Front Coilover Question

[kyngfish]

Hi all,

Reading through some old threads asking about coilovers as I start doing a suspension refresh on my 86 911. I had a few questions:

- People on here seem to go back and forth about reinforcing the front towers. This car is pretty much all street, all day, and the Rebel Racing kit isn't too far off the price of a higher end torsion bar setup, what's the downside here? Do I need to reinforce the front?

- What's the weight savings?

- My Bilstein shocks are new-ish, I'm assuming those cores fit in the coilover setup?

- For the rear, I'll switch to the 935 spring plates and reinforce the rear shock towers (unless someone on here says it's really not worth it), and since it's a bit of a higher investment, I may wait on that one 6-10 months after I do the front. Any downside to running coilovers in the front and leaving the rear stock for a bit?

I ran a search for threads, and I saw a few discussions, but nothing comprehensive and recent. If I missed it, my bad, and feel free to point me to the right place.

[Walt Fricke]

I don't think coilovers save weight. They are a spring, torsion bars are a spring. I've not seen this given as a reason for changing, but maybe I've missed it.

I think all of the coil is unsprung weight. I don't think any of the torsion bar is unsprung. And coilovers have to raise the CG, but not so much that guys don't prefer coilovers for race cars where rules allow.

The only downside of doing one end at a time is that your car's balance will be off when you drive it during the interregnum. Because what's the point if you pick spring rates which duplicate, or are very close to, the effective rates of the TBs. If your sways are adjustable, maybe you could correct that with them.

For stiffer spring rates, at some point you need shocks with less compression (the springs deal with that) and more rebound. But if you have Bilstein struts, your Bilstein shock inserts should work. Not sure about Bilstein for Boge shocks - they have a sleeve so the shock can be attached where a Bilstein has to be - at the bottom of the strut. Someone will know that.

The reason to use coilovers on a 911 is that even the very biggest bars aren't quite optimal. And coilovers are a snap, relatively speaking, to change, so you can experiment. And easy to deal with ride height and corner balance.

[Harpo]

I'm in the same boat. I'm only doing the front suspension with coil overs this spring. Rear suspension will have to wait to next year.

David

[Sboxin]

We did the coilover conversion a while back and have been satisfied with the results in my son's race car (see details in Pelican garage). The car does ride smoother over the track curbs. We also have adjustable sway bars and other modified suspension parts.

We did not reinforce the front body and have had no problems. We did reinforce the rear with the Elephant kit.

Chuck at Elephant was very helpful in determining the spring weights and rebuilding the Bilstein dampers to match the springs - a very important step in the conversion.


The 935 rear spring plates also are a big help in getting the toe/camber settings correct to match the tire we are using.


As Walt said - there is probably no car weight savings but the adjustments are a lot easier to make for corner balance and ride height.

Hope this info helps,
Regards,

[emcon5]

Originally Posted by kyngfish

This car is pretty much all street, all day, and the Rebel Racing kit isn't too far off the price of a higher end torsion bar setup

They why are you doing it? The only real reason to go with coilovers is to have spring rates higher than you can get with Torsion bars alone, and that is too much spring for a street car.

Unless your goal is to be able to say at Cars & coffee, "I have coilovers"

[kyngfish]

Originally Posted by emcon5

Unless your goal is to be able to say at Cars & coffee, "I have coilovers"

I think I put the reason in my OP. My understanding is there is some weight savings. So if dollar difference is minor and there is a weight savings + giving the car some adjustability - it might be a worthwhile investment. But yeah. Maybe deep down my goal is to be a jerk and cars and coffee. That was helpful.

[kyngfish]

Originally Posted by Sboxin

We did not reinforce the front body and have had no problems. We did reinforce the rear with the Elephant kit.

As Walt said - there is probably no car weight savings but the adjustments are a lot easier to make for corner balance and ride height.

Hope this info helps,
Regards,

Yes, this does help, thank you.

I've seen some threads that discuss the weight savings, but honestly, I can't find it now. Mainly I was just making price comparisons, and if new torsion bars are 400 give or take, and the coilover kit for the front is 500-ish... then why not go with the more flexible system. I was mainly curious about whether there was anything I was missing about the changeover or hidden costs.

[emcon5]

Originally Posted by kyngfish

I think I put the reason in my OP. My understanding is there is some weight savings. So if dollar difference is minor and there is a weight savings + giving the car some adjustability - it might be a worthwhile investment. But yeah. Maybe deep down my goal is to be a jerk and cars and coffee. That was helpful.

You asked what the weight savings would be, you didn't state that was the reason. You car already has plenty of adjustability.

Yeah, coilovers would be a little easier for playing with the ride height and corner weights, (and faster to change spring rates, but that is more of a race car thing) but how often do you actually do that?

Any weigh savings would be negligible, and the weight that is there would be moved up, raising the CG a bit. It could actually increase weight, depending on how you do it. The sleeve coilover conversions can be used with standard T bars, so you have a lighter spring on the strut, and less load on the shock tower, which wasn't designed to support the weight of the car anyway.

Is weight savings the reason you want to go to a 935 suspension in back?

If you go through with this, how bad the handling gets sodomized by doing the front before doing the back will depend on how heavy you go with the front springs. If the spring rate is close to stock (which goes back to the why-would-anyone-do-this-to-a-street-car? question), then it shouldn't change it much.

Generally speaking, increasing the spring rate in front without changing anything else will move the car even more towards understeer than it already is.

[kyngfish]

Originally Posted by emcon5

Is weight savings the reason you want to go to a 935 suspension in back?

If you go through with this, how bad the handling gets sodomized by doing the front before doing the back will depend on how heavy you go with the front springs. If the spring rate is close to stock (which goes back to the why-would-anyone-do-this-to-a-street-car? question), then it shouldn't change it much

I mean, I feel like in your mind I have to justify why to put a coilover suspension on a mainly street car.

Honestly - if I plan on keeping the car forever, why not just generally improve it? I just wanted to know if front reinforcement was necessary.

935 spring plates look simple to maintain, and don't the bushings that need periodic pain in the ass replacement. Overall, just feels like a cleaner system.

If it's a question of an extra 400 or 500 here or there for a car I am - as far as I know - keeping for as long as I can, I'm just wanting to make sure when I spend the money for a full refresh, it's for the most versatile system I can make it.

[Walt Fricke]

Reinforcing: I have a '68 based GT car, which came to me with coilovers. The rear shock mounts had been reinforced with a connection to the roll cage. I still have had to weld up cracks in the caps. I would recommend reinforcing, using the "standard" RSR type gusset.

The fronts had been modified, and I had a thick steel plate, with slots for the shock mount and two beside it for adjustment. (this was a mistake - I should have used a commercial product allowing for caster adjustment as well as camber). So I don't have personal experience. Stiffer springs will increase the G loads. Tracks generally are smoother than highways, and no pot holes. But bouncing off curbs is often the fast way around. The current Porsche sports cars use a cast aluminum shock tower, and there are reports of these towers breaking running over a pothole on an Interstate. However, since you have reports here of cars converted without reinforcement, it sounds like you could skip that, at least for now. Just keep checking for cracks. The front is much more accessible for reinforcement purposes than the rear, where you really want the engine out.

Coilovers get pretty close to the shock tower tub area. Cars frequently have part of this cut out, and metal welded in to increase clearance. This can serve as reinforcement. At the same time, shock tower cross braces of various designs can be welded in, or mounts added for adjustable/removable ones.

I am currently on crutches from breaking my pelvis skiing, or I'd go fish out a torsion bar and weigh it, and weigh a coilover spring.

However, you can pretty easily find the length of the torsion bars. You know their diameter. So calculate their weight, using the weight per volume for steel.

Springs might be a bit more difficult to calculate (as opposed to just setting on a scale, but maybe some catalog seller gives a figure. My GT car has 2.5" springs, but 2.25" may be more common and fit on Bilsteins? The Bilstein coilover strut for the front and shock for the rear has the lower part threaded already, so the strut/shock ought not to weigh more.

You'd have to add in the weight of the bottom adjustable perch, and the top hat. If you are going to use tender springs, add that weight (minimal) and the weight of the separator.

You could deduct the weight of the steel splash guard stock shocks have up top, as you can't use that with coilovers (is that a minus for a street driven car?), But without going to coilovers you could just do what the racers do with that part on their torsion bar suspension: toss it, and maybe replace it with a rubber bellows on what you have. I don't know if there is a bellows one could fit inside the coilover - I don't recall seeing a race car with one, but never had that in mind when looking at race cars.

Rear weight differential calculation is same as the front.

Some of us would be interested in what you calculate as an abstract matter, but it is racers mainly who do the conversion so they can get the very stiff spring rates which allow them to corner at a few 10ths of a G faster, and have more flexibility in experimenting with rates to achieve that goal. Overall weight reduction is approached with removal or replacement of lots of other parts of the car, where the reductions can be substantial.

[emcon5]

Originally Posted by kyngfish

I mean, I feel like in your mind I have to justify why to put a coilover suspension on a mainly street car.

You don't have to justify anything.

The questions you are asking lead me to believe you don't really know much about suspensions in general and what changing things will do to the handling/balance of the car.

Originally Posted by kyngfish

Honestly - if I plan on keeping the car forever, why not just generally improve it? I just wanted to know if front reinforcement was necessary.

Why do you think coilovers are an improvement?

Sure, big changes to ride height are easier, but aside from extremely rare exceptions, that is not something anyone actually does. The reality us you can reach the adjustment on a stock suspension without even jacking up the car.

If you want to stiffen the springs, you can more than double the spring rate without re-engineering the car by going to thicker torsion bars, and put the stress where Porsche designed the chassis to carry it.

Keep in mind that if you go too stiff, you will need custom valved shocks to match the springs. IIRC less bump, more rebound, but I could be wrong on that.

Originally Posted by kyngfish

935 spring plates look simple to maintain, and don't the bushings that need periodic pain in the ass replacement. Overall, just feels like a cleaner system.

It is a compromise. Sure, you won't need to replace the bushing every 20 years, at the expense of noise and vibration. That isn't something that matters on a race car, but I wouldn't want that on a street car.

Plus that whole re-engineer the car thing. To me it seems like a really cool and expensive solution to problems that don't really exist on a street car.

Originally Posted by kyngfish

If it's a question of an extra 400 or 500 here or there for a car I am - as far as I know - keeping for as long as I can, I'm just wanting to make sure when I spend the money for a full refresh, it's for the most versatile system I can make it.

Well the 935 spring plates alone are ~$850, that doesn't count the rear coilovers that are required.

I went through this exercise years ago. I did a lot of research and read a lot, talked (and listened to) people who knew more about it than I did, and came to the conclusion that I should leave the springs alone. I essentially ended up with your stock suspension. I replaced my SC sway bars with the thicker factory bars from a 86, turbo tie rods, and new Bilsteins, HD in front and Sport in back.

It is nice to drive, goes where I point it without being uncomfortable, and is still faster than I am on the track (or would be, if I still went to the track).


What problem are you trying to solve? What about the current car handling do you want to change?