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wheels to match RE71’s - harness - data r7 slip angle vs re71
Hey guys I have a base bare bones 997.2 c2, and want to run RE71’s. I’ve been searching nonstop for wheels that fit my car that re71’s will fit, but for the life of me I can’t find anything! I’d prefer an 18 over 19, but at this point I’d even be willing to sacrifice and run 19’s.
Ill probably put some ohlins in the car, and if I need LCA’s or Camber plate top mounts to make a wheel fit, I’m fun with that, and of course the added benefit of running a more aggressive setup. I’ll see if I can find the data for you so you can see.
Also while I’m at it, an unrelated question. Is there a relatively easy rollbar or even just harness bar that can come in and out of the car? I’m very against taking a not caged/log book car to the track, but if I have to, I’d at least like to have the ability to use my hans.
BTW, I did data testing of the slip angles on re71 vs r7’s and in turn in grip they are almost identical, after that they fall off compared to r7, but what was amazing in my race car the setup I use for r7’s was almost identical to my re71 setup!
I recently bought a set for my 987 and can fit 255/275 fr/rr RE71r. It might be a little different for your 997...
As far as harness bar vs. roll bar, I'd suggest double checking w/the groups you frequent events with. A harness bar w/a fixed back seat is not a good idea.
I had a custom roll bar made for my 987 for a very reasonable cost (much cheaper than the GMG bolt in bar for example). Removal would likely require cutting the bar though.
Fresh R7s I'd think have more to give than RE71Rs, but take-offs/heat cycled R7s have been fairly comparable to RE71rs in my experience. Consistency through a session is a completely different story though...
I just wanted to add that, in my notes, I find myself consistently adding rebound when I switch from RE71rs to R7s. Not sure I have the skills to objectively quantify this change, but subjectively I prefer less rebound on RE71rs
Last edited by dgm8138; 11-17-2018 at 09:44 PM.
Reason: additional detail
Appreciate the response! I looked at the apex wheels, but still can’t figure out a size for re71’s. May need to look at the width range.
Sounds like the bars are like most and hard to take in and out, so will probably not track the car.
Just yo be clear, I was saying the re71’s had similar initial turn in grip, but of course wouldn’t hold after that. I have to find the data, but my old aim logs are all messed up. We just added a little more resr bar and it was fine (on s2000)
BTW, I did data testing of the slip angles on re71 vs r7’s and in turn in grip they are almost identical, after that they fall off compared to r7, but what was amazing in my race car the setup I use for r7’s was almost identical to my re71 setup!
Just curious how you did that. Can you please share output, methodology and math channels?
My coach/setup data guy set up the math channel using the steering pot sensor and I believe G-Force. As I said my AIM is kinda messed up right now, but I’ll check. The formula is also in the famous data book (which is in my trailer that I’m cleaning up today and can grab).
Im sure Peter knows it, and can post it up, but I’ll search a bit more and worst case should be able to get it by EOD after I can actually walk around and get things in my trailer 😬.
Just curious how you did that. Can you please share output, methodology and math channels?
by the way this is the book, it’s awesome if you don’t know it: A Practical Guide to Race Car Data Analysis https://www.amazon.com/dp/1456587919/ref=cm_sw_r_cp_api_wQx8Bb76K7F4Q
also I’m sure Matt also is familiar with the formula
I have the formula and have the book and was just looking for insights to your practical application. For example, steering angle is not the same as wheel angle so that needs some calibration and it is cumbersome. Never mind...
If you install a steering wheel sensor (string pot wrapped around the shaft or a linear displacement sensor on the rack) then you can get close to and under/oversteer look without a yaw sensor.
What you know based on the GPS data:
Corner radius
Lateral G
What you can calculate based on corner radius:
The amount of required steering to make the measured corner radius.
What you measure from the car:
Actual applied steering
Wheel base
The equation is:
Required steering angle = (wheelbase / (Speed^2 / lateral G))*57.3
This is all laid out in the book: "The Competition Car Data Logging Manual"
The actual formula that is in the analysis package looks like:
If you install a steering wheel sensor (string pot wrapped around the shaft or a linear displacement sensor on the rack) then you can get close to and under/oversteer look without a yaw sensor.
What you know based on the GPS data:
Corner radius
Lateral G
What you can calculate based on corner radius:
The amount of required steering to make the measured corner radius.
What you measure from the car:
Actual applied steering
Wheel base
The equation is:
Required steering angle = (wheelbase / (Speed^2 / lateral G))*57.3
This is all laid out in the book: "The Competition Car Data Logging Manual"
The actual formula that is in the analysis package looks like:
I don't have the book at home to check, but you wouldn't normally calculate slip angle. To measure it, it takes a setup like shown below (borrowed from OptimumG).
From what you described and put for formulas (and my recollection of what is in those books), you are looking at expected steering versus actual steering. This is a good way to put a quantitative value on understeer. For a quick visual of understeer, you can plot steering vs lat G in an XY plot. That gives you a graph that you can see when the lat G falls off in comparison to steering input, you have understeer. You can also see an oversteering car here too. The second image shows the XY that is easy to see what is going on. Calculating the values is a great way and good for comparisons of one setup to another, changes over time, and as a comparison of what different drivers like, but can be more difficult for some people to pickout.
Yep, Matt’s hit on it. Expected steered angle versus actual steered angle.
You are plotting the numerical differential, but more than that, the rate of change of that value in order to extrapolate the amount of that change within a certain distance. Usually corresponding to the period between stable LongG to stable LatG. In one corner at a time.
Matt’s graph is typically used to show the amount of understeer or oversteer, but that graph is tracking events over a whole lap.
Yep, Matt’s hit on it. Expected steered angle versus actual steered angle.
You are plotting the numerical differential, but more than that, the rate of change of that value in order to extrapolate the amount of that change within a certain distance. Usually corresponding to the period between stable LongG to stable LatG. In one corner at a time.
Matt’s graph is typically used to show the amount of understeer or oversteer, but that graph is tracking events over a whole lap.
Drew may share, may not. Rallo, too.
How and/or when you look at the numbers is really dependent on what you are trying to accomplish. On multi driver teams, you will compare the metric against fast lap for each driver to see who likes the car with what amount of understeer/oversteer. Based no those numbers, you can determine which way to bias the car. Like many data metrics, it's not usually used for one corner, but certainly can be. It is very valuable on a lap basis to compare changes in setup (alignment, bars, springs, even tire pressures). I've found it to be a great tool to work with drivers developing their kinesthetic feedback. While making chassis changes or doing doing setup sweeps, it's very handy to be able to give the driver an actual number for the amount of understeer. Then they can take the numbers for the two sessions/laps and then put a more quantitative analysis to their qualitative feel. It can really help a driver with the quality of their feedback.
To Peter's point, you can fit a line to the XY graph to get a slope (best fit line) to compare things, but AiM (and lots of other analysis packages) don't do this. That is more often done in Excel of Matlab. But, it's not totally useful as the line is not linear.
Another good use to use measures like this to compare driver style. You can quickly see how things like cheating the entry, rolling the turn in, hand speed, etc can impact how the car handles.
But, for quick and dirty comparisons, the visual of the XY is great. Most times the quantitative analysis is used in review (in my experience). I'm sure some folks do it differently.
Great discussion! Hopefully Justin can report back if they were doing this (understeer) or something else.
I thought Justiin, with Drew's help, was identifying and comparing the rate at which the tires "turned in." That was the context of the original post.
The formula posted was from a Race-Technology forum demonstrating how understeer/oversteer might be plotted well, so Matt expanded on that very nicely with an example drawn from some of the information John Block presents.
My read of the original post was that Justin and Drew were not attempting to plot understeer/oversteer, but instead, track the initial response of the tire. Not tracking the sustained balance, but instead looking at the difference in what it took to get the car turned.
I can see how this could be done by comparing the initial steered input, comparing it to the expected steering input and then, comparing the delta between each tire...
But I could be wrong with that intent by the OP.
Good discussion though.
__________________ -Peter Krause www.peterkrause.net www.gofasternow.com
"Combining the Art and Science of Driving Fast!"
Specializing in Professional, Private Driver Performance Evaluation and Optimization
Consultation Available Remotely and at VIRginia International Raceway
11-17-2018, 11:23 AM
wheels to match RE71’s - harness - data r7 slip angle vs re71
