein sport shocks will help as well, but with todays tires you need more spring rate front & rear, and the shocks to match.

ddombrowski

I'm not the OP, but I'm having similar issues and I am just started to work out my different theories on what is happening.

Our 944 race car has a tendency to oversteer and I feel the rear is losing grip. I don't recall off the top of my head which rear bar we run, but we run it on the softer setting, which has shown to be an improvement. The rear of the car, when bouncing it up and down by hand, feels soft, and always has. Tires and shocks are good, which leaves geometry and rear suspension stiffness - a known issue on our car as the frame is slightly tweaked from a wreck.

To the OP, what alignment specs/ride heights are you running? We found our car, when we first bought it, to be ridiculously tail-happy. During our first alignment we found it had a ton of toe-out - who knows waht the previous owner was doing?

924srr27l

Where's Your oversteer occurring ? as soon as your turn and tip in to a corner ? or mid corner or exit?


The best thing I learnt from 10+ years Racing (Continuously) was to get a good balance it's often best to upset the good axle as opposed to looking at and making changes to the axle that's losing grip first.


For example, you could go stiffer front ARB and this should make a difference and begin or eradicate your oversteer.


The general rule is whichever axle slides first, Soften or stiffen the axle that's not a problem to equal the balance...


Factor's also play a part on what shock absorbers and springs and the front ARB size you have? also road or slick tyres? Rim and tyre sizes profiles etc..etc..


I see a few answers here have mentioned the word SOFT a few times, this should help with an issue for oversteer, and I wonder if "Body Roll" is the real characteristic that's happening here and not an actual loss of grip?


R

marc abrams

My are shocks/struts are Sachs Super Touring, the 4 Kumho are 215/60-15 on 7 inch cookies cutters are all less than 4 years old and about 6000 miles. The front and rear bushings and ball joints are about the same age also with me doing all the work. I haven't had the wheels align because I honestly don't know any place that would really do a real alignment. It's "set the toe and let it go" type of alignment around here. So I may be leaving some on the table. The tires are wearing good so I haven't bother with it.
What brought this up was me and my wife going through our house, throwing stuff out (we're going to be selling the house) and came across a old book "How to make your car handle" by Fred Puhn. He wrote " the Datsun 510 rear suspension is typical of the semi trailing arm type. The arm pivots make an angle with the centerline of the car. The result is a considerable amount of toe in change with suspension movement. This is not ideal for racing and works reasonably well only if the suspension is ultra-stiff".

Spencer, I think you pretty much summed it up.

924srr27l

Ok, so you read a book about a trailing arm application on a Datsun, but before this ? or after you started thinking the car's rear grip is not good? or ?


I'm 99% sure the rear suspension trailing arm design and geo is superior and well thought out on the Porsche more so than the Datsun.


This car's design brief from VW was a car that would be the Company's Flagship Coupe Sports Car, for Porsche it was to be their entry level Sportscar to replace the 914.


These chassis has a very high regard for good handling back in 1976 and also 41 years later.


What exactly has or is happening when driving your car to of made you post you have handling issues with the rear ?


R

StoogeMoe

Fred Puhn's book is an awesome book. Some may disregard it because it only has old cars in it. But physics is physics and you have to look beyond that.

The trailing arm suspension certainly isn't the most ideal. It has many flaws. But I'm sure Porsche chose it because of cost and packaging reasons. It actually works quite well. The trick to make it work really well, is to limit the motion of the arms with stiffer torsion bars or springs or anti-roll bars. The more they move, the more negative things happen.

I race in BMW Spec E30, which has a similar setup to the 944 except the trailing arms are a bit shorter. These cars go into wild oversteer conditions which makes them wicked fun to drive.

Perhaps Marc you are so used driving that front wheel drive car, that your normal feel is understeer. Then when experiencing a more neutral car you equate that with losing grip. I would start with a proper alignment. These cars are sensitive to toe-out. Or maybe have someone that is familiar with these cars to take yours for a spin to see what they think. It may be you and not the car.

JustinL

Start with an alignment before looking for other problems. You may have some fundamental alignment problem that is leading you down the wrong path.

marc abrams

Actually I always felt this way. I recently put my Daytona back on the road and noticed that the grip was better. As far Datsun vs Porsche trailing arm suspension goes my take is a trailing arm suspension is a trailing arm suspension. How can one be better than another. When the 944 is up on 4 jack stands you can even see the rear wheels toe in. The trailing arm suspension isn't bad. It's lighter than other suspensions. I just question Porsche's logic. They must have known about the toe in problem. So wouldn't it have be prudent on Porsche to limit body roll to limit the toe in?

V2Rocket

how old are the bushings in your suspension? original?

because of the way the suspension works, camber = toe at extreme suspension movement
if you have negative camber (top of wheel going towards middle of car) at extreme compression you would get toe-in and at extreme droop you'd get toe-out.

if you are getting toe-in with the wheels drooped (up on stands) you may need to adjust your camber.

MistaX

You need an alignment. Need to know the wheels are pointed in the right direction before anything else.

924srr27l

(Daytona) You noticed the "Grip was Better"
Do you mean the Body Roll was less?


or does the rear of the 944 actually loose grip and slide? if so when ?




It's best the rear toes in and not Out, this assists and stops an oversteer situation like the known E30 rear axle.
Porsche designed the 928 Weissach rear axle before designing the 924 which toes in exactly to stop any lift off oversteer.





The Datsun 510 looks to have Full Trailing arms, not Semi.


The axle attachment points are on a angle, where the 924/944 & 968 are not at this same degree angle which is crucial,


I think Porsche has a better reputation here than Datsun and they will of NOT had a situation where there is a Handling problem from an unforeseen design issue. Sure it was Beetle parts & design as designed by Ferdinand before he had and set up his own company.










You must have an issue here if your car is oversteering ? or is it not body roll your describing?






R