Yeah, I don't think that there is any doubt about it. Your data shows exactly what you are saying, the car with 18s accelerates slower than with 17s. Your rear toe setting is minimal so excessive scrub is probably not an issue, but I still would do a roll down test to see if there is any difference there. It would be another data point to help figure out what is going on.

 

I did a coast down test with my bmw just for scale. 18 seconds from 60 to 40mph on a flat road. Ill have to plug in the numbers and see how much hp it takes to do this. (thats rolling and aero, but the aero is not that huge due to the relative slow speeds)

Im leaning toward an engine issue at this point as it is physically impossible for 10lbs of rolling weight to effect acceleration by the amount seen.

Interested to see what it eventually turns out to be.

mk

QUOTE=Larry Herman;6665024]Yeah, I don't think that there is any doubt about it. Your data shows exactly what you are saying, the car with 18s accelerates slower than with 17s. Your rear toe setting is minimal so excessive scrub is probably not an issue, but I still would do a roll down test to see if there is any difference there. It would be another data point to help figure out what is going on.[/QUOTE]

 

am I reading the throttle position wrong, when I see that the faster line (black 17s) has the throttle applie much earlier in to the turn. I dont know what your steering input was at that exit, you could have been scrubbing speed on the exit as well. (just tossing out ideas)

bottom line, it is a solid fact that the heavier weight wheel cannot effect acceleration by the amount shown. as I mentioned and showed with some simple equations, the effect of 10lbs rolling, is like 20lbs if it was in the car. (an extra 3 gallons of gas would have this sae weight effect)

I dont think its head wind, as there is a difference off the exit of the turn down the first straight until the gear shift.

On thing I dont know for sure, is the effect of new race rubber and its effect on Coefficient of friction. aero effects we can predict. weight increases as well, but the friction part im not convinced it could be an issue. Ill be doing some testing of my own this weekend as I have a similar set up 275s going to 315s of a stickier rubber as well.

Off racing! See ya!

mk

 

I don't see a big mystery here, the small increase in weight and loaded rolling radius result in a small decrease in acceleration and slightly worse aero which results in a small decrease in top speed, even though you are carring more speed through the corners.

 

Now that I look at the graph a little more critically, I see that the shift with the 17s occurs at a lower speed than with the 18s. Like Bill suggests, maybe the true rolling radius of the tire is being overlooked.

 

Some of this due to squirm. That is most common on RA-1 with deep unsupported blocks. 4/32 vs worn RA-1 you don't get the squirm in the corners, but stil feel a bit of the weight. full tread R888 don't squirm if at all, but are heavier. The way I can tell squirm vs weight is the corners where as the over weight I feel powering out of the corners. Remeber also when you have only 134 hp getting down to the wheels 2lbs of tire weight is is much bigger percentage than if you have 290 at the wheels. As for hp I can also tell how HP changes in Summer, winter and morning vs afternoons. My 4-5 shift point can move 500 yards based on air temp alone. This a big deal since in cool weather I need to go 4th while still in a turn, but when it is warmer I can wait till I get straight and some times run down the straight and almost not need 5th.

 

Actually what I see is that on 17's you seem to get on the power fractionally earlier. Not much, but since you slow down more it seem like the car is gathered up and you can get on the power harder and sooner. On track it is probalby a car length. On the 18's you carry more corner speed, but that cause a pause in power application which impacts oveall top speed.

The real key is to look at the angle of cruves. The steep ness of each one. They seem to have a very similar slope, but the 17' curve seem to be shifted left a little bit. This tells me you got on the power sooner. So what this really means is that for maybe one car length you are still turning while on 18's, but have gotten back to power on 17's. Thus down the end of the straight you are going abit faster.

I think what you need to try on 18's is put the power on sooner. It looks like you were very close to the same min speed on 18's, but if that occured 1 car length farther down the track it will impact acceleration.

 

M758,
I agree with you, I think it might come down to driver not physics or aero or anything but mental.

I will say that the shift points of the zoomed in chart are different but overall, when looking at every lap you can see they are usually very close, just not on that zoomed in chart.

Thank you everyone for your time and energy in looking at my issues. I'll work on getting to throttle sooner with the 18's and not slowing down as much.

 

[QUOTE=Larry Herman;6663701]I'm tired of all of the math mumbo-jumbo. I can't tell if you're right or not. Time for a real world test.

Hey Brinkley, why don't you do a roll-down test. QUOTE]

Woah! Until I read the rest I thought Larry was advocating rolling a fatty and meditating on the real reason for this problem. Whew!

Back to your originally scheduled program..........

 

That would be a roll-up test.

 

And that would not be tire or oil smoke, I presume.

 

I don't theenk so maannn.

 

I'll just add that when I raced my 944T I went to a smaller tire. Gave up about 4mm/tire in contact patch but shaved about 5 lbs per tire. Same rolling circumference, but I was faster by .5 - 1.0 second consistantly at TWS.

Same brand tire and I had data for both new and worn for each tire size. The Michelin Slide deck on Pilot Sport cups says to use the narrowest tire you can for a given rim width.

Measure the actual tread width/height with the tire mounted and you may see that the tires are different in dimensions.

Back to Larry's original point, I'll add, what is the difference if you optimize set up for 17's vs 18's?

 

The 17s do look to accelerate faster, but only slightly as we've expected. I think the bigger deviations seen are driver inducted. A few quick notes.

Tower Turn: definitely at WOT sooner with 17s which gives you the extra speed on the straight.

Back Straight: You've applie more throttle earlier with the 18s and for the first half of the straight the difference is small. It is noticeable that the 17s are slightly quicker, but the difference is marginal. The shift is what gets you. It much quicker on the trace with the 17s.

Exit of Turn 17: Again you've gone WOT sooner with the 17s and that gives them the advantage in speed at the end of the straight.

Overall it looks as though you're probably still learning the 18s. There going to be subtlely different and it looks as though we're seeing it in the trace as well. For most corners it looks as though you're back to the throttle stronger and sooner with the 17s. Although, the 18s do appear to have a serious advantage is some spots, after Turn 1 and Turn 15-16, where you get to WOT with the 18s where you hadn't with the 17s. You give up some straigh line acceleration with the 18s but they do look to have some advantage and with some time you'll probably we able to get quicker laps.

 

Did you not see the throttle input that is later with the 17s? It could be the gap that carries through all the way to a later shift and more speed diff at the max speed. ( Look at 10,700 foot postion where it shows the difference in throttle position)

Here is the bottom line. 2 lbs of tire weight each, on a 25" diameter range, is worth the EXACT same effect acceleration wise as if 20lbs was in the car. (or about 2hp in this case with a near 2500-3000lb car with near 300hp)

I just cam back from laguna seca where i had a practice session to focus on speeds down straights along with changing tires 4lbs different each! (16lbs over all, 32lbs as if it was in the car as far as accel effects) we have 125mph straights and redline just before braking zones which I was able to hit with both sets. Point is, 5mph is a huge differenece, which I have only seen when my HP has jumped up by near 50hp.

Now, if you think that 2lbs a tire can effect the acceleration of a car off a same starting corner speed, thats your choice. BUT, it is physically impossible for that weight to cause that kind of near 50hp effect on acceleration. talk to anyone that knows his stuff. Its a really simple set of factors and equations.

the 5mph difference was due to something else, not the 2lbs of wheel weight.
the speed data can be off based on my actual measurements of the tire size in question. (26" actual vs 25.5" mentioned on a 10" wide rim. Also, we dont know the actual off the 275x35 )

aero, again, there is not much showing in terms of frontal area. maybe about 6 square inches on the front tires, and something less for the rears due to their position in the air flow. so, aero couldnt really contribute a significant amount.
the only wild card here is rolling friction. Dont think thats it, as I just went from real sticky hoosiers that picked up every pebble and piece of rubber on the track vs pretty dry toyos, and top speeds were near identical on both sets.

Anyway, good topic. Sure would like to know what caused the difference of top speed though.