Does a tyre change its CIRCUMFERENCE when underinflated?

Do people tend to use DSG-equipped cars in auto or manual mode mainly, I wonder? I have always found that the big problem with automatics is that they change down at the wrong time, eg during acceleration, especially when half-way round a corner or when accelerating out of a roundabout. A transmission that makes gearchanges more seamless while still allowing the driver to choose when to change gear (eg during brief gaps in acceleration when the change is less noticeable) sounds great. I think I'd tend to use it in automated-manual mode - the best of both worlds - rather than letting the transmission choose when to change.

Some time I'll have to test drive a VW with DSG and see how easy it is to drive compared with a) a fully manual gearbox, and b) a planetary-gear/torque-converter automatic box. At least modern VWs don't suffer from the problem that (I think) the Mark V Golf TDI had, where the engine was notoriously easy to stall - rather than being almost stall-proof, as most diesels are, it seemed to stall even more easily than a petrol. That Golf is the only car (petrol or diesel) that I have repeatedly stalled when setting off on a test drive - and the salesman said I was not alone. It's almost as if the ECU detects that there is insufficient fuel and therefore insufficient engine torque to be able to accelerate the car so it deliberately cuts the fuel altogether rather than letting the engine labour slightly to give the driver chance to increase the throttle a bit. I imagine that if you drive the car all the time, you soon get used to it, but for occasional drivers it catches you out.

How easily do people find it is to control the speed accurately at very slow speed (eg when manoeuvring or when crawling forward in a queue of traffic). It may be that I've never developed the muscle memory to be able to control a car's speed solely on the accelerator, when in a manual car I tend to control the speed also with the clutch, with a more constant engine speed. For me, the best combination would be a DSG that had a clutch pedal that was only used for finer control of speed for very slow manoeuvring and which was not used for gear changes. With a (conventional) automatic, I'm always scared stiff, especially if the accelerator pedal/linkage sticks slightly as the car gets older, of the accelerator pedal moving suddenly and the car surging forwards. When I have driven automatics occasionally (eg as hire cars on business journeys) I've been very aware of this possibility when reversing out of a parking space and the need to release the accelerator and/or touch the footbrake the instant that I detect it happening - a reflex which I bet elderly drivers who drive into shop fronts haven't got. It's the only time that I left-foot-brake: as a precaution when inching forwards/backwards in a car with no manual clutch.

Reply to
NY
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It *did* make me a bit nervous, but Tim Downie on here somewhere had had the same. The Leon was approaching 140k when I sold it.

Reply to
Chris Bartram

My first one was 'chauffeur smooth' for a year, then as good as a decent driver with a manual for 3-4 years, very enjoyable.

Then, on cold mornings, it sometimes gave a bit of a thump in your back the first time it came to a halt as it was changing down into 1st, as though it wasn't sure of the bite point, I had it recalibrated at next service it was better for 6 months then started doing the same again, it then occasionally put the gearbox into "limp mode" for few seconds after long journeys on hot days, only ever a warning, not an error. Main dealer option at that point was a "DSG rebuild kit", not cheap, and not much guarantee.

OBD cable showed it was occasionally unhappy with pressures and temperatures of hydraulic clutch valves, so it got chopped in, maybe I'll resist using the launch control on this one :-)

I thought they were pre-selector boxes?

Reply to
Andy Burns

Any auto only changes down if the speed is too low, or you demand extra acceleration from it. If you are driving so fast on a slippery surface that a gearchange mid corner is going to cause problems, then it would make sense to select the gear you want before. But most modern boxes will learn when you're pressing on and stay in a low gear much longer than when driving normally.

On a twin clutch DSG box, there is effectively no interruption of power flow when it changes gear. Far far less so than with a manual box.

Modern autos are so good there is usually no need to use the manual function. Except for fun.

Reply to
Dave Plowman (News)

The only time you notice is if you 'confuse it' e.g. it's in 4th and preparing to change up to 5th, but you boot it so then it needs to find

3rd in a hurry.

no disagreement there :-P

Reply to
Andy Burns

yes it is.

Reply to
The Natural Philosopher

100% wrong in every respect.
Reply to
The Natural Philosopher

It would be nice if you would just this once back up your claims with some evidence that you are right and everyone from the OED to the Royal Mint are wrong.

Reply to
Robin

I think that was so they would be acceptable in slot machines.

Cheers, T i m

Reply to
T i m

He can't. He is a left brainer and because *he* thinks he's right, he is. ;-)

Cheers, T i m

Reply to
T i m

I am. Seriously. Today I have learnt that my cat has a deeper understanding of the constant diameter of 20P and 50P coins than you do.

Reply to
Richard

What part of the concept of 'an effective radius' can't you get though your thick left-brained head?

Given that there IS a peripheral distance (because under your rules there isn't a true circumference either, it not being a circle or ellipse and all) you *must* also have 'an effective circumference'. That would be the distance a tyre *actually* travels per revolution.

That's the very thing it MUST do for any of the millions of iTPMS to work!

So, the tread under the load point shrinks reducing the unloaded (true) circumference to an effective circumference and the real unloaded radius to an effective radius.

I bet you can understand i or j and how it represents the square root of a minus number (something that is impossible in straight maths) but you can't accept how a tyre can change it's effective circumference / radius when it running at a lower pressure??

Bizarre.

Cheers, T i m

Reply to
T i m

Looks like we have a problem explaining it to Turnip so I think he is just trolling us all now (he asks the question but he neither wants nor cares for the answer).

Let's agree on some things we can probably all agree and consider to be facts because of what works ITRW (iTPMS).

1) The unloaded circumference of a car tyre is irrelevant to anything unless you are just about to take off. ;-) 2) The loaded peripheral length must be different to the unloaded one and different again under low pressure conditions (for iTPMS to work). 3) The tyre isn't slipping on the rim nor 'skidding' (in the traditional understanding) on the road whilst in normal use. 4) Steel belted tyres are constructed with the wires set on the diagonal (as they zig zag across the tread, around the bead and back again) and form a parallelogram (like a electric train pantograph or pop rivet gun). 5) One of the FACTS about how a parallelogram works is as it gets WIDER it becomes SHORTER ... and as it gets longer it gets narrower.

So, as a (car) tyre rotates, it flattens out when it is in contact with the road and the tyre construction pantographs (because as it flattens it gets wider) causing the tread to *shrink* longitudinally (around the periphery) as shown on here ...

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Figure 12 Longitudinal compression or bunching of the tread

... and therefore cause the tyre / wheel to rotate faster than it would if you compared it with it's unloaded circumference.

So, within practical constraints, from the typical correct running pressure to something no more than 25% less (to conform with the TPMS regs), the tyre shortens is peripheral distance (let's call it the 'effective circumference' and of course, an 'effective circumference' would have an 'effective radius') so that iTPMS can do what they do.

No (in significant terms) scrubbing, no slipping of the tyre on the rim, no magic, just plain mechanics and physics that even I can fully understand!

Cheers, T i m

Reply to
T i m

Yep, mIne gave me a fright when it stopped going up through the gears as it should have. In the end though it wasn?t a gearbox problem but a faulty wheel speed sensor feeding the gearbox erroneous information. Oddly this didn?t show up on the normal computer scans. It was only when it came up as a brake or ABS problem that the relationship became clear.

Tim

Reply to
Tim+

In message , T i m writes

Snip..

This might be the answer:-) The actual circumference varies with pressure irrespective of the axle road distance.

Reply to
Tim Lamb

Might? ;-)

Yes, the pressure affects the peripheral distance (effective circumference) and that in turn affects the height of the axle over the road (effective radius). ;-)

Cheers, T i m

Reply to
T i m

1989 first introduced From Wiki Despite such features, the interior's basic structure was the same as the Range Rover and virtually all the switchgear and instruments came from other Rover Group cars such as the Maestro and Montego. Similarly, the Discovery utilised several Range Rover body panels, as well as headlights from the Freight Rover van and taillights from the Maestro van. The latter would continue to bear the Austin Rover 'chevron' logo on their lenses until production of the first generation Discovery ended in 1998, ten years after Austin Rover ceased to exist.

The designers of the original model had been forced to economise and use the "parts bin" of the then parent-company, Rover. The 200 series used the basic body shell structure from the Range Rover, door handles from the Morris Marina, tail lights from the Austin Maestro van, and interior switchgear and instrumentation from Rover's surplus parts.

Doesn't sound like they would have anything so sophisticated as a steering angle sensor.

Reply to
bert

sabino56's post helps clarify some of the misconceptions occuring in this thread:

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and in brief:

1) the diameter does change with load/pressure/speed

2) the effective radius does change with load/pressure/speed

The effective radius has more affect on gearing than on diameter.

Reply to
AnthonyL

I never had the nerve to use luanch control, and the Leon got to 140K with no significant problems with the box itself. It would cometimes lurch a little into 2nd when very hot (as Dave experienced with PDK they're very keen to get out of 1st), but my current one shows no such problems, being very smooth all the time. I did run the basic settings on the Leon (I have VCDS) a few times, and it would make a *little* difference for a while. Not serious enough to worry about, and no fault codes.

The briskoda forums had someone with nearly 600K kilometres on a DSG Octavia. Certainly, all the earlyntales of them failing at 60k seem unfounded.

Not now. I think there's some DCTs, but lots of fairly conventional autoboxes.

Reply to
Chris Bartram

Yes, ISTR I dropped you an email and you pointed me in the right direction, and then a few days later my car did exactly the same as your Touran, lighting up the ESP and ABS.

Reply to
Chris Bartram

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