Repeatablitiy of Tyre Data

[Menisk]

Just curious to start a discussion on the accuracy and repeatability of the TTC tyre data. I've also started a thread on the TTC forums which has some of the data I've been comparing (http://sae.wsu.edu/ttc/viewtopic.php?f=19&t=130) but I'd also like to start a discussion here as I think more people will see it.

Remember that TTC data can only be posted in the TTC forum, not here.

The problem I'm facing right now is the massive difference in the data for the control tyre between round 4 and round 5 of testing. I've compared the Hoosier 20.5x7-13 @ 10PSI on a 6" rim and found that the shape of the curves was essentially the same but the lateral force produced for a given normal force was remarkably different. Round 5 testing produced far more lateral force. That wouldn't concern me if that was the case for all tyres and you just had to scale data to take into account wear to the sandpaper belt or whatever it was that caused the difference. However when I compare the data for the same tyre @10PSI on a 7" rim the peak lateral force is similar, however there is a big difference in the load sensitivity and the shape of the curve is different. It's to the point I'm wondering if I'm somehow using the wrong data sets as they look like entirely different tyres. (In the TTC thread I've posted an explanation of which data sets I've used and how in the hope people can tell me if I've done something very wrong.)

So my question is, has anyone else looked at this? Does anyone have any experience with manufacturing tolerance on tyres or how the FY-SA curves change depending on the aggressiveness of the surface?

I imagine that not only the maximum force but also the curve of the tyre will change depending on the surface you're running on as the tyre will deflect differently if it's on a surface that makes less force. Is the tyre data even useful at all or is it unreliable and not representative of real world conditions?

[DMuusers]

When comparing round 4 to round 5: was the slipangle for the maximum lateral force the same or was it different? Have you also looked at the different temperatures of the tire/room? For the 7" rim: in what way is the load sensitivity and shape different? More sensitive to load? More dropoff after the peak? Also, were there significant changes in camber sensitivity?

It is also very well possible Hoosier changed something in the structure of the tire so it performs better/different on a certain rim width. Otherwise maybe the surface of the testdrum was of different material? It's very hard to say where the difference comes from without knowing the entire test setup aswell as the structure of the tire. However, in general testing on a drum is really hard to translate to a road. Even translating tire measurements from one type of road onto another one can be very hard. This is mainly caused due to the 2 ways grip is provided by the tire: adhesion and hysteresis. For a more coarse surface for example, hysteresis plays a big role in tires with a very soft tread compound and stiffness. Because it forms to the road better, more force can be created by it. For flatter surfaces adhesion can also play a big difference. Rubber sticks better to tarmac than concrete for example (even if they have the same surface coarseness).

In general, testing on drums can provide insight into how stiff or soft a tire is, so how easy it is to drive. It also provides some trends(!), for example camber sensitivity. But for exact values, in my opinion, it's of little use.

[Edward M. Kasprzak]

Doug Milliken and I are very busy this week, but we will review Menisk's question and reply next week.

I will note that all TTC data has been collected on the Calspan Tire Research Facility's flat belt testing machine. No drum testing has been performed.

[BillCobb]

I looked at the graphs you showed and have several suggestions for checking the results. The differences between the two tests is far greater than one would expect to be normal variation, even considering the likely hand built status of the two tires. This is just my opinion based on tests of dozens of tires with identical build recipes. Yes there are some yearly variations in the 'control tires' (yes there is a standard tire used in the industry to check out tire test machinery. These tires are not used on vehicles, but only to validate and verify the machinery accuracy).

First make a comparison of just the raw data points for a single load (maybe the 80% of rated load sweep). Do the same for a single slip angle and a load sweep. This will show you the range of each sweep and any transient hiccups that could confuse the fitting routines. Post it back for us.

Second, someone needs to verify that these are in fact the same tire (brand, size, construction, pressure, wheel rim and width, BUILT DATE and mounting orientation). The differences you show are highly suggestive of completely different build packages. The low slip angle comparison says (to me) tire structure is different (bead, sidewall and breakers). High slip angle area differences are usually tread and compound differences.

If CALSPAN makes calibration runs before each test group using an ASTM control tire, then let's see those results. That would indicate whether machine electronics or transducers are involved (Doug and /or Ed). I doubt very much whether any belt surface issues are open because the low slip results (slope and offsets) are so different. Think of it as a comparison of a pair of running shoes being observed at a slow walk. (A tire really acts like a millipede with a lot of little running shoes).

Don't be surprised if you find out the tires are actually a different formulation. Such things are/maybe commonly done to the unsuspecting. This is why you should 'stick' with a tire with known, stable, high volume and major brand volumes. Designing a chassis and steering system to fit around a 'good' tire is less stressful, more productive, more satisfying and more fun if the tires you buy for synthesis, testing, validation, practice and competition are always the same. All it takes is one goofball out of 4 on your car to cause a headache. I always say that if your target is moving, select a belt fed weapon instead of a single round.

That's my opinion and I'm unanimous with it.

[BillCobb]

Here's an example from production car tire testing. No specific reason I ran this size, just grabbed a random wad of data. There are 7 tires on this plot. You might expect a shift in the data because of the previous test series' effect on the belt surface, but that ain't the case. Good, fast, cheap. You get to pick only two...

[Menisk]

Thanks for the replies. I'll shoot Hoosier an email and ask them if there have been changes to the tyre between the two rounds of testing. In regards to the TTC licence is it okay to send some screenshots of the curves along with my email to Hoosier so long as it's only their tyres on the plots? Not sure if this violates the licence agreement as one would assume that Hoosier would know what their tyres should be doing to begin with.

[DougMilliken]

... I'll shoot Hoosier an email and ask them if there have been changes to the tyre between the two rounds of testing. In regards to the TTC licence is it okay to send some screenshots of the curves along with my email to Hoosier so long as it's only their tyres on the plots? ...

Hi Menisk,

As Dr. Kasprzak noted above (and I posted over on the TTC secure forum), we are looking into your results but we have to fit this in with our regular work. Hopefully we will post something this week.

It seems likely that differences between the tests (which were about two years apart) generally explain your findings. Our thought was to look at the TTC test records first, make sure we know everything possible about the two tests, and then contact Hoosier if it seems necessary. The engineers at Hoosier are probably busy too (since it's the middle of the race season in N. America)...

License question: The tire companies that donate tires to TTC are "industry members" (all except Avon, TTC had to buy Avon tires). Thus they have access to all the data.

-- Doug Milliken for the TTC

[Menisk]

I'll leave it with you guys for the moment then and see where things go. Thanks for being so helpful.

[Edward M. Kasprzak]

Doug Milliken and I have posted a response to your question on the TTC forum.

[Z]

Doug Milliken and I have posted a response to your question on the TTC forum.

Awww.....

I was hoping to find out just how (IN)accurate this tyre-data can be.

Any chance of an "executive summary" of the findings being posted here? Maybe just some curves painted with a 4" brush?

Z