"Wobbling" on the accelerator

[erdnase]

This has happened to me a few times, but I don't know what it's called. When driving on the road (I think it's always happened when I'm accelerating), I'll go over a bump or pothole in the road and jerk the accelerator slightly. This in turn causes the car to jerk, causing me to jerk the accelerator again.. and so on.

A couple of times I've been stubborn and tried to smooth out the action, but it doesn't work. Dipping the clutch for a brief second to remove the power seems to get it ironed out pretty quickly. So what's this thing called, what causes it, is it dangerous (apart from the obvious loss of balance/control), and what's the best way to deal with it?

Erd

[GreenV8S]

erdnase said:

This has happened to me a few times, but I don't know what it's called. When driving on the road (I think it's always happened when I'm accelerating), I'll go over a bump or pothole in the road and jerk the accelerator slightly. This in turn causes the car to jerk, causing me to jerk the accelerator again.. and so on.

A couple of times I've been stubborn and tried to smooth out the action, but it doesn't work. Dipping the clutch for a brief second to remove the power seems to get it ironed out pretty quickly. So what's this thing called, what causes it, is it dangerous (apart from the obvious loss of balance/control), and what's the best way to deal with it?

Erd

Could be drive line shunt, although that's something that tends to be worst with a neutral throttle. If it's something that is cause by your foot moving on the pedal as the car rocks, you may find it helps to brace the side of your foot against the throttle pedal to stop it moving. On some cars though, even on constant throttle the engine will shunt at low speed. The answer is to accelerate briefly or dip the clutch.

[JWH]

I think what you're talking about is the same effect that causes the 'kangeroo' start when people are first learning to drive. Pull aways sharply but with part throttle, longitudal accelerative forces cause foot to be lifted from throttle which in turn causes car to decellerate, longitudal forces reversed and foot goes back down harder on throttle. This causes car to accelerate again, forcing foot away from throttle and continue ad infinitum.

A similar effect on the pilots hand on the control column of ana aircraft causes an effect known as 'pilot induced oscillation' (PIO), shown externally by the aircraft continually porpoising as it pitches rapidly up and down.

Two easy solutions, floor it keeping foot hard down for a second or two before gently lifting off back to part throttle or lift completely, stay off throttle for a second or two and then gently reapply the desired throttle opening.

[RobM77]

It's called "pilot induced oscicallation" in the aviation world

I must admit, I've never had this happen, despite owning a Caterham (which has a sensitive throttle and hard suspension). I wonder if your foot is relaxed or tense? What car are you driving? Most modern cars have drive by wire throttle, which can cause problems like this. Most of these electronic throttles just have a time delay followed by a ramp function response (gradually comes up to the throttle you asked for - very annoying!), but some have a tiny time delay followed by a step function (Nissan 350Z for instance) - which is nasty and can cause this problem.

[rsv gone!]

Check you are seated properly and adjust the front of the seat for proper support, if you can.

[BertBert]

RobM77 said:

I must admit, I've never had this happen, despite owning a Caterham (which has a sensitive throttle and hard suspension).

I think it's quite common in the caterham world - when they have produced engines that are not mapped that well. R500 and CSR200 both had the problem.

Bert

[RobM77]

BertBert said:

RobM77 said:

I must admit, I've never had this happen, despite owning a Caterham (which has a sensitive throttle and hard suspension).

I think it's quite common in the caterham world - when they have produced engines that are not mapped that well. R500 and CSR200 both had the problem.

Bert

I've had a VVC and currently own a 1.6 Roadsport race car, so both are Rover standard engines, which might explain it. From what you're saying, and what I said in my post regarding electronic throttle, I think this is an issue associated with poor throttle response, where the brain gets confused.

[ph123]

You need to anchor the heel of your foot more securely to the floor.
In single seaters, we used to run heel bars to push and pivot against, which stops your foot bouncing.

[rousseau]

I had exactly the same problem when I test drove a Honda S2000. Seems to be something thats quite car-specific.

/Steve

[RobM77]

rousseau said:

I had exactly the same problem when I test drove a Honda S2000. Seems to be something thats quite car-specific.

/Steve

I had a good length test drive in an S2000 and didn't notice the problem, whcih is interesting. Having thought about this long and hard I think I'll park the blame at a combination of electronic drive by wire throttle and throttle technique. Everyone gets affected differently by the delay on the throttle in modern cars - personally it totally messes up my cornering, hillstart and gearchange technique, and for others it may result in this wobbling effect.

[7db]

RobM77 said:

BertBert said:

RobM77 said:

I must admit, I've never had this happen, despite owning a Caterham (which has a sensitive throttle and hard suspension).

I think it's quite common in the caterham world - when they have produced engines that are not mapped that well. R500 and CSR200 both had the problem.

Bert

I've had a VVC and currently own a 1.6 Roadsport race car, so both are Rover standard engines, which might explain it. From what you're saying, and what I said in my post regarding electronic throttle, I think this is an issue associated with poor throttle response, where the brain gets confused.

You can also use the side of the pedalbox to steady your foot if it does happen. I do a lot of town driving in mine, and I find that a higher egear helps avoid any chance of this.

[stuthemong]

Its a common 'problem' in E46M3's because of their electronic throttle and power and doggedness when cold. I stop it if it occurs by rolling my right foot out to the right, to brace my foot against the kick panel and acellerator at the same time - now if the car lunges or lurches, my accelerator posisition doesn't change, so the bumps die off almost instantly.

Stu

[erdnase]

Thanks for the replies, guys.

I drive a Z4, so perhaps it is the throttle or a car specific "fault". I've never had it happen in any other car I've driven. It's hardly a fault, since it only happens now and again, and never been a serious problem. Just one of those car specific things I guess.

Erd

[Plastic chicken]

This is a wild guess, and you don't say what your car is, but I had a jerky problem a few years back, and I just knew it wasn't my driving technique..turned out that one of the engine mountings had broken, causing the engine to rock around a lot more than normal. Because the throttle cable was mounted at fixed points (the inner wing & bulkhead),the engine moving about, especially over bumps, caused the attached cable to move in & out, without any input from me on the pedal. Result-jerky driving. Fitted new mounting-problem solved.
Failing that, follow the advice of others: keeping your heel on the floor gives you more control than having your whole foot floating around in mid-air; perhaps you may need to lower your seat slightly, if you can.

[RobM77]

erdnase said:

Thanks for the replies, guys.

I drive a Z4, so perhaps it is the throttle or a car specific "fault". I've never had it happen in any other car I've driven. It's hardly a fault, since it only happens now and again, and never been a serious problem. Just one of those car specific things I guess.

Erd

That ties in with my diagnosis. The Z4 has a delay on the throttle. Classic 'Pilot Induced Oscillation';, where you input doesn't match the output and you're forever chasing it. I wrote to Munich about it, but they told me that the car was designed to do it, so I sold mine and bought an Elise

[Stuart G Gray]

RobM77 said:

erdnase said:

Thanks for the replies, guys.

I drive a Z4, so perhaps it is the throttle or a car specific "fault". I've never had it happen in any other car I've driven. It's hardly a fault, since it only happens now and again, and never been a serious problem. Just one of those car specific things I guess.

Erd

That ties in with my diagnosis. The Z4 has a delay on the throttle. Classic 'Pilot Induced Oscillation';, where you input doesn't match the output and you're forever chasing it. I wrote to Munich about it, but they told me that the car was designed to do it, so I sold mine and bought an Elise

I've never heard of this, but I can only drive cable operated cars due to fiscal reasons. Is this a fly by wire problem, and could it be "dialled out" with a software change?

[RobM77]

Stuart G Gray said:

RobM77 said:

erdnase said:

Thanks for the replies, guys.

I drive a Z4, so perhaps it is the throttle or a car specific "fault". I've never had it happen in any other car I've driven. It's hardly a fault, since it only happens now and again, and never been a serious problem. Just one of those car specific things I guess.

Erd

That ties in with my diagnosis. The Z4 has a delay on the throttle. Classic 'Pilot Induced Oscillation';, where you input doesn't match the output and you're forever chasing it. I wrote to Munich about it, but they told me that the car was designed to do it, so I sold mine and bought an Elise

I've never heard of this, but I can only drive cable operated cars due to fiscal reasons. Is this a fly by wire problem, and could it be "dialled out" with a software change?

Yes, since about the year 2000 all manufacturers started fitting drive-by-wire throttles to their cars rather than cable throttles. Instead of a cable on the pedal directly attached to the throttle, the pedal is attached to a potentiometer (like a volume knob), which is connected to a computer, and when you accelerate the computer tells a stepper motor on the throttle to respond.

In most cars the fly-by-wire throttle results in a delay between pedal input and throttle actuation. I've tried to find out why this is, but can't. The explanations I've heard are: simple cost saving in the controlling computer, emissions reasons, and smoothing out wobbly right foot syndrome (ironically!). In most cars this delay is both when coming on to the throttle and when coming off, and seems to lessen with increasing revs. Strangely, diesel cars don't suffer as much. All the current petrol engined BMW range that I've driven have delays, whereas none of the diesels do.

The problems that this causes are as follows:

1) hill starts - the timing becomes hard

2) Changing gear - you have to come back on the throttle pedal before you've dipped the clutch to make the change properly smooth. When changing down, some cars won't actually allow a throttle blip whilst you're braking.

3) Balancing a car in a bend. Most good drivers will like to achieve balance in a bend by subtley altering the throttle position. This is essential in fast driving, and obviously makes for safer driving at lower speeds. The human brain can't cope with a delay when it comes to balancing something, so this isn't possible.

4) Jerky throttle inputs (computer induced) - most people don't judge the throttle they've applied via the pedal position, but by the way the car reacts. If the car doesn't react for a second, then it is hard to judge throttle inputs properly, so you often put in more throttle than you wanted. This results in jerky driving and, apparently (as I haven't heard of this before), wobbling on the accelerator. I'm sure that's the root of your problem.

Believe it or not, most people don't notice the delay, so car manufacturers continue making cars like this. Lotus and Porsche are the exceptions, and their throttles are fine on the cars I've driven by them (Elise, Exige and Boxster S) - though the Exige S that I briefly drove had weird things going on with the throttle.

I'd always driven cable operated cars when I bought my BMW 330ci two years ago. I stalled it twice on the test drive, but was only allowed five minutes round the block so never really got my head round what was going on (sounds unbelievable now, but it was early in the morning and the salesman was giving it the gab). I stupidly bought the car, and when I asked for my money back they refused. The car went in for the problem to be fixed, and the mechanic came out with me to show me that nothing was wrong. I got him to stop talking and put it in 3rd gear at 30mph. Then I said "accelerate, 1, 2, 3" then the car responded. He looked at me aghast, and we returned to the dealership to go out in another similar car. It was worse than mine, as I counted to 5 before it responded!

There are companies out there that claim they can fix this problem. I paid £600 to one such company, but the problem didn't go away at all. I had videoed my car before and after so that I had evidence for small claims court, but my friend lost the video tape...

In the end I sold the car at a £6k loss after 3 months and only a couple of thousand miles (it was so bad to drive that I took the train most of the time).

Modern cars have been ruined by this silly feature. Gone is the day when I could jump in a base spec Peugeot 205/106 etc and enjoy driving it. What we need to do is compain to car companies and until they change their mind, stick with manufacters that don't have the delay, scarce as they are.