Jimnys and wind
This is a real deep dive and brings out some physics.
One of the comments you often see about a Jimny is how they get blown around in the wind.
“Hi everyone, would love a bit of advice! I’m finding it quite scary and dangerous when driving in strong winds on the highway as it pushes the car side to side (don’t worry I slow down when it happens but still)”
I thought I’d do some exploration about this and dig into the physics of actually being blown sideways.
Setting up the problem
A Jimny is driving down the highway. Suddenly, a strong gust of wind on a previously windless day hits the car directly side on. How much does this shift the car sideways?
To work this out, we need a few pieces of information.
Area of the side of the car
This one is quite easy. I have some vector drawings of the car I use to illustrate various articles. Using my favoured bit of vector drawing software I can apply a drawing scale and ask it to give me the total side area of the car from the side profile drawing.
Call it 4.6 m2, then.
This is really a maximum side area, too. Not all of the wind will perfectly interact with it, and while the Jimny is quite boxy, it is not truly vertical. The wheels, for instance, will create some vortices and won’t fully bear the wind’s load in the same way the bodywork will do, so it’ll have an apparent area to the wind a bit less than this. For our purposes, to calculate a ‘how bad could it be?’ question, it’ll do.
Wind speeds
Because I live in Perth, but also because it’s the windiest capital city in Australia, I’ve decided to look into wind data from Perth. Fortunately, someone’s already done this for me and done a really thorough job of it. The recurrent 50Y maximum instantaneous gust is ~150 km/h, but bear in mind gusts rarely actually come out of nowhere; typically they are more like 30% higher than the steady wind surrounding them. For this we’ll pick a breeze that is exactly side onto the car at 100 km/h.
Note the scenario implies the worst case possibility: if the gust was not side on then it would add a headwind or a tailwind to the car, which will reduce the side wind loading.
Collecting it all together
Side area of the car: 4.6 m2
Calculating it out
Wind loading
This gives
Centripetal force and corner radius
So what’s going on with wind and a Jimny anyway?
Your sensitivity to accelerations
Pitch and roll of the car
Bump steer and/or suspension geometry stuff
Limited caster even at the best of times
Short wheel base cars turn quite quickly
Car steers when the suspension moves
Bump steer unsprung weight
Survival instincts
A huge thing that goes on for people is they’re worried about what the Jimny is like in the wind, and so they’re more acutely aware of how it might behave.
Survival instincts can get even the most professional trained person. An example of this is the crash of American Airlines flight 587 in 2001. This flight took off immediately after a larger plane, and encountered the turbulence from the wake of this larger plane. After flying through this turbulence, the vertical stabiliser of the plane separated from it, and the plane crashed killing all aboard.
The cause wasn’t the significant turbulence by the plane in front, but instead, as per the investigation into this accident’s findings:
The National Transportation Safety Board determines that the probable cause of this accident was the in-flight separation of the vertical stabilizer as a result of the loads beyond ultimate design that were created by the first officer’s unnecessary and excessive rudder pedal inputs.
When you correct the car against gusts of winds which will rise up and die down again, you are very likely overly correcting the car, and then correcting your own additional steering inputs. Hopefully the above bit of physics will convince you that the car will not veer off the road in the face of even an incredibly strong gust of wind; you can very easily be overly correcting the car.