mark kibort

Some observations based on the claims and the graphs that have me a little curious.

The claim is that the heavier tires accelerate slower
the claim also is maybe the diameter is different changing gearing
There is also a claim that rolling friction is the issue

Gear shift for the 17s vs the 18s (18s being thought to be slower) was a full 200ft earlier as well as being at a lower GPS speed (95mph vs 100mph at 11350, to 11550ft)
So, the question that comes to me is, forget about the final speed of 123mph (it looks to be only a 3mph diff from the graph), look at the 100 vs 95mph GPSspeed. And the slower curve (18's) shows a shift 200feet later. Now this is more significant because it happens only at 100mph/95mph and shows a near 5mph different shift speed. The other interesting thing about that, is that the 305s are a full 1/2" (1.5%) taller than was expected. It doesnt effect the GPS speed, but if the same rpm was used, the taller tires shift point would be 5mph faster, not 5mph slower.

also, look at the difference in the curves before the first shift, (below 100mph ) and after (near 120mph). Now look at the point where full throttle was applied at near the same exit turn speed and position. Its a drag race from 10650feet and 60mph to 12600feet, However, "Mr 18"" waits to put on the gas by 25 feet at 10725. Couldnt that be the smoking gun you are all looking for??

For all the physics guys out there, the graphs tell us, we have a change of velocity of this 3000lb car from 60mph to 120mph over 1950feet (.37 of a mile) of 3-5mph in the neighborhood of 10 seconds. How much hp would it take to change that top speed from 120 to 125. Running the ball park numbers, I get near 50hp or an average of 40ft-lbs of torque from 4500rpm to 6500rpm. Anyone care to play?

Brinkley

Mark,
While I can't play, regarding the math, I can agree that I think my differences are down to, when I go to WOT and When I shift. I have been trying to shift at the same RPM. I'm attaching another chart that may give you more to review. This is a zoomed in view of Homestead, black is 17's vs. Red 18's. Notice the shift pts are closer and the top end are closer. Disregard the GPS speed clitch (small spike on run up to top end).

2BWise

The most notable thing would be the exit to the sweeper. Are you getting WOT all the time? It looks like the throttle might be slightly bound or hung up because throttle position is flat, but not as high as it is in all other spots.

Brinkley

Bill,
Its an E Throttle so it might be time to double check gas peddle wide open position with E throttle body wide open position. However, my point of showing this trace is to show that the two wheel sizes don't seem to make that much difference when both throttle position and shift pts. are matched better.

jrgordonsenior

FWIW.... I would try the 285/30's in the rear with the 245/35's up front. Better balance and less aero drag....

mark kibort

Again, the aero drag is easy to see. we are talking about someting the size of sticking your hand out the window. Remember, anything hidden by the fenders, is not frontal area . Only the block below the chassis counts for aero.

a 275 315 is a good match of tire. pretty big in the rear, unless you have at least 250-300rwhp and then it work pretty well

mk

mark kibort

It still looks like the throttle is the cause for any speed difference at any time.
Again, this is a plain and simple fact. 10lbs of rotating mass is like 20lbs as if it was in the car. (think about 3 gallons of gas). next time put 3 extra gallons of gas in with the 17s and see if that slows you down any. It wont. Its very simple and absolutely true. 20lbs linear will equal 10lbs rotating on a 24" diameter. It in NO way can effect acceleration by 5mph over the distance and speed based on your weight and power. So, since we know that, lets examine other factors, because for sure, it is NOT in the 2 lbs extra weight of each tire.
Aero? No as well.

It can be you power on that lap or session ( which could be effected by tempurature, or other engine operational variances.) It could also be your line through the corner. All sorts of things, but not the 2lbs on the tire. Sorry, physics on this matter doesnt lie!

Anyway, the graphs are very interesting. What are the colors for the throttle postion green is red and black is black? hard to see. If green is red, then you are consistantly late to the throttle by 50 feet or so with the 18s, and that certainly would be the difference.

Also, going back to one of my points, if the the GPS is correct, then if your shift points are later for the red, the speed would be quite a bit faster for the same RPM as the tire and wheel diameter would be 1.5% greater than you think based on actual measurements. 26" for 305s on 18" 10" wide rims vs more like 25.25" diameter for 275s on 17s.. Gearing changes could change top speeds based on certain track straight lengths and prior turn exit speeds. However, it wouldnt favor all tracks, only some. Gearing is an optimization tool.

mark kibort

It also looks like you are faster in , slower out with the 17s but on the gas sooner. It could be your line, but the net net is that it is giving you slightly faster top speeds down the straights. So, in the end, its not the wheel weights, nor aero losses , and unlikely rolling friction differences. The line and throttle applications are MUCH more dominant factors. ( an order of magnatude greater)

Can you verify that the red and the green are the same outing for speed vs throttle position?

edit: I just noticed something else. at 2200ft vs the turn at 4200ft, you are doing the opposite with fast in slow out. so that isnt it. BUT, look at the throttle position for both turns for the 18s (red) and 17s (black). 17s always get the throttle much earlier, and subsequently, your top speeds are higher. I think that is probably it. 50feet earlier throttle for that first turn and 25ft earlier for the second turn, plus it looks like the throttle is at a higher position too with that comparitive run.
Are these same day runs on the different tires?

Brinkley

same day runs different tires.
Confirming Green Throttle equals Red Speed and Black is Black. All colored lines are with the 18's and all black lines are 17's.

srf506

In my olden day experience with MOTEC they ran off of magnetic wheel sensors to calculate speed. But that's been almost 10 years ago so I wouldn't doubt they now have some fancy schmancy GPS hooked into the IMU to calculate an average speed.

mark kibort

sounds like Brinkley has a system that shows speed as a function of GPS. those are pretty accurate and certainly relatively accurate for comparison.

If the speed is off a magnetic sensor, then the difference is perfectly clear. tire diameter differences. (18s with 305s on 10s are .5 " taller than 275s on 10s 17" rims). the 18s would be going faster for the same speed output of the system.

from the look of the throttle output, that seems to be the likely difference in top speed. 25-50 feet difference in timing of full throttle application.

mark kibort

if thats the case, its the throttle timing based on the motec report. If the speed is actually measured by a speed sensor as I said, then the differnence is due to tire size. In no way could the rolling weight contribute to that much of a top speed difference. They could if the tires weighted 50lbs. more each!

mark kibort

Someone else did my dirty work for me. I thought that it would be more believable if someone else besides me, ran the numbers.

So, you might want to print this out or save it for future conversations.

This is an example of the effective HP it requires to accelerate rolling mass (tires and wheels) over dyno type acceleration time periods. (i.e. 7 seconds from 1000rpm to 2000rpm at the tire which could be from 60 to 120mph or 80 to 160mph depending on the tire diameter and how much hp and weight you have) (note:2000rpm on the tire would be 142mph if the tire was 24" diameter and 7 seconds from 70 to 140mph is pretty generous. so, the hp would actually be quite a bit less in this range of speeds, but would reflect dynojet accel times)

What the calculations below show, is that the effective HP difference of shaving 10lbs off of each wheel in the rear, would dyno less than 2hp . since all 4 wheels are spinning on the track, less than 4hp.
This number will change with different diameter wheels, speed ranges, and times. (as well as shape of HP/torque curve as we assumed a flat one for simplicy of the calculations) But make no mistake, by changing tires and rims, you are effectively only giving up or gaining less than 4hp in most extreme cases for all 4 tires and wheels.

enjoy!

Mark

>>>>>>>>>>>>>>


Originally Posted by 76FJ55
Okay assuming you are running along a flat torque curve with constant acceleration from 1000 wheel RPM to 2000 wheel RPM over a period of 7 seconds the following shold determine the amount of power consumed in accelerating the wheels at the 1000 and 2000 RPM points.

Equations:
Torque: T = Iα
α=angular acceleration (radians per sec^2)
I=moment of inertia of a disk = ½ M R^2
M=mass (lbm)
R=radius
Power: P(hp) = (T(lbf*ft)*RPM)/5252

With numbers:
α=1000RPM/7sec= 1000RPM*2πradians/60sec/7sec=14.96 rad/sec^2
M=mass in slugs=lbf*sec^2/ft
. 100 lb wheel: M = 100 lbm
. 80 lb wheel: f M = 80 lbm
I=moment of inertia of a disk = ½ M R^2
. 100 lb wheels: I=.5*100lbm*1ft^2=50 lbm*ft^2
. 80 lb wheels: I=.5*80lbm*1ft^2=40 lbm*ft^2
R=radius=1ft
Torque: T = Iα
. 100 lb wheels: T = Iα =50 lbm*ft^2*14.96 rad/sec^2 = 748.0 lbm*ft^2 /sec^2
. 80 lb wheels: T = Iα =40 lbm*ft^2*14.96 rad/sec^2 = 598.4 lbm*ft^2 /sec^2
. Converting lbm to lbf
. 100 lb wheel: 748.0 lbm*ft^2 /sec^2 * sec^2/32.2ft=23.23 lbf*ft
. 80 lb wheel: 598.4 lbm*ft^2 /sec^2 * sec^2/32.2ft=18.58 lbf*ft
Power: P(hp) = (T(lbf*ft)*RPM)/5252
Power at 1000 RPM used to accelerate the wheels:
. 100 lb wheels: 23.23 lbf*ft*1000/5252=4.427 hp
. 80 lb wheels: 18.58 lbf*ft*1000/5252=3.538 hp
Power at 2000 RPM used to accelerate the wheels:
. 100 lb wheels: 23.23 lbf*ft*2000/5252= 8.846 hp
. 80 lb wheels: 18.58 lbf*ft*2000/5252=7.077 hp

2BWise

Moment of Inertia of a disk assumes equal mass distribution through the entire piece. A wheel and tire assembly does not have equal mass distribution. Mass relative to the center of rotation matters. Easiest way to find MOI of a wheel/ tire assembly is to measure it. Although you could do the requisite calculations instead.

mark kibort

Hey, I just wanted to get a baseline out there so EVERYONE can at least relate to the effects of wheel size, weight, inertia and its effect on available power.

Help yourself and make a donut out of the weight, (making ALL the weight in the tire, and nothing in the center) as you will soon see that the values are well below rounding errors on a dyno. remember i used 10lbs per tire and wheel. I also used 7 seconds for the acceleration rate from 70 to 140mph. so me a car that does that and Ill show you a car with north of 600hp. I also used 24" diameter, a little shy of the 25.5" we are talking here.

so, if you look at the calculations, feel free to inject your situation and conditions. what you will find, we are talking about much less than 2hp for most situations if we are talking about a 5lb per tire range at a super speedway road course near you. Again, I pointed out the major factors that probably led to the straight away speed difference. he was on WOT about 25 to 50 feet later in most all instances. telemetry does tell a lot!

mk