How I Cut My GTI’s Weight Where It Matters Most: Unsprung and Rotational Mass
Simplify and add big brakes.
One of the few gripes I have with my 2010 VW GTI is how heavy it feels on its feet. And it is heavy, for a hot hatch. Tipping the scales at around 3,100 pounds, it’s a couple of hundred pounds too porky in general. But reducing unsprung weight in particular really improved the car's performance.
The iron-block EA888 TSI engine that hangs (literally) in the engine bay makes most of the curb weight figure sit in the very front of the car, relatively high up. After wrecking a set of Michelin Pilot Sport 4s' in 8,000 miles, I decided it was time to do something about it, focusing on the front suspension, brakes, and wheels.
As any self-respecting car owner does, I was Googling “mk6 gti racecar sick” one night and found some interesting info on forums. The actual VW-factory Grand-Am PQ35-based race cars run aluminum uprights (the part that holds the wheel bearing) and aluminum control arms. PQ35 and its variations are the platforms that 2004-2013 Golfs, 2008-2012ish Passats and CCs, and Audi TTs run on.
No Golf ever came with aluminum suspension for cost savings. Mk5 Golfs used a cast steel front lower arm, and a cast steel upright. Mk6 Golfs upgraded the lower arm to a stamped steel unit. So where did that aluminum stuff on the race cars come from? It didn’t look like bespoke motorsport hardware; it was suspiciously mass-produced looking.
I started spitballing with Google, going back and forth with various Volkswagen platform-mates. I looked up “VW Passat aluminum uprights” and hit the jackpot. These looked exactly like the parts on the race car. I went to a parts catalog, input a 2009 Passat 2.0T, and found a part number that cross-referenced to just one other car: the VW CC.
These cars are on the stretched PQ46 platform, basically a longer PQ35. As cars that are heavier and exist more upmarket, some bean counter at VW green-lit an engineer’s plea for weight-saving aluminum suspension members front and back. A decade or so later, the dumb kid writing this article is going to take full advantage of it.
I browsed my local junkyard listings and found a few Passats and a CC, but I'd advise you to never bet on any boneyard car having the parts you need. I always make sure there are a few options before I make the 40-mile trip to my favorite yard. That said, this yard has some ridiculously tasty parking lot oranges so the trip was never going to be a total loss. Both Passats were a bust, but I got what I needed from the husk of a CC.
I was so excited to get the old parts of my car to weigh the difference that I just sat and examined the new upright for an hour in my garage. I had to wait until the next morning to dig in. I did buy another wheel bearing that night, to make the front a matching pair of new ones.
The next morning I got up bright and early, shotgunned a rose milk tea, and tore the front suspension completely apart in 45 minutes. I had my strut-spreading technique down from a particularly rough night with my Bilsteins... but that's a story for another time.
Once I got everything off I was able to weigh stuff up. It worked out like this:
- Mk6 GTI stamped steel control arm: 5.2 pounds per side
- CC Aluminum control arm: 3.4 pounds per side
- Difference: 1.8 pounds per side for a total of 3.6 pounds saved
- Mk6 GTI cast steel upright: 9.4 pounds per side
- CC aluminum upright: 6.4 pounds per
- Difference: 3 pounds per side for a total of 6 pounds saved
- Total weight saved: 9.6 pounds unsprung non-rotating weight
Another hour of re-install and my nearly 10-pound weight reduction was finalized! And since that weight was unsprung, as in, not held up by the car's suspension (it is part of the suspension after all) it has an even greater affect on the car's performance. In a good way.
Since this topic comes up a lot while discussing the weight of wheels, there are some misconceptions floating around regarding the difference between unsprung mass and rotating mass. Shaving rotating mass, reducing the moving weight of your wheel and tire set and driveline bits that physically spin, is hugely beneficial for performance and efficiency. The less weight that the engine has to spin, more power actually makes it to the ground.
Unsprung mass is a little less clear, but it definitely changes the natural frequency of the suspension. I've felt no ill effects from this modification yet, though I'm not quite ready for driving impressions as there are more exciting mods afoot.
The suspension swap cut my unsprung mass, but for a reduction in rotating mass I bought myself a set of used 17-inch Wedsport SA-97F wheels. UpGarage USA had them in a warehouse for $380, and they were perfect fitment for the GTI. The stock 17-inch Denver wheels weigh 26 pounds without tires. The Weds weighed 19 pounds without tires, a huge rotational weight saving. I used the same sized Michelin PS4S tire as a control tire.
With that, we have 28 pounds of rotational weight savings. That's huge, and that’s before my next magic trick…
To further the mission of unsprung and rotational weight saving, I got myself a used Stoptech ST-41 big brake kit. The VW's stock front calipers weigh 14 pounds apiece with pads. The forged ST-41 calipers weigh 8 pounds with pads, so that’s another 6 pounds unsprung shaved from each front corner. The new single-piece rotors work out to about the same weight, so no savings on rotating mass.
So let’s add it all up. I saved 9.6 pounds from the aluminum suspension stuff (which also have the side effect of increased rigidity), 28 pounds of rotating mass from the new wheels and another 12 pounds of unsprung weight from calipers.
In total, I saved 49.6 pounds of total weight from the front suspension. Scientifically speaking, that's a metric fuckton!
As for how it affected the car's feel – the short story is that the difference is unbelievable. It smacks you in the face the first time you release the clutch and roll away from a stop. My GTI used to feel on the heavy side, and the dual-mass flywheel didn’t help.
It’s now much easier, much more fluid. Floor it in second, and the difference in acceleration is startling. The car was fast-ish before, but slow to build momentum. Now it feels like the turbo is spooling faster, and there's less delay between flooring it and the drivetrain taking up the slack and responding. It’s hard to describe lightness, but it’s all in transient response.
Ride quality went from rough on Bilstein coilovers to actually nice. Not surprising, but a welcome change.
Turn-in is my favorite upgrade area. The sharpness in transient response is what I really wanted... "Transient response" refers to how quickly something can change inertia, direction, or position. It changed the whole character of the car from a lazy old grand touring hatchback, to razor-sharp hot-hatch.
It's the rate of change that's the crucial bit with weight savings, it makes the biggest difference when stuff is changing speed quickly. The more quickly you accelerate with lighter wheels, the more you're gonna notice the difference of light wheels.
Everybody, go get light wheels. Make your cars lighter. It is worth every dollar. Seriously, skip that $600 tune, or the $1,000 exhaust. Spend a day in a junkyard, $500, and totally transform the character of your car.