Optimal front sway for a 1.7 ? Options

[Literati914]

I believe, if I’m not mistaken, there was a 13mm front sway bar for the 912 cars.. and I was wondering if that wouldn’t be more appropriate for a 1.7L 914 rather than the stock oem 15mm. I’m not sure if anyone bothers with a ARB on a 1.7L, but the 13mm ones are fairly easy to come by, cause everyone upgrades to bigger. So that’s a plus. Thoughts?


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[wonkipop]

I believe, if I’m not mistaken, there was a 13mm front sway bar for the 912 cars.. and I was wondering if that wouldn’t be more appropriate for a 1.7L 914 rather than the stock oem 15mm. I’m not sure if anyone bothers with a ARB on a 1.7L, but the 13mm ones are fairly easy to come by, cause everyone upgrades to bigger. So that’s a plus. Thoughts?


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porsche thought they were.
and fitted them to just about all the 74 1.8s after they got full control of the project.
i'd have to look up whats in mine, but its standard out of the box since new.
front and rear sway bars. 74 1.8.

i ain't got no complaints. its sweet.

[914Sixer]

Factory set it up for the best possible all round driving. Why mess with it? Reproduction 15 mm bars are now available.

[mepstein]

The 13mm will be a lot better than no bar.

[Superhawk996]

Anti roll bar size has nothing to do with engine size given that for all practical purposes all T4 engines are the same weight and don't fundamentally change the weight distribution of the vehicle.

I see a lot of discusion about adding front bars but almost no one is stating why they want one, what problem they are trying to solve, and what their current setup (spring rates f/r, presense of rear bar & what size, tires, etc.) are.

FWIW - as a generalization, adding a front bar will increase understeer, adding a rear bar will increase oversteer. So how many are really trying to increase understeer?

[Superhawk996]

The 13mm will be a lot better than no bar.

To the point above, a 13mm bar very well may be better than a 15mm bar if you don't have a rear bar as part of the system.

Again, all depends on what problem is trying to be solved.

Highly recommended reading

[914werke]

Ive done at least one 13mm Front/No Rear update with some 140# springs I though it handled pretty damn good.

[mepstein]

Ive done at least one 13mm Front/ No Rear update with some 140# springs I though it handled pretty damn good.

Yea, heavier rear springs can serve as a bit of a substitute for no rear sway bar. Like superhawk996 has said in the past, suspension is a combination of parts but the front ARB and stiffer rear springs is a great fix over stock.

[Superhawk996]

I'm going to (IMG:style_emoticons/default/stirthepot.gif) just a little more.

The discussion around Anti-Roll Bars (ARB) that presumes less body roll is an indicator of better handling. That is a generalization that does not hold in all circumstances.

I have seen instrumented handling data on vehicles that shows that they have less body roll than a benchmark competitor that is perceived to handle better, yet the competitor has MORE body roll. Keep in mind, this is as measured with precision instrumentation, not just some seat of the pants perception of body roll.

Likewise, I've seen objective test data on cars that handle BETTER than a compeititor that that has less body roll.

Food for thought. (IMG:style_emoticons/default/idea.gif) Need to understand what the end objective is.

[Superhawk996]

Ive done at least one 13mm Front/ No Rear update with some 140# springs I though it handled pretty damn good.

Yea, heavier rear springs can serve as a bit of a substitute for no rear sway bar.

Indeed, it's becase the 140# rear springs add to the rear roll stiffness. However, the stiffer springs come with a ride harsness penalty. Using a rear ARB with softer springs would yield the same hadling result without the ride degradation.

As stated, the suspension is a system. Nothing can be changed indepently without affecting something else.

Here's a fun fact. A race car may often be purpose built around a specific (spec) tire. All aspects of the suspension deign are tweaked to give that tire exactly what it wants. Sometimes that is more weight transfer, sometimes it is more camber, and so on. But the bottom line is the rest of the suspension is optimized to give the tire what it wants since the tire is the interface to the road and ultimately determines handling.

So how would you know what the tire wants? Via lab testing of the tires. Tires can be characerized on flat track machines to determine where they perform best. What is the normal load that gives the highest grip? How does the tire respond to the rate at which normal load is transfered to it? Characterization of slip angles vs. load, caster, camber, toe, temperature, on & on.

[Literati914]

Anti roll bar size has nothing to do with engine size …. So how many are really trying to increase understeer?

Superhawk, I based my question on (my) understanding that a front bar is basically there to add a bit of understeer which helps counteract the oversteer situation of a given car.. I certainly may have that all wrong, I’m no suspension expert (& thanks for the book recommendation). But if that’s close to being right, then in my mind a smaller bar would be more appropriate for a simple 1.7L street car than the 15mm one used with a 2.0-2.4L type IV power plant (for example - since they weigh the same) - because 1.7L would theoretically produce less oversteer (all other things being equal).

Anyway the car will get a bar while torn down for bodywork, and the 13mm seems logical enough to me, I’ll probably give it a try. BTW, did any of the 1.7L cars come from the factory with sway bars?

Also, @wonkipop (thanks for your input) says the 13mm bars are on all 1.8L cars, but I don’t recall reading that 914-4 used anything but 15mm - perhaps that was a European/Aussie thing? Anyone?

I wonder what Porsche’s thinking was with giving the 912 a 13mm front bar, it being a more tail happy car than a 914.. but the 914-4 getting a 15mm up front?


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[Superhawk996]

... my mind a smaller bar would be more appropriate for a simple 1.7L street car than the 15mm one used with a 2.0-2.4L type IV power plant (for example - since they weigh the same) - because 1.7L would theoretically produce less oversteer (all other things being equal).

Let's untangle the powertrain aspect.

There is steady state and transient handling. Both matter but generally we attack steady state 1st when tuning. Think of steady state as the skid pad. Big circle, constant turn radius. Does the vehicle track the circle naturally and linearly with respect to steering wheel input?

I think you're alluding to power on oversteer - a transient condition. Otherwise engine doesn't matter. Power on oversteer really isn't going to get addressed by bar other than to the extent that rear roll stiffness (and to a degree front) can help keep the rear wheels planted preventing a loss of lateral grip as the rear tire with the least grip (usually inside) spins up under power. This is a bit of red herring since an open differential tends to do the one wheel peel on the inside, lightly loaded tire. This isn't where the majority of the lateral grip is anyway - it's on the outside tire due to weight transfer. Now, let's consider an extreme example; 400 HP engine. It won't matter what size rear bar you have or how much front vs. rear stiffness you have, the outside tire will break loose (on power) and the vehicle will be inclined to oversteer and/or spin.

So let's not chase bars to solve transient power on oversteer regardless of engine.

With respect to your question on the 912. If you wanted to reduce body roll on a rear engine car that already has an oversteer tendency due to its rear weight bias, you definately would not want to use a rear bar to limit body roll.

So that leaves the front as the logical place to put the ARB. So why not a massive front ARB?

Now we are back to steady state vs. transient handling.

The rear weight bias of the 912 or 911 is always there. However much good the rear weight bias does for acceleration and increased effectiveness of the rear brakes, it is a HUGE liability for transient handling. We all know about 911 lift throttle ovesteer. I've never driven a 912 but everything I've hear is that it actually oversteers less than a 911, due primarily to the lighter T4 engine.

What happens is that if the front has sufficient grip, and turns in quickly at the start of the corner, the (heavy) tail end wants to keep going straight. As the front continues to turn in, at some angle, the back of the vehicle has a sufficient moment arm (leverage) around the center of gravity and the back end begins to "swing" on its own due to that moment arm and inertia. The result is oversteer.

The front ARB helps limit the inherent oversteer to an extent, reducing that tendency for the front to turn in too quickly for the rear to follow. But again this is tranient, at the very start of the turn.

You can't put a big enough front bar on it to completely kill the inherent transient oversteer. If you try, you'll just end up with a car that doesn't want to turn it at all and which continually understeers in more gradual corners once the vehicle (yaw rate) is in a steady state condition. It would lead to a loss of steering confidence.

So not being a 912 guy, I suspect that Porsche found the 13mm to be just enough to settle the rear (during transient conditions), but not so much that it killed the liveliness of the vehicle under steady state conditions. Since I don't have experience tuning a 912, I'm open to criticism but that is the way I'd look at it from a theoretical tuning position.

Note: I should also add that Limited Slip differentials can add another whole set of variables to both transient and steady state handling but I don't want to go there since so few of us have them.

[mepstein]

My 912 came with just the 13mm front bar. The 912/616 engines are not only 200lbs lighter, the weight isn’t as far back as a 911. Less pendulum effect.
But that will be all out the window when I convert it to a 3.2SS.
The nice thing about 914’s are they are so neutral out of the gate. No need to “fix” anything, just improve.

[Literati914]

Wow, always a wealth of knowledge offered up so enthusiastically here on the world .. thanks for the education (everyone), I appreciate it! (IMG:style_emoticons/default/beerchug.gif)

Anyone know if 1.7L cars ever came with sways at all, from the factory? I doubt it, but would like to know for sure…





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[mepstein]

Wow, always a wealth of knowledge offered up so enthusiastically here on the world .. thanks for the education (everyone), I appreciate it! (IMG:style_emoticons/default/beerchug.gif)

Anyone know if 1.7L cars ever came with sways at all, from the factory? I doubt it, but would like to know for sure…



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The early cars did not come with sway bars though they did come with mounts on the a-arms.

[emerygt350]

Having a little roll and compression is a good thing if you are thinking about pushing the weight of the car around. In autocross it is used all the time to increase traction. If you have a powerful engine and powerful breaks you often need very stiff components, but you still get dive and roll because physics. Take those same components and put them on a car with a weak engine and 50 year old brakes, not so great for handling.

[horizontally-opposed]

Great discussion, and I've long wondered about adding a rear ARB.

Consensus when I added a front Weltmeister ARB and 140lb rear springs in the 1990s—hard won by a super active autocross and time trial community around the 914, presumably based on lap times + handling dynamics—seemed to be:

> Weltmeister front ARB, no rear ARB
> 140lb rear springs
> Konis (red for more street, yellow for more track)
> Lowered and aligned
> Widest rubber you can fit

I followed that recipe and also thought the car handled really well, both for autocross and for back roads. But this isn't 30 years ago, and I'm here to learn!

[wonkipop]

Anti roll bar size has nothing to do with engine size …. So how many are really trying to increase understeer?

Superhawk, I based my question on (my) understanding that a front bar is basically there to add a bit of understeer which helps counteract the oversteer situation of a given car.. I certainly may have that all wrong, I’m no suspension expert (& thanks for the book recommendation). But if that’s close to being right, then in my mind a smaller bar would be more appropriate for a simple 1.7L street car than the 15mm one used with a 2.0-2.4L type IV power plant (for example - since they weigh the same) - because 1.7L would theoretically produce less oversteer (all other things being equal).

Anyway the car will get a bar while torn down for bodywork, and the 13mm seems logical enough to me, I’ll probably give it a try. BTW, did any of the 1.7L cars come from the factory with sway bars?

Also, @wonkipop (thanks for your input) says the 13mm bars are on all 1.8L cars, but I don’t recall reading that 914-4 used anything but 15mm - perhaps that was a European/Aussie thing? Anyone?

I wonder what Porsche’s thinking was with giving the 912 a 13mm front bar, it being a more tail happy car than a 914.. but the 914-4 getting a 15mm up front?


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not an aussie /uk thing.
914s never sold in australia.
my car came from maryland usa.
never modded.
out of the box with the roll bars.
standard show room stuff.

1.8 has even less horsepower than a 1.7 unless you are unlucky enough to own a 73 cal 1.7. (IMG:style_emoticons/default/stirthepot.gif)

the 74s are interesting - as i understand it this is how porsche-audi north america decided to set the base models up to improve tham. it is during this period that porsche gained marketing control over the joint project. theoretically you could despec a 74 1.8 and get rid of the performance group package but you would have to order it that way.

to clarify - far as i know its 15 front and 16 rear.
i could try and double check that.
i didn't mean it had 13 mm sway bars.
i meant it had front and rear sway bars, not just a front one.

this is what porsche thought they should have for street.

i've driven an earlier car a quarter of a century ago without the bars.
i preferred mine.

but then again i run skinny 165 tyres so most blokes would consider me a retard.
i do run bilsteins, but that is because boges are not available in the modern world.
it runs stiffer than it used to. i'm not complaining about that. its far superior out on second rate old country roads and stays planted with the bills. but the spriings are soft and standard - and comfortable. we don't have smooth tarmac once you get off the main interstate highways in australia.

if you want the evolved set up for the base cars that porsche thought appropriate thats the set up.

but then again you could experiment.
note - they appeared to have the slightly stiffer bar on the rear,
which kind of goes with your idea of a single softer bar on the front.
but that will be more aggressive effect than the factory set up.
depends what you want to do with the car.
you can also get a lot out of setting it up level or a little bit nose down.
or as the factory did, a little (or a lot) nose up. that made them understeer.

[Superhawk996]

I followed that recipe . . .

The only real problem is that what is good for autocross isn't always what you want for the street. AX tends to favor cars that are easy to rotate at low to moderate speeds. You very well find that ability to rotate isn't appreciated on a big highway cloverleaf sweeper taken at 2x or 3x posted speed. (IMG:style_emoticons/default/dry.gif)

Don't get me wrong, we all follow the recipe to a degree. The moral of the story, don't just follow blindly, tune to what suits you and know why you're doing it.

Following that AX recipe forces some ride degration with the 140# springs that might otherwise not be needed had we opted for a softer spring and a smallish rear bar.

Don't get me wrong, I largely followed the same recipie you laid out when I got started back in the early 90's with suspension and chassis tuning and didn't fully understand the tradeoffs I was making. Not everyone want's to go to all the time, expense, and trouble to to learn how tune. That's OK as long as they know they are making tradeoffs in the process. (IMG:style_emoticons/default/beerchug.gif)

[Superhawk996]

note - they appeared to have the slightly stiffer bar on the rear,
which kind of goes with your idea of a single softer bar on the front.

Can't judge bar stiffness just by diameter. Although the rear bar appears larger (by 1mm) it is a far less effective geometry than the front bar. I'm sure the overall rate of the rear bar is less than the front based on experience though I haven't done the math to calculate the rates.

The 15mm OEM front bar is basically a stright bar with two very short lever arms down to the LCA's. That aspect makes it highly effective.
However, the drop links are long, thin and probably flex substantially reducing effectiveness some.

The 16mm OEM rear bar has very long lever arms. But then the rear drop links are very short and probably don't flex much at all. But on the whole, the stiffness lost to the long lever arms won't be made up by the short, effective drop links.

Bottom line, I don't know the engineering rates (wish I did - next time I'm bored, I'll do it) but don't judge an ARB just by it's diameter.