Not a relevant question for most owners, but for those looking to max the factory adjustable camber or toe settings, the front and rear subframes can affect the alignment adjustments based on if the subframe might be shifted a bit to the left or right side. In my case ('21 GT4) I noticed significantly more camber available on the passenger side, both in the front and the rear. In the front, I can balance the camber side to side with the upper strut mounts but having available lots of camber on the right but less on the left got me thinking if I could shift the subframe toward the driver side, some added camber would be available on the left, and some camber be decreased on the right. Anybody have some good (or bad) experiences with trying to re-align the subframes?

I tried moving the rear subframe today, loosen all the bolts I thought might be useful, and at best might have shifted the subframe to the driver side 1-2mm. Not much, and perhaps my skill is the limiting factor. Will try the front in a few days, but just wanted to toss the question out into the group. Thanks for any feedback!

 

I would not move the subframes. Shim the front lower control arms and the upper strut mounts to get what you want. Max rear with the factory adjustable part. If tracking the car you need to increase rear toe in which will require an aftermarket part.

 

Done, done, and done.

Thanks for your response.

 

Shifting the subframe sounds like the right type of cheating for SCCA stuff or something else that needs to pass tech inspection for a specific racing class. I'd just shim and aftermarket the parts as necessary if that wasn't the case.

 

I've been involved with SCCA Solo classes for 25+ years, and heard my share of "cheating" stories. Correcting a misaligned subframe within the tolerances of the mounting holes is as legal as it gets. It may be negligible in adjustment, and not worth the effort, I really don't know yet. Thus, I posed the question to the forum. Again, I've done the normal adjustments, but the range of adjustment on left vs. right side is quite inconsistant. Maybe I have to live with it, fine. Not exactly something that's mentioned in the owners handbook, but small amount of play in subframe positioning is not uncommon in many manufacturers, Audi is an example.

Anyway, hoping perhaps someone with some technical experience in the suspension can chime in.

 

Hence the comment, the right kind of cheating. Now that we've established why you're doing this I think you won't get a relevant answer without going to a rack and shifting the subframe to assess whether a desired change is measurable on your car.

Have you checked the other usual suspects for inconsistent settings?
ride height
tire pressure
shim stack height variance
eccentric bolt runout & position
alignment done with drivers weight
swaybar preload

For this level of specificity you're probably in the wrong forum, maybe even the wrong site (Try the Racing & Drivers Education Forum or Cup car forum here)

 

My 981 GT4 was a little off-side to side. Attempted to shift the rear subframe as well and ended up with no usable difference. You're talking .1ş maybe...very negligible and not worth the time spent.

 

Thank you for your info! Your experience with your car was exactly what I was looking for. I doubt there is much difference in the subframe mounting design from the 981 and 718 chassis, so I am not expecting a large gain in the left/right balance of alignment settings. As mentioned extensively in other alignment threads, rear camber is limited by amount of rear toe-in available. My situation doesn't allow me to use aftermarket rear toe links, camber plates, or front caster pucks. My left rear has run out of toe-in adjustment, while the right rear has plenty more toe-in adjustment. Same goes for the camber adjustment. I can get 1.5-1.6 deg camber in rear with very minimal total toe-in, but despite the amount of time/labor I'm just trying to squeeze whatever alignment gains I can get from the subframe situation. I'm in the middle of it now, have already checked ride hts, tire press, shims, eccentric bolt (3500 miles), alignment and cross weights with driver wt, disconnected sway bars, so I am running out of options. Will look into moving this thread to another forum (perhaps Autocross) where it may be more appropriate.

Thanks everyone who have tried to contribute info. Appreciate it and hopefully some more info will be added to this thread that may help others too.

btw, I think the 718 GT4 is an awesome vehicle. Probably best purist driver I've owned. Bought it for not only the wonderful driving experience, but also the fairly adjustable suspension. Not quite as much as my C6 Z06, but much more than most BMWs.

 

I would only consider this if you have maxed out both sides of your rear negative camber with shims of the same amount. On my previous Cayman S we were something like -2.7 and -2.9 in the rear and my very experienced mechanic loosening the bolts and torqued on them for a minute. When done, we had right at about -2.8 for each side. I would not have anyone play around with this type of setting, unless they have done it many times before. You don't want to be their guinea pig. If you don't have rear toe links and visit the track, then don't even think about this. It is only for extracting that last bit of negative camber for the rear axle, in a balanced manner .

 

Thanks for the feedback! I am also looking at simply trying to balance out both sides of the rear camber/toe. As you know, the camber and toe are each affected by the other. There is plenty of camber to be had, it's the limited toe adjustment that is an issue. Can the subframe adjustment gain some toe-in for an already maxed side? Would like to see. Glad to hear your experience seemed to work out well for you. I'm also very hesitant to trust my car to a shop to try this "adjustment". Not sure why you mention the necessity of having rear toe links if trying something like this? I'm thinking if toe is maxed already on one side, then an adjustment of the rear subframe in that side's direction might result in a small increase in available toe-in, if camber is kept constant afterward.

It does certainly appear that the limited responses thus far indicates at best a very small gain from trying to move the rear subframe. On the order of 0.1 deg camber each side. I'll update if I gain more info.

 

I was told that there was some adjustment that could be done to try to even the two sides out (I'm at -1.7 on the drivers side and -2.0 on the passenger side,) but haven't yet tried it. I'm honestly not sure that I'm even going to worry about it. The car was fast enough to win Nationals this last year had I not been off my game on day 2.

I'm not familiar with the Porsche subframes, but my previous experience with Miatas is that the mounting points have at least half of them being "centering" style which has only micro amount of capability of movement. That's obviously on purpose to make everything easy. I'd bet our subframe mounting is the same.

Ron

 

Thanks for your input Ron! Followed much of your success last year with your GT4. Congrats on your Nats finish! My rear alignment specs are fairly similar to yours and I believe the movement of the subframe is very minimal at best. Although not a top tier driver as you and others, I have the time and space in my garage to pursue these challenges. Keeping me busy. Wouldn't pay a shop to chase 0.1 deg camber changes!

Hope to say hi to you sometime this season. Perhaps Crows or Packwood

Don

 

Just wanted to update my efforts to gain some alignment values from shifting the rear subframe. Indeed, movement appeared very minimal, but there definitely was movement. Total effort was not bad, never felt concerned about the subframe dropping too far since I only loosened the mounting bolts a few turns and supported the assembly with a bottlejack/2x4. Before the process, my rear alignment was limited to 1.6 deg camber on left, about 1/16" toe-in at best. Right side was set at 1.6 deg also, with approx 1/16" toe-in also. Right side had plenty of additional toe-in adjustment, left side was maxed. Of course I maxed out the camber, as much as I could while maintaining a slight toe-in total. Honestly, my total toe-in was closer to 1/16". Either left or right side could be seen as basically zero toe. Never set at a toe-out at either side. I have tried running the car with zero toe in rear also, but really prefer a bit of toe-in, for the feeling of stability. All drivers are different, so that's just me.

After the subframe shift, and the pic attached looks a bit crazy, but I thought in terms of like dental braces, keep some positive tension on the left, persuade the subframe to shift toward the driver side with lever, BFH knocking on a 2x4 at several points of the subframe. Got the rear numbers to 1.8 deg left/1.85 deg right, a full 1/16" toe-in per side or 1/8" total toe-in. Pretty much my goal for toe. FWIW, I think the subframe shift made the L/R rear eccentric bolts (for camber and toe) closer to each other in the final positions, not exactly the same though. I'm happy with the results, and perhaps not a procedure worth the effort for most, but for me and my access to equipment, it was totally worth it. btw, I shifted the front also, and noticed a more balanced front alignment, gaining a slight amount of camber on the left, giving up a bit of adjustment on the right (it had plenty to give).

 

Wait... are you saying that at max camber you can only get something like 1/16" toe-in at the rear per side? That's all the adjustment there is? Or, do you mean that's where it went to when you were done and you just left it there?

 

The camber eccentrics (and shims if so inclined) can give well over 2 deg camber per side. The problem is with that much camber the toe goes extreme toe-out. The toe eccentric bolts are very limited in the amount of toe-in adjustment when compensating for that amount of camber. Of course, I wanted the max camber possible, WITH at least a minimum of 1/16" toe-in per side. If I adjusted for zero toe total, probably could get almost 1.9 deg camber per side (just guessing, didn't actually try). Camber changes affect toe, toe changes affect camber. I bounced back and forth with adjustments till I got the best of each. Not able to use aftermarket toe links, otherwise this whole effort would not be needed. Would love about 2.3 deg camber in rear, but not gonna happen

Hope I understood your question!