mark kibort

Still feel this way, even after PROPERLY analysing the data?

2BWise

I took a look at the data, and really need to stop for the evening but I've got a couple initial findings.

There does appear to be a difference in the dynamic rolling radius of the tire. For equal rpm there is an definite difference in speed. For 5th gear about 1mph at the top end. Not much but there. It also explains why the shift pts are slightly off.

The second is that the run onto the straight, exit of 18, is the real money maker down the straight. On the quick lap with the 17s apex and exit speed are up and help with the boost down the straight.

The addition of these two factors are the big reasons for the time loss

mark kibort

1mph would be due to a 2/10" difference in tire diameter at 130mph. I measured my tires at 25.5", but its not to say they dont change in diameter because of pressure or just being a different set of tires vs the 25.4" tires on the 17s.

I think we certainly have found the reasons though. Nice job!

quickxotica

Your first sentence above I agree with. The cause of the speed delta (and hence segment-time delta) appears to be primarily a result of when/where Brinkley gets on the gas. My first post argued that, given the only change to the set-up was the wheel/tire package... it would seem logical that something about the 17s was enabling him to achieve that earlier full-throttle application (and quite possibly a different racing line as well -- which can't be seen in the screenshots posted so far). I never argued that the 17's lighter weight was enabling faster acceleration on the straights (we agree here too... that single, narrow metric is not responsible for the delta). But assuming this data is representative of lots of Brinkley's laps and not an anomaly, then some other dynamic caused by the wheel/tire change is most likely the causal factor. I offerred some guesses, but I don't pretent to know what that dynamic is. Nontheless, I wrote in all-caps that the wheel change is reponsible because I bet it is... not because of the weight impact on acceleration, but because of some other impact we aren't aware of.

The second snip of yours above shows you returning once again to your acceleration vs. weight model. Why? Everyone has agreed pages ago, you especially, that the wheel-WEIGHT cannot be the determinant factor, so why do you keep bringing it up? It only obscures the relevant parts of this discussion that are now buried under 10,000 words about that. Obviously you're free to write about whatever you want. I just got frustrated reading so many posts about a dead-horse, when I really wanted to see people figure out what was making the 17's faster in that one segment that was helping the laptime so much.

We still don't know the underlying reason Brinkley was on the gas earlier. But, as I said in my first post, I would guess that something about those 17's was changing his line, the car's attitude or his perception of the car's stability... which in turn enabled him to get on the gas sooner.

So YES, I still feel the 17's are responsible (it's not a sure-thing, but if this data is consistent across lots of laps, then it's a reasonable conclusion). Not their weight, repeat, NOT their weight... but some other change they bring to the table.

mark kibort

Thats the problem with email discussions, as its hard to get a clear picture of the message. Good, sounds like we are on the same page.

I only bring up the weight issue, so that we understand the effects. Now as far as the actual tires, flex, contact patch, etc, thats all good to look at too, and could cause issues on entrance and exits of turns, thus causing top speed down straight issues. However, the graphs really needed to be disected and I think we did (2bwise and brinkley) in looking at the real cause of the speed deltas. Lots of factors, but I still dont think its the tires. I think its a results of different approaches to the straights. I think by capturing the straight line runs at the points we did, rules out the tires as the issue.
However, there are many factors as you point out, but its nice to have so many of them on a graph to narrow down the possible causes.

2BWise

There is a very high potential in the rolling radius being different.
17s vs 18s will have a different tire vertical stiffness and therefore they will react slightly different.

The easiest way to confirm this is to roll the car. With a marking utensil (sidewalk chalk or so) mark a spot on the ground and a corresponding mark in line with the mark on the ground on the tire. Roll the car so that the wheel rotates ten times. Make sure it goes straight. Measure the distance it traveled. Do the same for both 17s and 18s. This gives you the dynamic rolling radius of the tire. Its likely that there will be a slight difference which will correspond to speed differences.

quickxotica

Two things:
1. You guys are correct to investigate the heck out of a potential difference in rolling radius. Because even if the difference is slight it could skew the accel.

2. We know the 18's are competitive, or even better than, the 17's for most of the lap. So I want to know more about this one corner/sequence preceding the last straight where the 17's appear to offer some advantage. Is it bumpy? Do you drive over a berm? Or seam in the concrete? I ask because one factor that hasn't been discussed much here is the change in total effective spring rate of the suspension (at the road surface) caused by the shorter sidewall of the 18's. It could be that this one corner is rewarding the softer/taller sidewalls of the 17's with more grip, allowing Brinkley to get on the gas sooner (as we have seen).

2BWise

Maybe a little clarification. On the quickest lap the 17s were roughly .8s quicker, almost all made during the back straight. The second quickest lap was only about .15s quicker. On the flyer posted above the apex speed in Turn 18 is 2mph faster. On the laps with .15s, both are about 2mph slower at the apex. The big key here is the apex speed. Mid corner speed is up and correspondingly then exit speed is up as well. It then plays out all the way down the straight. It appears that on the quick lap there was just a significant improvement that allowed the increased apex speed which then made a huge improvement down the straight.

mark kibort

#2 is worth looking at , because all other things being equal as far as power, or cost of power due to weigtht, the only thing left is everything else and driver. I think Ive felt this difference a few times when I first made the change to 18s. with a little time on the tire, and playing with air pressure, I was able to match the prior lap times pretty fast, in all areas of a given track. However, at first, the stiffer sidewalls (I think) was causing the car to be upset with the same driving style and tire pressures.


#1 the slight difference in diameter would also not be a major factor, UNLESS the speed numbers came from the cars Speedometer. If its GPS, over a long enough straight and using multiple gears, the trade offs can pretty much cancel out depending on the speed ranges. remember, that acceleration is all about usuable HP over any speed range. (no to dive in to the HP/torque discussion though ) Personally, ive gone from a tire with an extreme diamter and seen no real difference in lap times. In fact, i was able to push just a little harder, because I wanted to use my same shift points, or redlines, with the taller tire, which forced me to push the envelope a little. Even then, those differences were very subtle, but able to be responsible for 1-2 tenths a lap. That driver change in technique and style, far outweighted any gains one would think there would be by a smaller radiused tire.