I came across a rule or rule of thumb that if there is a fixed object such as a wall or pillar beside your car, if you drive straight forward so that your (the driver’s) shoulder is level with, or forward of, the object, you won’t hit it no matter how tightly you turn the car in that direction.
I’ve tried it and it appears to be correct, at least for my car and my driving position. I’m just wondering how universal it is. With all the different types of car on the road with different shapes, wheelbases, steering setups, and seating positions, it wouldn’t surprise me if it works for some cars and not others. Or is there some universal reason why it works?
I don’t think there can be any fundamental, universal reason for it to work. After all, there’s no fundamental reason for the position of the driver in a car: You could have a car with the driver right at the front, or one where the driver is all the way at the back.
It also depends on the amount your front wheels can turn in steering: If you had a car with front wheels that could turn completely perpendicular to the car’s axis, then you’d clearly hit the pillar.
And it also depends on how close you are to the pillar in the first place: If you’re already brushing right up against the pillar, then any amount of steering before you’ve completely cleared it will hit it.
All that said, with most real-world cars, there isn’t very much variation in the position of the driver or the amount the front wheels can turn, and there’s a limit to how close most folks will let them get to an obstacle in the first place, so it works out to be a pretty good rule of thumb.
I’ve read this over and over and can’t picture what’s going on. What does it mean to drive such that your “shoulder is level with, or forward of” a wall or pillar?
Well, at least until the centre of your rear axle has cleared it (for practical purposes, absent the perpendicular steering angle you mentioned).
I agree, but although academically interesting, it seems practically useless to me; if you’ve got enough clearance for a rule of thumb to be useful, you can just use your normal driving experience and judgement. For really tight spaces, you need to adjust your mirrors and go carefully.
Imagine you are parked parallel to a wall. You’re so close to it that your outside mirror on the driver’s = left side of the car is only an inch or two away from the wall. Further, imagine that the wall ends a few feet forward of where you are parked. Once the wall ends, the road surface opens up so you could turn 90 degrees left or right.
That’s the geometric set-up.
The OP’s contention is that in any car you could drive forward parallel to the wall until the end of the wall is abeam your shoulder. Then you could crank the steering wheel full left or full right while continuing forward and no part of your car will impact the wall.
As others have said, it can’t be strictly true for all possible vehicles and geometric set-ups, but it’s probably true for the majority of ordinary cars and ordinary car-to-wall spacing.
Okay, that makes more sense. I don’t see how that could be possible. In fact, I’d think that most cars would hit it. It seems easy enough to (non destructively) test out. Set up some cones or even draw a chalk line. Drive until your shoulder is ‘level with’ the end of the cones/chalk line, cut the wheel, pull up a few more feet and open the door to see if you drove over the cones/line.
You could even do it in your driveway. Pull out of your driveway, cutting the wheel as soon as your shoulder is past the end of it (or from being parked on the road pulling into the driveway) and see if you can cut over the corner of the grass.
If you pull forward out of a parking spot, I feel like you have to be further out than your shoulders before you can start turning.
This should essentially work with all sedans and coups (and likely even long bed trucks) because the inner turning radius of a car is generally about 1.3 x wheelbase in order to provide Ackerman steering geometry (preventing the rear, non-turning tires from scuffing, or for rear drive vehicles overworking the differential). This doesn’t apply to a much longer vehicle like a bus or a stretch limousine, or an articulated vehicle, however.
Given a particular car, and a particular driver who “cranks” in a certain repeatable way, there’s clearly a spacing from the wall where they can turn towards or away just as the end passes their shoulder and not have an impact.
The question then becomes as the car-to-wall spacing decreases, will that driver ever park so close to the wall that they’d hit it? Or would the simply refuse to park that close and implicitly avoid the crunch scenario?
I’d never heard the OP’s contention before today. I could imagine it’s true for many people in routine parking situations.
I lnow for a fact it’s not true the way I park in constricted spaces.
Watching my fellow drivers attempt to park or maneuver in confined spaces it’s obvious most have little idea how to drive, nor any reliable awareness of where the edges and corners of their vehicles are. For them a mindless rule of thumb might avoid 30 seconds of floundering indecision.
Certainly, when I’m pulling out of a nose-out parking spot with other cars beside me, I’ll pull out until my body is at the front of the car next to me before I start turning the steering wheel. But this is as much about me getting good sight lines to the other cars in the lot as it is about turning radius.
Driving position needs to be considered also - I reckon in most cars, the difference between the nearest and furthest seat position could be the difference between a scrape and no scrape when using this rule.
It would only be true if the rear wheel tracks over where the driver is in a tight turn. I don’t see how it can be true for any car. Surely any amount of turning will bring the rear wheel inside the driver’s seat so if you start close enough to the wall/pillar you will scrape it. It would be easy enough to test with your own car using a painted mark rather than a pillar or wall.
And it doesn’t. The center of the turn is intersected by the axles of all four tires:
This means the inboard rear tire is the closest tire to the center of the turn. Since the driver is forward (and slightly outboard) of the inboard rear tire, the inboard rear tire is closer to the center of the turn than the driver is. The only way to guarantee no contact with an adjacent obstacle during a tight turn is if it’s already in line with your vehicle’s rear axle (and not in line with the driver’s shoulders). If the obstacle is forward of that line, then there’s a risk of contact when the vehicle moves forward while turning.
The OP definitely wouldn’t work with my 1995 F350 pickup truck. It’s a crew cab (4 door) with an 8 foot bed and dually rear wheels, which means that the rear fenders flare out quite a bit.
Because the truck is so long, if there is a pillar anywhere near the cab, it’s fairly easy to turn into the pillar. In fact, if the pillar is right next to the driver’s door, or even next to the rear door on the driver’s side, just going straight forward will hit the pillar against the rear fender.
Flared fenders aren’t common, but the OP doesn’t work for any vehicle with flared fenders. It also doesn’t work for any long vehicle where a pillar anywhere near the driver is still a good distance in front of the rear wheels, as my F350 or the limo mentioned in the 2nd post demonstrate.
The OP actually has a practical application. If I turn my F350 too sharply to the right coming out of my driveway, I would probably hit my mailbox. since it is just off of the corner of my driveway. I have to make sure to pull straight out far enough that I won’t hit the mailbox and then turn right.
This is the theoretical side behind the practical correct answer. It isn’t your shoulder and applicable to all vehicles, it’s the rear axle (assuming you have a single rear axle).
While not fully accurate, the lesson I teach is that the vehicle pivots on the inside rear wheel. As @Machine_Elf said above, it’s a little more complicated, but the simplification works well. When your back tire passes the obstruction, turn the front wheels all you want, you’ll never hit it
If you have two rear axles, the pivot point is centered between them, again on the inside of the turn. Pivot the front wheels enough and the rearmost axle will drag horizontally around that point (scrubbing the tread off of your tire).
If you assume that the wall/pillar is on the left side of the car then turning left when the rear axle is level with the wall/pillar is fine. But when turning right it will depend on how much bodywork extends beyond the rear axle.
