DRC Warranty

[PetrolDave]

But if you hit a bump at high speed with no shocks then the force doesn't have time to affect the mass of the car? All the impact is taken with the spring movement so you have a nice smooth ride.

The kinetic energy of the movement of the car relative to the bump is stored as potential energy in the spring, which then has to be released back into kinetic energy (it can't stay stored in the spring for ever). The shock absorber (damper) controls the rate of that release, so if it's faulty or worn then the car will be forced upwards at a rate that may be sufficient to reduce the contact pressure of the wheel on the road - resulting in loss of grip and traction. If one shock absorber is worn and one isn't (As when only 1 on an axle is replaced) this will result in a yaw moment on the car (unwanted steering in effect).

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the spring is compressed
when it it rebounds it will propel the car upward
unless of course it can push the earth down out of orbit
physics 101

Only true if the bump is a step say like a curb with no down side. If you hit a speed bump then by the time the spring has compressed from the inital bump the wheel would have cleared the obstical and the spring will exert the same equal force in both directions. As the wheel is substantually lighter than the car it will move further and faster. As such the effect of the impact to the car is minimal.

[ArthurPE]

Only true if the bump is a step say like a curb with no down side. If you hit a speed bump then by the time the spring has compressed from the inital bump the wheel would have cleared the obstical and the spring will exert the same equal force in both directions. As the wheel is substantually lighter than the car it will move further and faster. As such the effect of the impact to the car is minimal.

nope, true under any situation in our inertial reference frame...
the car will rebound as the spring extends

you are making a lot of assumptions concerning re; velocity, time, height, geometry to fit your argument...

say the car hits the 'bump' and compresses the spring...
the car clears the bump and 'gravity' makes the car fall faster than the spring extends...(or the spring is still compressing as the car falls)
so wheel on ground, compressed spring...
what happens?

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Didn't think I was making any major assumptions - a car heading in a straight line hiting an average speed bump dead on?

What you're saying doesn't fit real world though- I know from experience that the attitude of the car will change more when travelling slowly (say 5-10 mph) over a speed bump than at a much faster speed (30+).
The force on the car from the initial bump has to come through the spring but the effect is a product of both force and time applied. With an impule the force may be high but the time is so small the total effect of the force is negligable.

The scenarios where a car would be falling quicker than the spring is pretty specific - the car would need to be already falling and then hit a bump.. a more complex example than the one above.
I mean, car clears bump so car, spring and wheel are effectively airborne and falling under gravity. As such all three will fall at the same rate, however the spring wants to extend so will push the car up and the wheel down. Given that the force on both is the same but the wheel much lighter it'll be pushed down way quicker than the car. Also the car would have some upward momentum from the initial bump (if quite small)

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Here's a basic case study
http://engin.swarthmore.edu/~jshin1/e12/lab04/
http://engin.swarthmore.edu/~jshin1/e12 ... /case1.jpg

One of the cases tested was for halving the damping and the effects of the bump were dramatically reduced.

[Jonah]

So, 12 days in and I notice a slight knocking sound when going over any sort of bump/hole in the road coming from the front nearside I think.

My car went in to Audi yesterday and on collecting, they informed me the Near side front & off side rear shockers needed changing. This is being done on warranty. I questioned whether both shocks on the same axel should be changed at the same time, I was told as the DRC works in diagonals this is what they change. The guy seemed pretty clued up about this issue and am booked in to get them sorted when the parts arrive.

[PetrolDave]

I questioned whether both shocks on the same axel should be changed at the same time, I was told as the DRC works in diagonals this is what they change.

So why did Portsmouth Audi replace both front shock absorbers (and the DRC valve) on mine in July but neither of the rear shock absorbers?

It seems that each Audi dealer does it however they want - there's certainly no evidence of a common policy from Audi UK.

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Absolutely agree there!
I guess I can see arguments for both so you could argue one fails all need doing but not surprised Audi doesn't go down this route straight away.

Have to be honest, I'm looking at dumping the DRC's should they fail again. Nearing the end of the 5 year (june 11) and will get them checked before then but I've already budgeted a swap to adjustable Ohlins from Stasis. Based on the other thread of 2.7k to replace some of the DRC (not all!) this isn't far off at all what you'd pay for top spec ohlins. I've used them on other cars for years and quite frankly they are stunning.

[ArthurPE]

what you are saying is that the compressed spring will extend (uncompress) faster than the car will 'fall'
that is absolutely not true

assume:
4000 lb car
150 lb suspension wt
300 lb/in spring rate ~ k
bump of 4"
impulse of 2, so net force ~2000 lb/corner

using Hookes law
spring compresses ~ 6.7"
f of oscillation ~ 1/2Pi sqrt(k/m) ~ 1.3 hz
or it will take 0.4 sec to extend after the impulse
this assumed undamped, with shocks this will be longer...

now, how long does it take for the car to fall ~6.7"?
0.18 sec

so what actually happens the car falls ~ 4" and the spring extends ~2' and they intersect, therefore cancelling/absorbing the force
you try to design to be equal & opposite so no net force to the car...

the point is gravity is 'faster' than the spring (which is governed by gravity less the spring resistance and shock dampening)

[pmo]

I did have the car booked in at Oxford Audi but according to them there is no 5 year warranty or other extended service applicable to the DRC shocks and it's a fully payable item.

Did you make any further efforts with Oxford or did you just go to Aylesbury in the end? I usually use Oxford and I was planning on contacting them about the knocking I'm hearing when going over bumps but if they're likely to be dicks about it then I'll maybe try Aylesbury as well.

Though my car's under its Audi Approved warranty so maybe it won't be a problem.

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No I ended up going with Aylesbury and was impressed with everything.
If a service dept needs that kind of arm twisting then I doubt be taking the best of care with the car. Aylesbury didn't even need prodding but straight away volunteered information on the process including how Audi have a list of pictures for different cases along with the approved action. Also they could see me sooner with a loan car and commented "we fixed one last week". The other thing I liked was that they pretty much only do servicing and seem to do it well. Got lots of apologies as I had to queue for 4 minutes as one of their computers had failed. Sure, not the be all and end all but all add to the feeling that they know what they are doing. Also did additional recall work that was not requested by me.

I have a tracker on mine that I can view and could see it move about the dealership and down the road. No speeding and no excessive miles. Even told me it was finished and being cleaned - and it was

Even though Oxford are closer I'll be taking all my future dealer business to Aylesbury.