993 Lowering problem

[JPP]

Need suspension guru's help. Today I had Bilstien HD's and H&R sport springs put on my '95 993. The shop also performed a corner balance and 4 wheel alignment as well. The car is now about 1 1/2" lower all around. The car rode REALLY stiff and the steering felt kind of loose and weird during the ride home. I thought either I'd made a big mistake or maybe the shop had. I then took it out and really pushed it through some twisties and things got better. Seems like the suspension is 'settling in' for lack of a better way to describe it. However, the steering still feels erratic when the front end lifts up coming out of a dip/corner with lots of bumpy feed back in the wheel. Is that what people call bump steer? I've heard that if you lower the front end of a p-car that sometimes you need to install a 'bump-steer' kit. What causes the problem and what does the kit do to fix it?
Thanks for your help,
JP

[Bill Verburg]

It could be bump steer, probably is, but worn bushes, ball joints or even tire pressures out of spec can cause issues.

The oem heights for the 993 are as follows

as you can see the difference between stock US @ ~174 to RS @~124mm is just short of 2".

The factory bump steer correction is provided by uprights and tie rod ends that have a geometry suitable to the lower ride height. Bump steer is aggravated by deviation of the tie rods plane of operation from horizontal, lowering the car w/ stock uprights cause the rods to be angled down, the lower the car the greater the deflection.

You can read more about it, here

[Don Plumley]

Do you have your alignment specs? It would be interesting to see Toe and Caster in comparison to Bill's Chart.

[JPP]

The heights are in inches measured (I believe) to the fender tops:
Now: Front - 25 1/8" Rear - 25 1/2"

Was: Front - 26 7/8 Rear - 26 1/2"

Alignment Specs from the installers sheet:
Front Left Front Right
Now -0.3Deg Before -1.6Deg. Camber Now -0.4Deg Before -1.7Deg
4.4Deg 4.3Deg Caster 5.0Deg 4.8Deg
0.03in -0.37in Toe 0.04in -0.39in
19.8Deg 19.7Deg SAI 20.4Deg 20.0Deg
19.5Deg 18.1Deg Incl. Angle 20.0Deg 18.4Deg

Now 0.1Deg Before 0.1Deg Cross Camber
-0.6Deg -0.5Deg Cross Caster
0.07in -0.76in Total Toe



Rear Specified Range:
Camber -1.4 to -0.9Deg
Toe 0.04 to 0.13in
Total Toe 0.08 to 0.25in
Thrust Angle -0.17 to 0.17Deg

Rear Left Rear Right
Now -1.5Deg Before -2.0Deg Camber Now -1.4Deg Before -1.6Deg

Rear
Total Toe Now 0.52in Before 0.48in
Thrust Angle Now -0.27Deg -0.26Deg

Kraus Racing in Castro Valley Ca., did the install/alignment. I'm going to call them today to discuss.
JP

[hdemas]

If you started from stock height and lowered by 1.5 inches all around, which is the way it sounds, you really aren't low enough to detect any bump steer. This is especially the case if you are just driving on the street. Frankly, even at a lower ride height than you are at, I wouldn't even begin to worry about the bump steer issue unless you are driving on the track.

The car should be a lot stiffer as you just put stiffer springs on it as well as new and stiffer shocks. Stiffly sprung 911s will sometimes get a light feeling in the front, especially over bumps, as there isn't a lot of weight in the front as compared to the rear. As a result, they can feel a bit bouncy at the front. That is normal.

Another thing I would note from your alignment is you went from a fair bit of negative camber up front (-1.6 deg) to almost no camber at all (-0.3 degrees). This will help with tire wear but the car will not be as responsive or have as much grip when you turn into a corner. It may also change the way the front feels as in hard cornering you are riding an aweful lot on the very edge of the corner as opposed to having the forces more evenly distributed across the tire.

Finally, the toe settings are interesting. Is it 0.03in now and -0.37in before for the left front? If that is the case, and assuming here that the positive number is toe out (and the negative indicates toe in), then you went from having front wheels that were pigeontoed in (which is the way street cars come from the factory) to having front wheels that are aligned straight ahead if slightly pointed out. The reason the factory aligns a car with a fair bit of toe-in is that it helps make them track straight and true, and makes them very smooth and predictable in their response to driver inputs. A car that has the wheels pointed straight ahead or slightly out (the way race cars are often aligned) is hyperresponsive to driver inputs...great if you are paying attention and want the car to respond instantly to input, but not relaxing as the car will also tend to dart around from bumps and other imperfections in the pavement.

Of course, if there are clunking noises or anything like that, then maybe something is moving around and you have a dynamic alignment that changes as you drive...generally not considered a good thing.

Hope this helps.

[Don Plumley]

Harry hit it on the head - what I was thinking about toe.

Ride height to fender tops is just a ballpark figure - you really need the factory measuring points as Bill's chart has. Kraus is certainly a well-respected shop. But I don't understand the loss of camber combined with neutral toe.

[JPP]

Yes, this is making sense and describes most of the problem. I talked with Kraus and am taking the car in Monday for some readjustment. I'll talk to them about toe-in.
Thanks for your expert help, I really appreciate it.
JP

[JPP]

Another quick question about Toe and Camber. My main goal with this project was to get the car lowered down about an inch all around and increase responsiveness and feel a little more over stock. I drive this car on the street daily, so alot of hyper response from the front end on the freeway is not what I'm looking for.
The car was fairly level before lowering as well, and Kraus has a 3/4" rake on the car now which means the front springs are compressed that much more. I think I'm going to have them raise the front back up to level to regain some spring response. The car will still be an inch lower than when I started which I'm happy with.
What Toe and Camber numbers should I be asking for to get the car back toward a more docile front end responsiveness. I don't think I want to go all the way back to stock but want a 'compromise' somewhere between where it is now and where it was. Camber was adjusted -1.3Deg. from stock and Toe was adjusted +.40In. left and +.43In. Right. Should I divide those in half and adjust Camber back -.6Deg. both sides and Toe -.2In. Left and -.215In. Right and see what that feels like? Not very scientific, but it's a start...
What do you think?
Thanks,
JP

[hdemas]

Your idea isn't bad. For toe, just put a decent amout of toe in. You will notice that the toe setting for all the factory cars is the same...see Bill's spreadsheet. If you want it to track nicely on the freeway, just shoot for something fairly close to stock. You could get a little closer to straight ahead (less toe-in) but when you get around zero toe it will start to get a bit darty. I always run zero toe in the front, even on my street cars, as I like the responsiveness, but you probably should put a little bit of toe in to be more comfortable with it if you are doing a lot of highway driving.

For camber, the factory settings are really conservative. I would run at least 1 degree of negative camber in the front. The car is light in the front so you should be able to get away with 1 degree and not have too much wear. 1.5 to 2 degrees, which is where you were at before, is also pretty good for the street and will give you even better grip in front, but you will start to see somewhat more wear in the insides of the front tires and if you are doing a lot of highway mileage you probably don't want this much. Mind you, the handling even at 1 degree, if you are just driving on the street, will be a lot better than almost zero degrees which is where you are at. Using 1.5 go 2 is really more for someone who is going to do the occasional DE event or autocross.

For the rear I'd probably leave it where you have it...negative 1.5 or so. I wouldn't go any less than negative 1.5 in the rear as the purpose of that camber in the rear is to make sure the rear stays planted. You will notice the factory settings have similar rear camber across all the models. It is the front that gets more negative camber as you go to the more aggressive RS model. If you run negative 1 in the front and negative 1.5 in the rear, that is a safe street set-up and you will get good mileage out of the front tires and not so good out of the rear (but the rear you will never get good mileage out of as there is a lot of weight back there). If you run negative 2 in the front and negative 1.5 in the rear, you have a set-up that would work pretty well as a compromise between road use and DE events. Hence, just leave the rear as you have it and adjust the front to something in the 1 to 2 range depending on where you want to be on the wear/handling trade-off. One thing to note, though, is that you may not be able to get all the way to negative 2 degrees with the factory strut tops...at some point the only way to get more camber is with camber plates, and that point changes depending on how high the car is. In any event, if you decide to go for more negative camber, if the car will only give you negative 1.6 that is fine and you shouldn't view 2 as a magic number or anything like that.

Most of the cars that I've seen lowered have somewhat more rake than the cars have stock. Having 3/4 more of a drop in the front than the rear sounds fairly normal to me. The big question with the rake is whether you are happy with the way it looks. For driving on the street you won't really be able to tell much of a difference from very small differences in the rake. The spring response should be the same regardless. What I usually see is the cars that are lowered for the street have somewhat more rake in order to get the fenders to be the same height above the wheels, but then when the cars are lowered down to race car level the rake is somewhat reduced.

[JPP]

This sounds like a plan I can work with. I'll show Brandon at Kraus Racing your post and discuss it with him. I'm sure from these suggestions we can work out settings that will create the handling I'm looking for.
Thanks again, I really appreciate your input. You guy's are great
JP