Cowl shake

[intoflatlines]

Can someone explain what cowl shake is and how bad it is in the E85? I've heard in some reviews that the E85 does not have much noticeable cowl shake, but the problem is, I don't even know what cowl shake feels like.

I searched online and people have said that the windshield twists or something? How is that even possible?

What I do notice is that at very low speeds if there are dips or "tracks" in the road (don't know how to best explain it) and I drive by them, it feels like the car is steering/pulling me into the dips (and the steering wheel does pull into them), as if it really wants to keep the wheels inside these dips. Is this cowl shake or due to the alignment or something? Sorry, I came from a 2011 GTI so this Z4MR is my first truly sports car.

[StickMon]

Steering/pulling is "tram-lining"
Cowl scuttle is when the car twists.
I don't know what it really feels like either, but it makes the body creak when you go up an incline at an angle or over a speed bump with one wheel at a time.
It will also twist in high G cornering, which affects handling.

[psygnosis]

The Z4 has one of the stiffest chassis BMW has made, and the coupe even more so by a significant amount.

[chickdr]

Cowl shake is typically seen in cars that were originally designed as hardtops and converted to a convertible. A good example would be the VW Cabriolet. I had an 83 Rabbit Convertible and it had lots of cowl shake. When you hit a bump you could actually feel the steering wheel shudder in your hands as the chassis went back and forth. The dash would also visibly move side to side over undulating road surfaces. Very disconcerting to experience. Today's cars are much stiffer in general so it isn't too bad. If you want to experience it I would suggest trying a Chrysler 200 convertible. It probably still has decent cowl shake. The Mustang has a bit too, but it is much better than the older gens.

[elerner61]

Get an '80s Mustang GT convertible and then drive it at speed over a very bumpy, curvy road and you'll know more about cowl shake then you ever wanted to know. And you'll learn about the downside of a solid beam rear end at the same time (you may want to wear a pair of Depends if you do this when the road is wet).

[dre99gsx]

Same with Toyota Supra targas. Just driving down the road, you can feel the steering wheel column jiggle laterally, as well as the rear view mirror. This is simply torsional flex between the front half and the rear half of the vehicle. As your front shocks compress, the chassis will give a bit more (than a typical hardtop vehicle) and take some of the compression load. Terrible for handling.

I had once cut the top off of my Eclipse GSX, ran into terrible cowl "shake". If you attempt to take aggressive turns, you'll notice the chassis flexing a lot more.

[Huz-Z]

Originally Posted by psygnosis

The Z4 has one of the stiffest chassis BMW has made, and the coupe even more so by a significant amount.

That is absolutely correct.

In fact, the chassis of the Z4 roadster is stiffer than some sedans and coupes that BMW made. There is data on this in my Z4 Design book, which I'll dig out and quote later today.

So as far as convertibles go, the Z4 is one darn solid car.

[Walkurie]

One of my former cars was a '53 MG-TD. The body panels were literally laid on a wood frame, although to be fair, there was something that looked like a roll bar hoop under the cowl. The function was to stiffen the body and frame in that specific area. Still, the cowl shake was so bad that in order to calm my nerves, I would fold down the windscreen and it would seem as though it was reduced, ha-ha. But it was lots of fun at low speeds.

[psygnosis]

Originally Posted by Huz-Z

That is absolutely correct.

In fact, the chassis of the Z4 roadster is stiffer than some sedans and coupes that BMW made. There is data on this in my Z4 Design book, which I'll dig out and quote later today.

So as far as convertibles go, the Z4 is one darn solid car.

Found a list

BMW E36 Touring 10,900 Nm/deg
BMW E36 Z3 5,600 Nm/deg
BMW E46 Sedan (w/o folding seats) 18,000 Nm/deg
BMW E46 Sedan (w/folding seats) 13,000 Nm/deg
BMW E46 Wagon (w/folding seats) 14,000 Nm/deg
BMW E46 Coupe (w/folding seats) 12,500 Nm/deg
BMW E46 Convertible 10,500 Nm/deg
BMW X5 (2004) - 23,100 Nm/degree
BMW E90: 22,500 Nm/deg
BMW Z4 Coupe, 32,000Nm/degree
BMW Z4 Roadster: 14,500 Nm/deg

[Huz-Z]

Originally Posted by psygnosis

Found a list

BMW E36 Touring 10,900 Nm/deg
BMW E36 Z3 5,600 Nm/deg
BMW E46 Sedan (w/o folding seats) 18,000 Nm/deg
BMW E46 Sedan (w/folding seats) 13,000 Nm/deg
BMW E46 Wagon (w/folding seats) 14,000 Nm/deg
BMW E46 Coupe (w/folding seats) 12,500 Nm/deg
BMW E46 Convertible 10,500 Nm/deg
BMW X5 (2004) - 23,100 Nm/degree
BMW E90: 22,500 Nm/deg
BMW Z4 Coupe, 32,000Nm/degree
BMW Z4 Roadster: 14,500 Nm/deg

Yes!

MY source is BMW Z4 Design, Development and Production by David Lightfoot. It notes on page 45 that the Z3 had 5,600 Nm whereas the Z4 exceeded that without adding weight. Prior to that, BMW's best was the 1990 8 Series at 11,000 Nm.

So, the Z4 is very stiff.