Does lowering 911 front end require realignment?

[winders]

Originally Posted by nickelplated5s

There's a thread over on Early911 about someone with handling problems with a 67. My user name over there would find it. I'm skeptical but here's the claim; OP cut 1" off of the shock rod and re-threaded the same. Thought about it and the only way I can think to do it is run a die down existing threads. Double nut the top to hold it for the die. Interesting idea and might work. My solution was short throw shocks with internal strut bushings. No need to raise the spindles.

If you lower the body of the car in any way, you are lowering the roll center unless you also raise the spindles. The last time I checked, the control arms are attached to the bottom of the chassis. So, if you are lowering the car by shortening the dampers, you still need to raise the spindles....

[KTL]

Agreed 100% with the fellas who are being “idiotic” about the height change. The old 911 front suspension is very sensitive to height change. If you don’t reset the toe when you’ve futzed with the height, whatever was your previous toe setting has changed by a lot



The above is a graph from Porsche (provided by Bill Verburg many many years ago) that shows how the toe changes with front suspension movement. The vertical is height, the horizontal is toe. Trust me, the graph shows a substantial change in toe as the suspension moves. So when you change the static height of the car, you’re then making that toe change tht much more when the suspension goes up and down. This may not matter much to folks just driving around on the street, as they will never really notice it from a feel standpoint. But eventually your tires will show it to you.

In a more telling situation, here’s a case in point of what it does at a faster pace. My buddy decided he didn’t like how his car looked and lowered the front to his visual liking. Next time we were at a track day event, he complained how his car was understeering like crazy. He’s a guy who was always faster than me but not today. I asked him what do you think changed, like are your track tires toasted? He says, well I lowered the front end. I said oh well there you go your toe is all out of whack now, you’re welcome. Reset the toe and all was back to normal, unfortunately for me…..

Lastly, a long long time ago I rebuilt the suspension on my ‘87 911 3.2 and set the heights visually, then got it aligned. It seemed to drive fine, had fun with it at the track days. But man on corner exit in some corners It would lose grip and spin the inside tire quite a bit (open differential, not a limited slip) and I just thought that was because of the open diff. I drove it with that condition for a long time and just blindly assumed that’s just how it is. NOPE.

A track day friend asked another friend (accomplished 911 racecar guy) if he could weigh his ‘79 SC on his corner weighting scales because he was simply curious how much it weighed. So we made a get-together out of it. I weighed my ‘87 Carrera to see how much fatter it was than the SC. What we also found was my right rear was about 300 lbs light. That’s a LOT. The car was basically like a table with a short right rear leg

Point of sharing this is that I put a few turns of raising the left front suspension to see what it would do- transfer some weight to the right rear. What I should have done is work on the right rear but that’s more involved…... Shame on me but whatever.

Wow what a difference next time on the track, even though I had compromised the toe. After that discovery, we set the corner weights to something decent (can’t get the weights spot-on ideal with the original Porsche fixed length sway bar links) and I got a full alignment with more aggressive camber settings. The car was transformed. It drove SO much better and felt much more stable My confidence also went way up because the car was easier to drive and my lap times were faster.

^^^ Take the above for whatever it’s worth. Just one dipshyt’s experience, shared on the internet

[Garen]

Those saying "you don't have to re-align" are surely just having fun and pulling the op's leg, right? Common sense (to anyone familiar with anything suspension related), as well as simple grade-school geometry would prove without a doubt that there would be changes to the alignment with a 1.5" ride-height change without raised spindles.

Plus, how on earth do you verify the setup is symmetrical? By trusting a tape measure at some point on the fenders? This is all assuming the work is being done on level ground with a square and true car. Sometimes people just crack me up. Or do they just crack up trying to wind people up.

OP, do yourself a favor and get that car aligned by an expert and reap the rewards of the improved driving dynamics. You might even get better tire wear as a bonus. I'm assuming you don't align cars, otherwise you'd just check and align after your ride height drop.

[KTL]

A set of toe plates is a cheap tool and an easy, very effective way of resetting your toe accurately

[nickelplated5s]

Originally Posted by winders

If you lower the body of the car in any way, you are lowering the roll center unless you also raise the spindles. The last time I checked, the control arms are attached to the bottom of the chassis. So, if you are lowering the car by shortening the dampers, you still need to raise the spindles....

I'm confused. What's the problem with lowering center of mass? Shorter throw shocks only means you don't bottom them out.

[Bill Verburg]

The Roll Center Height affects the lever arm length causing cornering roll.

The only things that you can sortta easily adjust that affect roll center height are
ride height
spindle height

The effect of lowering on RCH is illustrated here, an eyeball metric is the angle of the A-arm, the more down slope the higher the RC, the shorter the RC arm A, the less roll, it's just like using a stiffer t-bar w/o the bad side effects



The other separate but related thing to watch is the steering rack and steering arm heights

2 ways to adjust the steering arm height
rack spacer (RS)
adjustable steering knuckle

this table shows the relationship between using a notional 10mm rack spacer(actual thickness is more like 11.5mm) and ride height, ideally to retain stock steering geometry the sum should be ~110+/-, an adjustable steering knuckle is only needed if the car is lowered to RSR height, ~160mm.

[nickelplated5s]

It's a long thread but take a look. My ball joint photo is on page 11. OP's is page 1 or 2 I think which explains his problem. I defer to BV but there are other ways to skin a cat. Yes, stiff suspension. Roll, no. In other words my suspension compensates for the lower CG.

https://www.early911sregistry.org/forums/showthread.php?173561-1967-handling-problem/page11

[David Inc.]

Originally Posted by Bill Verburg

The Roll Center Height affects the lever arm length causing cornering roll.

.....

I love suspension geometry stuff. It makes sense but it's absolutely not immediately intuitive. I'll say that raised spindles and bringing the front end back up a bit has really helped the front end.

For a little more on the concept--the roll center is the imaginary point in which the body tries to rotate (at that axle). The distance between the center of gravity (where side forces are applied) and the roll center is like a moment arm, so the higher the distance between CG and roll center the more torque there is on the body.

With a wild enough roll center you could actually get the body to roll opposite to cornering forces, but everything else would be terrible.

Interestingly, I saw in Formula 1 that at one point Ferrari had a neutral RC/CG at the front end to remove roll entirely. They stopped using it, though, because they were getting a ton of sideways movement at the front wheels under braking which upset grip (the wheels would move away from the body under braking load).

[winders]

Originally Posted by nickelplated5s

It's a long thread but take a look. My ball joint photo is on page 11. OP's is page 1 or 2 I think which explains his problem. I defer to BV but there are other ways to skin a cat. Yes, stiff suspension. Roll, no. In other words my suspension compensates for the lower CG.

https://www.early911sregistry.org/forums/showthread.php?173561-1967-handling-problem/page11

If you mean to say your suspension compensates for the lower roll center, no, it does not. The only way to compensate for the lower roll center is to raise the roll center. If you want to keep the front end lower, the only way to do that is to raise the spindles.....

[Bill Verburg]

Originally Posted by nickelplated5s

It's a long thread but take a look. My ball joint photo is on page 11. OP's is page 1 or 2 I think which explains his problem. I defer to BV but there are other ways to skin a cat. Yes, stiff suspension. Roll, no. In other words my suspension compensates for the lower CG.

https://www.early911sregistry.org/forums/showthread.php?173561-1967-handling-problem/page11

The red line approximates the free body line of action for determining the instant center, which is under ground here, the roll center is also probably under ground, not good

[Bill Verburg]

Originally Posted by David Inc.

I love suspension geometry stuff. It makes sense but it's absolutely not immediately intuitive. I'll say that raised spindles and bringing the front end back up a bit has really helped the front end.

For a little more on the concept--the roll center is the imaginary point in which the body tries to rotate (at that axle). The distance between the center of gravity (where side forces are applied) and the roll center is like a moment arm, so the higher the distance between CG and roll center the more torque there is on the body.

With a wild enough roll center you could actually get the body to roll opposite to cornering forces, but everything else would be terrible.

Interestingly, I saw in Formula 1 that at one point Ferrari had a neutral RC/CG at the front end to remove roll entirely. They stopped using it, though, because they were getting a ton of sideways movement at the front wheels under braking which upset grip (the wheels would move away from the body under braking load).

There are 2 types of weight transfer in roll
Elastic: through the springs and shocks
Geometric: through he suspension linkages

The issue w/ geometric is it happens instantly, elastic takes some time as the springs compress, as the roll center is raised the ride becomes more harsh w/ a tendency to hop, F1 cars keep their roll centers low because of this effect

lesson: you want roll center high bur not too high

[winders]

Originally Posted by Bill Verburg

There are 2 types of weight transfer in roll
Elastic: through the springs and shocks
Geometric: through he suspension linkages

The issue w/ geometric is it happens instantly, elastic takes some time as the springs compress, as the roll center is raised the ride becomes more harsh w/ a tendency to hop, F1 cars keep their roll centers low because of this effect

lesson: you want roll center high bur not too high

Yes. You end up with too much jacking forces. This is same for anti-dive and anti-squat. If you have too much anti-dive or anti-squat or too little roll, the load transfer is too fast which causes the tires to lose grip...

[brianlay]

Originally Posted by Bill Verburg

The Roll Center Height affects the lever arm length causing cornering roll.

The only things that you can sortta easily adjust that affect roll center height are
ride height
spindle height

The effect of lowering on RCH is illustrated here, an eyeball metric is the angle of the A-arm, the more down slope the higher the RC, the shorter the RC arm A, the less roll, it's just like using a stiffer t-bar w/o the bad side effects



The other separate but related thing to watch is the steering rack and steering arm heights

2 ways to adjust the steering arm height
rack spacer (RS)
adjustable steering knuckle

this table shows the relationship between using a notional 10mm rack spacer(actual thickness is more like 11.5mm) and ride height, ideally to retain stock steering geometry the sum should be ~110+/-, an adjustable steering knuckle is only needed if the car is lowered to RSR height, ~160mm.

So based on this table if I'm using spindles raised 19mm I should not be using the 10mm rack spacer? I put the spacer in when I originally lowered the car with stock struts.

[stownsen914]

There's a lot to chew on when you start talking about moving roll centers around. Lowering the car will move roll centers around. Changing spindle location will move them around. So does moving suspension pickup points, like a 935 style suspension kit does.

As big or worse though is that when you move the suspension substantially away from its normal range, extraneous movements become more pronounced, like:
- side to side movement of tires as they travel upward and downward (scrub)
- toe change / bump steer gets worse
- bottoming can be an issue when lowering the car

Roll center changes can also become more extreme, but it becomes secondary to the above. As I believe David said above, the things that change roll center aren't always intuitive. The only way to really know where your roll centers are is to measure everything and map it. But it's not usually necessary to do it for a 911, as the mods that work are pretty well understood.

If you play around with suspension modeling, one thing you'll quickly realize is that everything is a compromise, and getting the "optimal" setup winds up being very specific to a given set of parameters (ride height, tire/wheel sizes, etc.), and as soon as you change a parameter, you have to change other things to re-optimize.

Raising the spindles addresses the issues pretty well in most cases when lowering a 911. 935 suspension kits take it a step further.

[Bill Verburg]

Originally Posted by brianlay

So based on this table if I'm using spindles raised 19mm I should not be using the 10mm rack spacer? I put the spacer in when I originally lowered the car with stock struts.

correct, but remember the table is based on a notional 10mm rack spacer, The last one I measured was 11.5mm and I don't know if they are all that size or not.

also remember the factory specs for street 911 ride height are +/-5mm, there is a 10mm range for all the values associated w/ it

In other words I wouldn't worry about it too much

[winders]

Originally Posted by stownsen914

Raising the spindles addresses the issues pretty well in most cases when lowering a 911. 935 suspension kits take it a step further.

The ERP 935 A-arm kit doesn't do anything to change roll centers. Well, if you use it to change the front track it might change the roll center a bit but it's not going to be much. If I visualize it correctly in my head, widening the track actually lowers the roll center.

[stownsen914]

I've always understood the 935 style kits move the rearward A arm mount inward. And possibly upward as well. (Can't speak for the ERP kit specifically.) Moving suspension pickup points definitely moves roll center.

[winders]

Originally Posted by stownsen914

I've always understood the 935 style kits move the rearward A arm mount inward. And possibly upward as well. (Can't speak for the ERP kit specifically.) Moving suspension pickup points definitely moves roll center.

It depends in what plane you are moving the mount points. The point of moving the rear pickup point up as is done in the 930 chassis is to increase anti-dive, not change the roll center. The angle of the plane formed by the three points of the A-arm (the joint on the strut and the two chassis mounting points) is what matters. Raising one end of the chassis mount points a very small amount (which is all the ERP kit does) is not going to raise the roll center much. Certainly not the extent that raising the spindles 19 to 30mm would accomplish.

[Bill Verburg]

Originally Posted by winders

T.. If I visualize it correctly in my head, widening the track actually lowers the roll center.

That's correct and it also increase the effectiveness of the springs, w/ the same 7" wheels and stock 18.8mm t-bar changing track from SC 1.38m to 930 1.432m raises the front roll rate from 236 lb-ft/deg to 254lb/ft/deg