Help me understand coilover suspensions.

[autobonrun]

I've got several questions on this subject. First is a coilover suspension better than struts and torsion bars or just an alternative? If coilovers are better, why did Porsche just use struts and torsion bars on the earlier cars, at least the SC's that I'm familiar with? Does the coil setup come with a matched strut or can they merely fit over existing struts. I have Bilstein sports currently. Just finished reading the Vic Elford Performance Driving book again and it got me thinking about my suspension.

On a related subject there's an interesting article in the latest Automobile Magazine about Delphi's new shocks that work using a magnetic field to modify the fluid's vicosity, giving a fully active suspension at each corner. Wonder when these will hit the aftermarket?

[john_colasante]

Putting coilovers on a 911 can be an expensive proposition. Usually the only reason why you would do it is because you need a higher spring rate than torsion bars can provide. You see, torsion bars are limited to how stiff they can be because of physical constraints. So if you have a 911 race car and you want something like 1000 lbs/in rates you need to go to coil springs.

[masraum]

Coilover struts are most often purpose made to be coilovers. I think I have seen some kits to add springs to regular shocks or struts, but they are generally "helper" springs, not meant to replace springs, but to augment the regular springs (like for a pickup that regularly hauls heavy loads.

I believe the primary reason torsion bars were used was to maximize the space available in the front trunk, and to some degree give more room in the rear as well. With torsion bars (springs) the bars are below the useable space of the car, so they don't subtract from luggage space, rear seat room, etc.... If you take the struts (maybe 3" in Dia) and add a concentric spring around them then you have made the whole thing now maybe 6" in dia. Where you once needed strut towers that were only 6" in dia you now need 9" or 10" dia strut towers which will intrude upon the trunk space and the space for the already very small rear seats.

As far as changing from torsion bar to coilover in a pre-89 car the problem is that the shock/strut towers are designed to withstand the force of the strut/shock pushing up on them, now imagine that force, plus the force normally supported by other parts of the car where the torsion bars mount resting on those points. Those parts of the tub need to be greatly reinforced to withstand the stresses of a coilover suspension.

I think one benefit of coilover suspension is the adjustability. You can easily change springs, adjust ride height. I think a benefit these days (most modern struts are a lot like coilovers with concentric springs) is manufacturing cost. The shock spring assembly is one peice that quickly and easily bolts in with only a few bolts.

Someone else will surely give you much better and more information, but this should have given you some food for thought until then.

I think there actually is already a vehicle with the active shock suspension technology. I think GM is using something like that in the Z06.

[masraum]

John makes a good point. Torsion bars can only be so thick and still fit in their place.

[89911]

Coilovers have been used in the racing series of 911's while they were still using torsion bars. I think that makes a good marketing pitch from these manufacturers to convert these to your t bar 911. Unlike a race car, the 911 does not have the needed bracing at either ends for coilovers, although these manufacturers while tell you otherwise, (Windward ). Most of the race prepped 911 had full body cages that used the shock points as part of the stregthening design. So unless your going to basically strip your car out and add the needed strengthening in these areas, I would'nt bother. I seriously looked into Coilovers the last suspension change I did and went the convensional 22/30 bars and revalved springs and the car drives great. Using the coilovers also will practically end the street use of the car. 500lb/spring rates on the back of cars doesn't feel good on railroad crossing and such!

[jluetjen]

I think that everyone pretty much covered it except for a couple of details.

1) A T-Bar's rate is a function of it's length and the square of its diameter. The length if fixed by the mounting points in a 911, so that only leaves the diameter to play with. As it was said earlier, you can only make them so thick.

2) An added "feature" of T-Bars in addition to their packaging (which incidentally is why most F1 teams use them) is that they can provide springing over a very wide range. Even all the way at full droop, a 911 still has some significant spring force. That is a lot of travel. It is more difficult to do this with coil springs (but not impossible) without making the length very long which then makes them soft.

3) Many people make the mistake of equating stiff springs with high performance. This is not the case unless the car is running out of suspension travel. You want the car's suspension to move -- it's a good thing! Cars which are too stiffly sprung do not communicate approaching limits to the driver and as a result are difficult to drive on the limit. It's like trying to balance on a knife's edge.

4) Adjustable shocks have been around for a long time. Mazda use to offer them in their 323 GT's at least 14 years ago. In that case servos changed the valving of the shock. Carrera started selling the electronically infinitely adjustable MagneShock about 3 years ago. Delphi was working on it in parallel (See Racecar Engineering Vol.11 #4). I guess the oil has metal particles in suspension which change their resistance through the shock's orifices based on the application of a magnetic field. It sounded interesting but keep in mind that many racing organizations do not allow driver adjustable shocks. Still the ease of adjustment would even be welcome in the pits.

I haven't seen very many teams using them which leads me to believe that there must still be some issues to be resolved for heavy duty application. Keep in mind the abuse that shocks take at the race track. You will often find that shocks are too hot to touch at the end of a race.

It's definitely an interesting technology which I'd consider when it comes time to buy shocks for track use.

[82SC]

can you expand on the fact that F1 cars use T-bars...I didn't know that!!!?? I thought they all had that horizontal shocks and springs layout (I think some refer to it as penske suspension??)

also I think T-bars lower the center of gravity which is a good thing too.

And in the POC circut it seems coilovers are great to squeeze that last tenth of a second out of a lap time...some of the top drivers are still using the tried and true t-bar setup...

[Elombard]

I was going down this road as well and decided not to because I did not want to weld in a cage to the rear shock towers. Then I was looking at the Starkey R-RS-RSR book and in a couple of the RSR engine shots I cant see the cage coming into the engine compartment?? Whats up with that? IF the factory did not need to tie the rear shocks in for coil overs why would any one else need to??

Ohhh - did the RSRs run Torsion bars? Maybe thats it - Dohhh

I guess the 935 spring plates and coil overs did not come around until the 935 which was after the RSR??

Anybody got a pic of the roll cage coming into the engine compartment?

[autobonrun]

As usual you guys know the background. From what I'm hearing, I think I should not consider springs. I'll stick with what I have which are the Bilstein sport shocks and stiffer torsion bars.

Probably the weak link in the whole equation is my own driving skill. My car in its present form probably has more capabilities than my skills can take advantage of. I'll leave the suspension alone and concentrate on the human element. Thanks.

[Chuck Moreland]

Originally posted by Elombard
I was going down this road as well and decided not to because I did not want to weld in a cage to the rear shock towers. Then I was looking at the Starkey R-RS-RSR book and in a couple of the RSR engine shots I cant see the cage coming into the engine compartment?? Whats up with that? IF the factory did not need to tie the rear shocks in for coil overs why would any one else need to??

I'm not sure what the RSRs ran, but people succesfully run coil overs rear without tying the cage to the mounts.

They do reinforce the mounts by welding the cap at the head of the tower all the way around its perimeter. They also add some welded on gussets to strengthen everything.

[Elombard]

Hmm... When I talked to the guy in Portland (Rennsport Systems?) I thought he indicated that the "box" that the shocks mount to in there is too flimsy and moves around. I saw in the Starkey book the pics of the gussets they weld in but I am suprised that is considered strong enough for a race car with the spring and the shock driving through there.

Have you seen Engineer to win? doesnt he say you dont want a suspension hard point or an individual roll cage bar hanging out in the air... Should always be supported in compression I believe.

I am not surprised it works like you said, probably a compromise and/or a vintage race rule or something.

Hope to order a pair of your new bushings when I get a few extra bucks!!

[island911]

Originally posted by jluetjen
. . .2) An added "feature" of T-Bars in addition to their packaging (which incidentally is why most F1 teams use them) is that they can provide springing over a very wide range. Even all the way at full droop, a 911 still has some significant spring force. That is a lot of travel. It is more difficult to do this with coil springs (but not impossible) without making the length very long which then makes them soft.
3) Many people make the mistake of equating stiff springs with high performance. This is not the case unless the car is running out of suspension travel. You want the car's suspension to move -- it's a good thing! Cars which are too stiffly sprung do not communicate approaching limits to the driver and as a result are difficult to drive on the limit. It's like trying to balance on a knife's edge.
. . .

Excellent post!

The one other thing I see missing in the discussion; CG. (Center of Gravity)
The torsion-bars help carry the 911 design theme of having the heavy elements of the car very low. The lower the CG, the better. It means it need less swaybar, which in-turn means more independent the suspension movement.

[cab83_750]

Originally posted by Chuck Moreland
I'm not sure what the RSRs ran, but people succesfully run coil overs rear without tying the cage to the mounts.

They do reinforce the mounts by welding the cap at the head of the tower all the way around its perimeter. They also add some welded on gussets to strengthen everything.

I have the inverted coil overs installed on my project car! I did plan on reinforcing the mounts --- even though I was told I do not need to as the car may never see DE (har, har, har1 -- famous last words).

Does any sell the reinforcement kit/units/parts? If so, where?

[Jack Olsen]

Funny you should mention that. TRE Motorsports does the reinforcement, along the lines of the original RSR mods (Pelicanite TRE Cup, the shop owner, used to own an RSR, so he knows the ins and outs of what was done). They also beef up the caps of the rear towers if coilovers are part of the plan.

They did mine today.

[CamB]

"They did mine today."

Hehehe - I like it Jack

Its not really an issue for this discussion, but I need them for the front of the BMW because the standard spring hat interferes with the strut tower at about 0.5 deg negative camber.

Adjustable ride height would be cool too...

[cab83_750]

Jack,

I was right imagining the reinforcements required on the towers. However, I did not imagine the portion pointed by the left. Awesome! Thanks for the post and especially the pics.

A pic is worth a million words (corny, huh?).

I just have another reason to buy a Miller or Hobard welding system. Plus, another reason to justify missing my deadline.

HAR, HAR, HAR!

[jluetjen]

can you expand on the fact that F1 cars use T-bars...I didn't know that!!!??

Here's a quick sketch of what I saw of the Ferrari rear suspension when Rory Byrn discussed it on Speedvisions coverage of the USGP. (Note that I didn't draw any of the frame elements. Only the suspension pieces)



(Forgive the poor artwork!)

Note how this concept moves the shocks to the bottom of the car and integrates the Torsion bars in with the bell cranks. They'll often run T-bars at the front too with a similar arrangement.

Nowadays the F1 teams rarely change springs since they've figured out most of their set-up on their 7-post shaker rigs back at the factory. The same applies to their shocks. They use adjustable shocks sometimes in testing, but at the track the shocks are preset. If they require a change, the send the data back to the factory, the factory runs the tests on the shaker rigs and sends the settings back. Then one of the crewmen will revalve the shocks and mount them to the car. This will apprently sometimes happen between sessions, but more often overnight. It's pretty incredable!

[TimT]

Notice to all about the fact that F1 uses Tbars

they are really short, and large diameter....

The travel in a F1 Suspension is minimal, tire sidewalls are now included..

porsche chose torsions bars for best utilisation of space when the 901 was designed ( gotta cary golf clubs)

The design restraints Porsche imposed upon itself required other methods of springing.... ie coil overs. after the limits of the torsion bars were neared

[Chuck Moreland]

While tbars do make a compact package with excellent trunk space, I don't know that adequately answers the question of why coil overs weren't selected in the first place. After all we can put coil overs into the tbar chassis without altering the towers or trunk space.

I guess that means coil overs can give a compact package with excellent trunk space too.

I suspect the decision to use tbars in the 911 was more about going with a familiar, proven solution. After all what did the previous cars designed by herr Porsche have? Tbars. Including the bug.

[Jack Olsen]

I think I once read that torsion bars were readily available to the company in the early 60's, and were considered easier to manufacture cheaply and consistently.

But I could be 100% wrong.