Today's suspension and tire experiments

[Bry5on]

This afternoon I ran a couple quick experiements, I'll go ahead and share the results here for those that are interested.
1. Can 20" rear wheels and tires be run on the front for a square setup?
2. How much travel is there in the rear until we hit the bump stops?


1. Like some of you may have noticed, this car tends to understeer when pushed. I prefer a bit more neutral of a car, and one way to do this while also squeezing a bit more cornering oomph out is to run a wider front tire. I'm on the 245/45/20 F - 285/40/20 R setup and was curious to see if a 275-285/40/20 tire would work up front. A little ballpark math guided me toward a ~10mm spacer up front to keep from impacting the strut.
So I tried the above setup - 10mm front spacer, 20" rear wheel mounted up front. Well I'll be damned, it fits and there's still clearance throughout the steering sweep. There is only about 3mm wheel/tire clearance to the strut and the tire pokes out a bit more than I'd like unfortunately, so it's unlikely that I'll run this setup, but dropping .5-1" inch off the wheel width and running a 275 would be a pretty good compromise up front. Steering sweep clearance was min ~5mm up front at full lock (the worst position) and ~15mm in back at full lock (again, the worst position). Some photos below:

2. Several folks here have talked about lowering their CT with lowering links. I tried this and found that I didn't like the feeling of smashing into the bump stops so I reverted to stock. Several folks debated whether or not the car actually hits the bump stops, and I wanted to put some numbers behind what I had felt. Measuring the front is not trivial, but the back is pretty easy so I started there.

With the wheel drooped, I started by measuring under the dust boot until I hit the top lip of the strut body, then I added a conservative 2mm to this measurement and marked it on the outside with tape:

This leaves you with ~145mm of strut travel from full droop until you hit the bump stop, seen in white/yellow at the top of the strut:

We then want to translate this distance to wheel travel. Since the strut picks up the lower control arm at about 15" and the wheel is about 18", we must multiply 145mm X 18/15 to get 174mm of wheel travel. Then by putting the wheel back on (droop remained unchanged when I did this, FWIW), we can measure from the top of the fender to see how much tire 'tuck' on the fender there is before bottoming out, to get an eyeball reference point:

What this shows is that there is about 17mm of tire tuck (.67") on the CT before you bottom out the rear suspension.

So what did we learn? If you lower your CT that has 20" wheels so that the tire is even with the fender trim, it'll take a .67" change in pavement to bottom out your suspension. Around me in the bay area, I regularly traverse roads with 1.5" changes in pavement. My stock CT in its lowest setting has about 1" of fender gap or so, and I notice a significant difference in the number of times I bottom out between "Lowered" and "Low" (lowest).

Now, some math to help those lowerers. If there's ~1.67" of total travel:
10mm: Lowering your car 10mm consumes 25% of your total travel available.
20mm: Lowering your car 20mm consumes 50% of your total travel available.
30mm: Lowering your car 30mm consumes 75% of your total travel available.
40mm: You're sitting on the bump stops

Hope this was helpful!

edit: If anyone has a gopro and some suction cups, it would be great to get a video of suspension compression while driving!

 

[bsclywilly]

Thanks for sharing these details.

The understeer is definitely noticeable and a square tire setup makes alot of sense for a 51% rear weight distribution. I can only imagine Porsche decided to downsize the front tires for efficiency and road manners, but who cares.

Was the 3mm clearance in your test to the tire or the wheel? The good thing is that for the laden wheel, that clearance should only get larger with camber compliance. You've got me rethinking my AM wheel spec.

I've also seen many reports of premature rear tire inner edge wear. Have you considered a more balanced alignment to get rid of some of the understeer? I took some camber and ride height measurements and am surprised at how much camber is in the rear. I couldn't find Taycan specific alignment specs but it seems 911s and Panameras alike are similarly spec'd with more rear camber. I plan to take out 1-1.5degs in the rear to help with balance and wear.

Note, measurements are with 20mm drop via lowering links. Height measured ground to fender edge.

I too have been somewhat obsessed with bottoming out after our prior discussions. I don't think the amount of travel directly correlates with the size of bump that can be absorbed before hitting the bumpers since you've got some warp and roll couple effects going on. I had attempted to find full compression with an articulation test, 3" blocks under diagonal corners. The car heaved so much due to the roll and spring stiffness that the compression travel barely changed visually and I didn't bother to measure. This of course doesn't take into consideration the inertia effects of hitting a bump at speed.

Gopro would be one way to confirm. Do you know if the bump stops and dust shield can be pulled down the damper shaft? You could go for a drive and check after if the bump stops have been pushed back up to the top.

One thing I've considered is that with the Taycan's 3-chamber air suspension and electronic damping, there is alot of factory tuning involved with damping and spring rates. At lower ride heights, a smaller chamber volume, you naturally run a lower pressure and an increasingly progressive rate (and the natural air spring rate is progressive because it's an adiabatic process, ideal gas law doesn't work here). It wouldn't' surprise me if damping rates are also adjusted to compensate for the loss of spring rate at lower ride heights, spring rate possibly too, and that contributes to a harsher ride you feel in low.

Anyone lowering their car needs to consider the roads they're driving on. You can't have the car slammed if your roads are crap. That said, my goal is to find the lowest height that's still comfortable to drive. The bump stops are also there for a reason for the occasional or unexpected event.

 

[Bry5on]

Thanks for the data! Rear camber looks alright to me based on similar mass vehicles, I just wish I could dial a bit more in front.
The 3mm clearance is actually about the same to wheel and tire due to the built in rim protection of the PS4 tire and the shape of the suspension upright.
275/40/20 and 245/45/20 are almost the same diameter, so I think that 275/285 really is a prime pairing here.

Anyone know much about cutting and widening wheels? I’m not an expert on welding cast aluminums, but I wouldn’t expect a lot of stress in the mid-barrel section..

 

[philbur]

another way to change the understeer is to stiffen the sway bars - this changes the grip balance front to back similar to changing the grip level with different witch of tires. you can also dial that in with tire pressures - lots of stuff to play with I personally have found that changing sway bars makes the most dramatic difference in balance dynamics. im sure the lawyers demand understeer for the general public.

 

[Bry5on]

I run -2psi front and +1 psi rear and got the dynamic sway bars with PDCC ? I hear you though!

 

[buruburu]

When I was autocrossing my Taycan 4S on the OEM 20" Summer tires, (245 front / 285 rear), understeer was pretty bad on the reccomended tire pressure. I ended up settling on 36psi (hot) all around in order to get the balance to be neutral and not roll over the tires. My plans is to run 275 front / 335 rear Goodyear Eagle F1 Supercar 3 as those are the only 220tw extreme summer tires that had the load rating that can support the Taycan.

Biggest annoyance so far is that I don't see any centrics that can be used to adjust front camber.

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