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View Full Version : OT: What makes the steering "twitchy" on a car?



winchman
07-12-2009, 05:28 AM
I've been looking for another vehicle lately, and I've driven about twenty new and used cars in the process.

By "twitchy", I mean the car is overly sensitive to steering inputs when going straight ahead. Two of the cars have been ruled out because they just wouldn't go straight.

The worst was the new Aveo. It was all over the road, going either right or left, but never straight ahead. A ten-minute test drive just wore me out.

The new Toyota Yaris was pretty bad, but the ten-year-old Toyota Echo was just fine.

The new Prius was marginally OK. It got better as the speed increased.

So, what is it that makes a car twitchy? Is it something that can be corrected by adjusting the alignment or changing the tires, or is it built into the suspension geometry, bushings, power steering, etc?

Roger

Doc Nickel
07-12-2009, 05:49 AM
Any number of things if it's a used car- worn tie rods, worn steering box or rack-and-pinion, worn ball joints, bad alignment, you name it.

If the car's new, though, it's a factor of the steering ratio, and thus not an easy fix. A "quick" ratio- say, 1-1/2 turns lock-to-lock, makes for fast handling. Fine for a small car driven by a person that likes to drive aggressively, but as you noted, a bit tiresome on just a commuter car.

A small car with a short wheelbase will also feel more sensitive than a longer-wheelbase car.

Something else you might keep in mind, which happened to me- if your current car is older and a bit "loose", and has a fairly "slow" steering rate, a newer car will feel twitchy and overly sensitive. Going from my old Toronado to a '76 Corvette- which feels like it has go-kart steering- is almost panic inducing. The 'Vette was a blast to drive through the twisties, but on long trips, can be physically tiring. The Toro on the other hand, takes more frantic movements on the wheel to do a fast turn, but is way more comfortable to drive a distance.

It might be that you're so used to- just as an example- say, a big Ford truck, with it's slow steering and long wheelbase, that the new smaller, shorter car feels twitchy even though it has only moderately fast steering.

Doc.

beanbag
07-12-2009, 06:24 AM
also caster

Doozer
07-12-2009, 08:31 AM
also not enough king pin inclination
also too much toe-out

--Doozer

Evan
07-12-2009, 10:22 AM
I have had new vehicles like that. I have driven a lot of new vehicles over the years as they were supplied by my employer. The worst was a Ford Aerostar van with four wheel drive. It would either head for the ditch or the oncoming traffic the moment you took your hand from the wheel. It was incredibly tiring to drive and I traded it within the company for a different vehicle as soon as I could.

Our PT cruiser is one of the best I have seen in this regard. It will maintain a hands off straight course in it's own lane for as long as 20 to 30 seconds before drifting to one side on a good road.

Look for a vehicle with variable ratio power steering. This gives the effect of many turns lock to lock when the steering is close to centre but the ratio speeds up for larger movements of the wheel for easy parking and low speed manoeuvres.

dp
07-12-2009, 01:11 PM
Front wheel drive cars are vulnerable to forced steering while accelerating and braking. The toe-in is affected by engine forces in a very noticeable way. This is not necessarily symmetrical, so you can have three tires pointing the right direction and the forth wandering skewed off. Roads here in the Seattle are are often badly rutted from trucks and studded tires, and my Jeep, with wide hard chine tires, wanders like a flock of sheep.

I've noticed a lot of vehicles, when in uneven road conditions, have wandering geometry in the front end. You can easily see the camber moving around, and I can't imagine that can happen without an effect on toe-in, and that can certainly cause wander.

I wonder too about the rigidity of the rubber mounted forward frames. They're loosely coupled to the chassis of the car rather like the rear end of a Harley bagger and there's not much as loosey goosey as a loaded Harley FLH going down the road. To the extent that there are after-market products available to minimize it. Certain models are subject to high speed wobble which is a lock to lock thrashing of the front end followed by a trip to the hospital and insurance company.

Ries
07-12-2009, 01:56 PM
One other thing to consider- new cars will have very hard tires mounted as stock.
A hard compound tire, generally, will give better mpg and last longer. So most new cars come with them.

But they are more twitchy, and dont corner very well.
When I buy used cars, I always splurge on higher quality tires, especially cause I like cars that perform well in the twisties- this means shorter lived, more expensive, stickier tires.

This may not be what you want, if you are looking at Yaris' and Aveo's, which are all about economy- but it may be part of the reason they drive the way they drive.

Falcon67
07-12-2009, 02:10 PM
Not enough caster (king pin inclination - like we use those anymore LOL ) and incorrect toe, most likely toe out.

darryl
07-12-2009, 05:18 PM
A new vehicle should not have any problems of that type, by my standards. Can you see the designers accepting a situation where their car behaves that way? Most likely it's a wheel alignment problem- there's no guarantee that it's been done properly from the factory. My opinion only, but I think that a new car should have some driving time on it before the wheel alignment is tweaked.

I've had more than one situation where I touched up the alignment myself to get the handling right. Once was right after a 'professional' alignment using the machines.

If a used vehicle, then worn suspension bushings, ball joints, worn or poor tires, and alignment problems are all probable causes.

kf2qd
07-12-2009, 06:14 PM
Maybe you are used to cars that are a bit loose and when you get a new one it has no slop and your driving technique needs a bit of polish to work with a new car.

saltmine
07-12-2009, 06:31 PM
Most newer cars suffer from directional instability because the steering geometry is optimized to present the lowest rolling resistance possible (for gas mileage). Combine this with a very light car, short wheelbase, and power boosted steering, and you get "twitchy"
Not much you can do on run-of-the-mill late model cars, except maybe "get used to it".
I know my old man can barely drive their Cadillac front-drive....He's used to the old American car and pickup steering with half-a-turn of play in the wheel and three tons of iron pushing it down the road....Every time he flinches, the car almost leaves the road. My Mother loves the way it handles, though.

tattoomike68
07-12-2009, 07:02 PM
A few more things,,,

Tire air pressure is a biggie. Or a car that just sat for a long time might have tires less than round. A few good burnoffs will round them out. :D

Around here our city is flat broke and has no money to fix the roads, they are getting bad so I dont care what you drive its a rough ride.

I see people hit curbs in the winter and that will bend a rim, A careless driver will run it like that for years and never have it fixed.

Lost wheel weights, if you ride a bicycle you see wheel weights on the side of the road all the time so its not uncommon for those to fall off.

I drive a little toyota tercell with small wheels and it grabs every bump in the road. I sure can tell a crappy road from a smooth well made road.

dave5605
07-14-2009, 12:02 AM
I'm gonna say that there is nothing wrong with the various vehicles you have driven. You just have to alter YOUR driving technique to fit newer/different suspension geometries.

In the last 50 years I have driven everything from a F40 Ferrari, to high performance Porches, various years corvettes, 24' box trucks, just about every pickup truck known to man, a crap load of foreign and american cars and worn out pickups with worn out suspensions. I owned everything except the Ferrari and Porches. Probably very comfortable driving all of them after 5 minutes on the road.

I have test drove a new Surburban, Camaro, and Toyota Camry in the last 3 months and they all 'seem the same to me'.

My driving technique tends to be (1) get the seat where I want it, (2)get all the mirrors where I want them, (3) decide where I'm going to rest my right elbow, and (4) grip the wheel with my right hand at about the 3 o'clock position. I am very relaxed and comfortable when driving.

I see a lot of people with their hands at the traditional 10/2 position, seat mal adjusted and having a death grip on the wheel. When they drive it looks like they are steering a NASCAR race car with the constant 'sawing' left/right of the wheel.

saltmine
07-14-2009, 01:57 AM
One thing I didn't mention about some late model cars (and trucks)....
Some of them have computer controlled, electronic pressure regulators on their power steering pumps. The car's computer picks up a signal off of the steering shaft in the steering column (EVO sensor). With this and an input from the speed sensor, the computer adjusts the output pressure of the pump, which varies the amount of boost the steering gets at any given time.
At high speed, the pressure is reduced to give the driver better "road feel" and at low speeds more boost to make the car steer easier in parking lots, and slow traffic. The steering input sensor also gages the speed and distance the wheel is turned and adjusts pressure accordingly.
It's a very well-thought-out system, but back in the late '90's we started getting Chevy pickups with complaints of "twitchy" steering... The problem turned out to be the "EVO sensor" that senses the rotation and speed of the steering shaft. These were failing, and the system was going into "limp home" mode, which automatically set the power steering regulator to maximum. Full pressure at highway speeds made the steering very sensitive.

Carld
07-14-2009, 10:26 AM
You would think that after about 100 years of making cars and race cars they could make a steering that would be neutral and balanced. There are still cars and pickups that have bump steer and swerving, etc. with no input from the driver. I know they can do it but they don't. My 2003 Ford Ranger has independent suspension with torsion bars and it has bump steer but otherwise handles good. It does swerve some on the concrete highways that have grooves cut in them.

There's not much you can do but adjust your driving style to control the vehicle. Remember, your driving the vehicle, the vehicle is not driving you, at least it shouldn't be.

saltmine
07-14-2009, 03:38 PM
Rangers always have had a "bump steer". I suspect it's mostly due to it's design. When I worked in a County Fleet shop, I had quite a few Rangers in the fleet, and I seem to recall it didn't take much of a hit to knock one out of alignment. We were always shuffling them to and from the dealer and alignment shop.
Another problem my brother had to deal with when he was a service manager at a dealer was people putting on these huge custom wheels with the tiny little, skinny tires. A well behaved, good riding SUV could be transformed into a twitchy, darting, rough riding POS in a matter of minutes. And when the car (or truck) became all but unmanageable, they brought them into the dealer, and wanted them fixed. My brother would look at the vehicle, and quietly tell them it's not a manufacturer's problem, take it back to the moron that sold you those huge chrome wagon wheels...

Carld
07-14-2009, 05:41 PM
Most cars have bump steer.

danlb
07-15-2009, 12:20 AM
The castor of the suspension ( a design feature ) causes the wheel to return to center. When there is too much toe-out you can have both front wheels fighting over which one will track correctly. This is made more noticable when you have worn bushings.

Some of the newest cars (like the Prius) use electric power steering so that it does not depend on the engine running. That comes with it's own complexities, but all in all I like it.

Dan