Audi GT3

[tosh]

Hello everyone,
we are fiddleing some setups for the Audi LMS. But overall we are struggling with the handling after lap 3 or 4. On the outlap and maybe +2 more laps, the handling is really great. After the tyre pressure and heat build up its feel wobble and does not react any more (fast and predictable). We are riding lowest pressure and cambers in front about 3.5 and 2.5 on the back. Is the handling of the other GT3 cars as well?

 

[Steve Olden]

I'd like to know how to sort this car too...I tried for ages to get it to last for more than 5 laps..in the end I gave up and just drove a Mcfaster car..

also as most GT3 cars run Ohlins suspension, why does the Audi not have seperate fast/slow damping like the others?

 

[mogwai]

also as most GT3 cars run Ohlins suspension, why does the Audi not have seperate fast/slow damping like the others?

There are differences in suspension configs for various series the car may be running in. Part of me thinks, maybe not having 4-way adjustable damping is a result of modeling a car that may not have carried it?

The documents I have show that the R8 LMS cars running to both 2014 FIA GT3 rules and 2014 IMSA GTD rules used Bilstein dampers. But the documents for the 2016 Asian LMS Cup car show they ran Ohlins though it appears they only had 2-way damping.

 

[mogwai]

Hello everyone,
we are fiddleing some setups for the Audi LMS. But overall we are struggling with the handling after lap 3 or 4. On the outlap and maybe +2 more laps, the handling is really great. After the tyre pressure and heat build up its feel wobble and does not react any more (fast and predictable). We are riding lowest pressure and cambers in front about 3.5 and 2.5 on the back. Is the handling of the other GT3 cars as well?

With regard to the Tosh's question, have you looked at any of the data in MOTEC or another telemetry tool to see what is going on with the tires or in particular looked at the corners where the car behaves unexpectedly? I haven't driven this car and have so little experience with the tires here to say if it is characteristic across all GT3 cars. It would strike me as odd that all the GT3 cars would suffer from this same issue so generally speaking I'd go look at MOTEC to find out what is going on.

For instance, is there a reason you are running with lowest tire pressure? I would think that after a few laps of hard driving you are simply going to overuse the tires, they'll get less responsive because they are overheating.

Also, the geometry of your camber will be affected by many other things in the suspension so using only temps and camber as a reference to why the car handle poorly after a few laps doesn't give much insight. Regardless, spool the telemetry then look in MOTEC (or somewhere) and find out where the car is behaving badly, what the tire temps are and what the optimal tire temps are. That would be where I start.

*edit - to be sure there are many things that could cause this handling issue though. using telemetry may be the most appropriate way to incrementally uncover what the root cause may be.

 

[mantasisg]

I drove it recently and it seems to slide more than others, obviously sliding results more wear. And perhaps because of that I like it a lot. Will be interesting to check whats up with it. I would suspect aero maybe.

 

[Daniele Vidimari]

Many guys reported bump steering issues in the rear of the Audi, this is probably the cause of the strange behaviour of this car.

 

[davehenrie]

For instance, is there a reason you are running with lowest tire pressure? I would think that after a few laps of hard driving you are simply going to overuse the tires, they'll get less responsive because they are overheating.
.

I raised this question several years ago. Clearly low tire pressures are faster, it has long been a known way to go fast with rF2. However, there wasn't any penalty for using that low pressure. As you surmised, you would think a squishy tire would soon overheat and cause the handling issues described. But I have never seen an announcement of a tire upgrade that addresses such behavior.

 

[mogwai]

I don't recall the R8 race spec cars having bump steer issues irl. A quick glance at the R8 owners group shows that some R8 owners have reported bump steer issues with the road going variant but that was as a result of changing suspension components it looks like.

It would seem odd if rF2 was modeled on a road going car though, wouldn't it? If bump steer is the issue, that's easily identifiable in the telemetry as well. Would be interesting to take a look at it if someone has the telemetry from areas where they think BS is occurring.

 

[mogwai]

I raised this question several years ago. Clearly low tire pressures are faster, it has long been a known way to go fast with rF2. However, there wasn't any penalty for using that low pressure. As you surmised, you would think a squishy tire would soon overheat and cause the handling issues described. But I have never seen an announcement of a tire upgrade that addresses such behavior.

That's good to know...now I know what to do I think iRacing suffers from that issue as well but I can't recall if the low pressures have some eventual penalty of overuse which offsets the pace you get. It would be interesting to see if there is greater wear as a result. Interesting stuff to be sure.

 

[tosh]

Hi guys, if you want, here is a motec file from a 1H race on Zandvoort. We have a mandatory pit stop (30sec.) but we are not allowed to change the tyres.

 

[mantasisg]

Interesting. I just drove in Zandvoort too, but I used less fuel load, and saturated rubber tarmac conditions.

My RL left was ~6percent of wear in 7laps. Soft tire. Laptimes a bit slower than yours, but fairly similar (under 1'38 or low 1'39).

I also attempted to drive with stronger diff preload, it did seem to make car less nervous and less slidey as expected, but surprisingly wear remained very similar, I couldn't at that point check laptimes, because I stupidly selected very short practice session.

Will be interesting to test "natural progression" conditions, and with full fuel load, to see if tire wear will indeed double.

 

[mogwai]

Hi guys, if you want, here is a motec file from a 1H race on Zandvoort. We have a mandatory pit stop (30sec.) but we are not allowed to change the tyres.

Thanks. Just a couple quick comments. This is interesting stuff.

If you're using hte DAMPlugin for rFactor, are you logging at rate 3? Higher may be better. Also, there should be an option to enable some optional channel groups which will give more detail on camber/toe/carcass, etc. You don't have to run again, but I would enable the optional data as the camber/toe may see if there is recurring bump steer. The other data that is good to capture is the environment data; track temp, air temp, etc. These temps will affect handling and performance. That is available in the optional data as well.

I'm not really familiar with rFactor in detail, but there isn't anything that stands out to me right away from what you posted. Maybe others with more experience will have some insight as to why the car seems to handle worse after a few laps. However, there's not an increase in rate of wear over the breadth of your laps, no meaningful change in rate of temperature changes, and there is no noticeable increase in the rate of change of the rate of wear (that's meta...isn't it).

Gotta get back to work, but hopefully this chart is as interesting to someone else as it is to me.

*edited for clarity

 

[mantasisg]

Yes, indeed interesting. As far as I know rF2 surface temperature influence, and ambient temperature influence is always static (correct me if wrong). So these variables are not needed now.

Interesting how tire wear rate is so stable. I would also guess that tire would wear less at the beginning and then more and more towards the end. But perhaps it is not the case. Would be interesting to check and compare other cars. It does make sense though, if tire keeps stable temperatures, and tread through it's depth is same compound and also with more wear as it gets thinner it will become little bit less squishy.. On the other hand as tire will wear, it will start slipping more and it should increase temps + wear exponentially. But then again, driver may start being more careful, and decreasing fuel load will also result in less work for tires...

This is interesting issue to learn about.

I will also look at toe settings next time, they may affect tire wear for obvious reasons.

 

[mantasisg]

And it is interesting how S397 tire wear is perceivable. Of course tires are different and everything is realistic there probably, well if simulated tires are different in the first place.

In AC and irl as far as I know, some slick tires would have protective layer, that wears in first lap, or even warm-up lap, or something like that. And that layer is not very grippy.

Also it is easy to confuse stuff. Perhaps in rF2 tire wear percentage, doesn't mean amount of grip ? I would guess that there is no real grip loss above 94percents, under that it feels like tire does become slippery. But it is still strange, about how soon it drops. I don't know how much driving you should expect from it, before it would start giving up ?

 

[tosh]

You have to hit the pedals more carefully as longer you run. But its almost linear after lap four or five. The dramatic drop of the handling is in the laps before.

Many guys reported bump steering issues in the rear of the Audi, this is probably the cause of the strange behaviour of this car.

That is the reason for an early dnf, because I lost the car on the crest for Scheivlak. Runs stable almost all the time, suddenly no control out of nowhere. I will test some other GT3 cars out of the pack for a comparison, with higher logging rate.

 

[mogwai]

Yes, indeed interesting. As far as I know rF2 surface temperature influence, and ambient temperature influence is always static (correct me if wrong). So these variables are not needed now.

Interesting how tire wear rate is so stable. I would also guess that tire would wear less at the beginning and then more and more towards the end. But perhaps it is not the case. Would be interesting to check and compare other cars. It does make sense though, if tire keeps stable temperatures, and tread through it's depth is same compound and also with more wear as it gets thinner it will become little bit less squishy.. On the other hand as tire will wear, it will start slipping more and it should increase temps + wear exponentially. But then again, driver may start being more careful, and decreasing fuel load will also result in less work for tires...

I'm honestly have no idea how the tires work or are modeled here.
Overall, i imagine a smooth driver would wear tires evenly over time so that stable increase in wear would appear to happen from a higher level. But, if I jump in the car and I'm erratic, would the wear rate be as stable or would it be more errative and higher than tosh. I suspect it would be higher.

I agree with you. Tires wear more the longer the stint goes on, but at this track with this car in these temps that appears to be most noticeable on the rears. The rate of change of wear (think how quickly the wear rate is increasing or decreasing) is the reason that I also created a trace of that data. I highlight 2 laps in the attachments here. Tosh's fastest lap and one of his later laps.

And you're correct, the wear rates later in the stint are higher, but only most noticeable at the rear. The rate of change of the wear increases notably on the rears as the stint gets longer and, reading into this (perhaps incorrectly), this could be because the rears have more wear and therefore move around more therefore wearing more. I'm assuming it is tire wear related because Tosh's laps over the breadth of the stint are fairly consistent.

• Wear Rate on rears on fast lap is approximately 1.51% (RL) and 1.17% (RR) vs
• wear rate on rears on Lap 22 is approximately 2.02% (RL) and 1.514% (RR)

So while wear appears to be constant from the "full stint" view, when looking at each laps average rate of change in wear, there is generally in increase in the rate of wear on the rears as the stint gets longer.

However, it is possible that in this car, at the track at at these temps, the tires lose optimal grip after a few laps and that may be one piece to what Tosh is describing. But, testing under similar conditions in another GT3 car and looking deeper at suspension geometry over time may flesh out other causes. Those causes may be more than the tires going off.

 

[mogwai]

You have to hit the pedals more carefully as longer you run. But its almost linear after lap four or five. The dramatic drop of the handling is in the laps before.

That is the reason for an early dnf, because I lost the car on the crest for Scheivlak. Runs stable almost all the time, suddenly no control out of nowhere. I will test some other GT3 cars out of the pack for a comparison, with higher logging rate.

Do you have data from that run Tosh? Attached is that corner complex from one of your late stint runs. There are a couple of reasons one can lose it there. To my eyes, you drive very consistently overall which is why I'd like to look at the data from that corner. I'm not certain it is related to the tires only so I'm curious. You likely know these things also but here are a couple quick thoughts...they all relate to compliance at the rear;

• Ride Height - any ride height issues will be highlighted in this complex. The reason is because the car changes ride height on average more in this 550m than at any other place on track.
  
• Rake - difference b/w front and rear ride height - as fuel burns off, the car gets more rake and combining this with more tire wear later in the stint could make the rear less predictable.
  
• Tire wear - as we discussed, tires wear more on the rears, in particular RL, so it will be looser over time in theory and considering how the dynamics of the car will change, this complex may highlight the shortcomings of the tires wearing
  
• Bump/Rebound/Spring - I think we established this Audi has only 2 way damping so maybe the rear is too stiff overall. As the tires at the rear lose grip, the lack of compliance in the suspension at the rear becomes worse over time
  
• Aero Balance - I presume we have some aero adjustments on this car. As the tires wear, the effectiveness of aero on the rear will suffer as well.

Just thoughts here though. And perhaps I'm just full of it this morning

*edit - track map on left of attachment shows this complex and the RH data on this complex. Red dot on that section shows where the vertical reference line is resting.

 

[mantasisg]

Plenty of stuff to go through indeed. But yeah seems like it needs less rear end roll resistance and probably more rear end downforce too.

 

[Andregee]

The whole thing is very simple. The lower the tyre pressure, the larger the contact patch and the higher the grip. By default, the front tyres have more air pressure than the rear tyres on a few GT3 and all GTE cars in terms of setup and there is a reason for this. They have a little less grip than with minimal pressure, but the balance is maintained.
The front tyres cool down faster between corners than the rear tyres and thus build up less pressure. If you start now with the same minimum pressure at the front and rear, after a few laps you will have more pressure at the rear than at the front and with it a greater loss on the contact patch of the tyre at the rear, which causes the balance to shift. Some vehicles are more affected than others depending on standard setup and tyre wear and heating.

 

[davehenrie]

I almost always ran the fronts 1 or 2 settings higher psi/kPa than the rears. Never knew why, just always assumed that is how it was done. huh.....