Why is quicker not faster???

Draco1340

Registered
OK all you brainiacks, explain me this...

Why do I consistently get more MPH at the quarter when I spin and get a fairly crappy 60'. For example:

60' 1.496 spun, backed off and didn't hit full throttle till the 330.
330 3.848
1/8 5.792
mph 126.2
1000 7.480
1/4 8.914
mph 156.83

Same day, next pass within an hour, air hadn't changed at all.

60' 1.415 instant full throttle,tire hooked good, with small wheelie
330 3.733
1/8 5.689
mph 124.9
1000 7.392
1/4 8.840
mph 155.24

This is just yesterday's example, but its something I've noticed that doesn't make sense. Guys at the track were trying to convince me that a really good 60' doesn't give the bike enough time to build speed at the end of the track.??? Because you are getting there quicker, the bike doesn't have the time to accelerate. I would think that if you are accerating harder, then you should be going faster. Thats not what the majority of the bar-bike guys were saying. Alright you physics majors... what gives?:please:
 
if your going to a point 1/4 mile away starting from a stop with a bike and car. the bike gets up to say 100mph in half the distance of the car. the car will need to be going at a higher speed to finish the 1/4 at the same time as the bike... :thumbsup:


hows that?
 
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if your going to a point 1/4 mile away starting from a stop with a bike and car. the bike gets up to say 100mph in half the distance of the car. the car will need to be going at a higher speed to finish the 1/4 at the same time as the bike... :thumbsup:


hows that?

Evil, I agree with your scenario, BUT

in the case of the same vehicle, with the same HP and torque, a better launch tends to drop MPH, and a weak launch can result in higher MPH. If you are accelerating harder, shouldn't you be going faster in the end.
 
If you are accelerating harder, shouldn't you be going faster in the end.



nope you just finish quicker... not faster thats why i use the car/bike example. the car because it doesn't accelerate as fast has to be going at a higher speed at the end to finish with the same e.t.


maybe someone else can explain it better then me...
 
My theory is dont worry about top speed be more concerned about your ET. Speed does not win races ,ET wins races. Speed is measured at the last 66' of the track. It doesnt matter how fast your going at that point. What matters is how quik(et) you get from start to finish.
 
I was at the track yesturday .My 1st two runs 10.86 @ 132.6 and my 2nd was 10.81@ 131. The 1st run my 60' was 2.01 the 2nd was 1.91
 
If you launch good there is less actual time for the bike to be at wide open throttle, thus it hasn't accelerated to as high a speed before the run is over. Make sense? Plus as said above, mph don't matter, unless you're trying to figure out horsepower.
 
It's not really important when you are running 8's on a pretty much stock bike... :laugh:


What have you got done to her...? :poke:
 
MPH = HP, so it does matter if you are trying to evaluate corrections to tuning, density altitude, and engine health. Dropped mph can mean anything bad from a bent valve to a bearing starting to seize, or a change in headwind.

Perhaps back half MPH (in this case about 30.5 mph in almost all cases) is a better indication of motor performance. I like to understand stuff like this, and I'm struggling with a good mathmatical answer to this. Vf = V60 + A*t is the simplified equation for final velocity = velocity at the 60 ft + average acceleration over the rest of the quarter times the remaining time.

Oh well, just trying to get people thinking:poke:

Bike is stock motor with a small 20 dry shot and regular pump gas. Its extended to about 65.5 inches. Its my daily commuter bike into Boston.:laugh:
 
+1 Mr Brown

if you accelerate the bike for 9 sec vs 9.25 sec at the end of which run will the bike be going "faster"

not necessarily quicker but faster.. ET and MPH are related but not always like you might think they should..
 
Draco,
How are you? your equation is an interesting idea what we need to figure out is a few secondary things such as coming up with a chart of frictional effects namely wind. What could be developed is a "chart" of constants at the end that are incremental in head wind MPH and if one felt so sasy cross wind with the angle of inclusion, this would relate into the discussion earlier about the effect of wind and MPH. This could all be ballparked with linear accelleration, what I don't have the info for but I believe you have is how linear that curve is (how linear is the engines power with things such as "ram air" with increased MPH), can you plot for us one of your runs with the change in MPH with respect to time, we could start at the time of a know run with your 60' time being the starting point. If you had one with no nitrous that would be a good baseline because as you know and I know you know from a discussion you started as the bottle pressure drops there is not linear power from the N2O. By the way I was hoping to spike your interest and hear back from you about the conversation of bottle pressure, I have an idea that I want to start working on a bit and would run it by you and since you have the ability to data log your bottle presure could make you and your bike a good guinea pig:laugh: If you where intersted out of curiosity sake PM me and let me know your interested I will give you the readers digest version and see what you think, I can tell your inquisitive nature and if nothing may be some food for thought. Talk to you later,
"O"
 
There is a difference in "quick" and "fast".

For example:

Lets say the top speed on my bike is 100MPH.
Lets say the top speed on your bike is 150MPH.
Your bike is FASTER than mine.

Lets say my bike will go from 0 MPH to 100 MPH in 2 seconds.
Lets say your bike will go from 0 MPH to 100 MPH in 2.5 seconds.
My bike is quicker than yours up to 100 MPH.

Hope this helps.
 
Oh well, just trying to get people thinking:poke:

Bike is stock motor with a small 20 dry shot and regular pump gas. Its extended to about 65.5 inches. Its my daily commuter bike into Boston.:laugh:

It was a joke... :laugh:

Enjoy this old article from a car forum...

1) Trap Speed will tell you about your HP to weight.
2) ET will tell you more about traction and your launch.

Of course ET is important to true drag racers, because the winner is the one that gets there first. However, we're not necessarily true drag racers in our attempt to get a power estimate. Honestly, ask 10 guys at the track "What kind of trap speed are you running?" and 8 out of 10 will answer with their ET - to one or two decimal places even. When you say, "No, no, I meant trap speed", they will fumble with a broad estimate with NO decimal places and might even have to pull a time slip out of their pocket to check. Try this question when you're at the track; it's almost funny.

THE DYNAMICS OF TRAP SPEED VS. ET

After running lots of quarter miles, it becomes clear that how well you do in the first 100 feet of the track is KEY to a good time. The last half of the track is KEY to a good speed.

Let's use an example of a stick-shift mini-pickup that on a perfect run, gets a timeslip of 19.50 seconds at 70.00 mph in the quarter.

Imagine that the light turns green, the truck moves two feet and the engine dies for three seconds. After restarting the engine, the driver proceeds to then complete a perfect pass. His time slip would show 22.50 seconds at 69.97 mph. The ET was 3.00 seconds high but the speed was almost unaffected.. why?? It's because his racetrack was 1318 feet long instead of 1320, and in those last two feet this truck usually gains an additional 0.03 mph. However, the clocks recorded the long time. My point? Much of a great ET is made by a great launch.

Now take this truck again, and the driver leaves right on the green light. However, he misses the 3-4 shift when he's at 1250 feet. He coasts for the last 70 feet while trying to find fourth gear. Now instead of accelerating another few mph in this final 70 feet of the track, he decelerates over this distance. His timeslip; 19.51 at 67.83 mph. Note how the et is almost perfect (only off by 0.01 second) but the trap speed is way off (over 2 mph slow)! On a good run, traveling that last 70 feet at an average of 69 mph, would have taken .692 seconds. At a 68 mph avg., that 70 feet takes .682 seconds. That's why his ET only varied by .01 seconds, yet the trap speed was 'way off'. My point here: the end of the track is critical to trap speed; shift rpm, missing a gear... these are the big players.

Hopefully these examples are clear. Neither of these runs are 'perfect' runs, it's just that one has an error at the start, one at the finish and the results are obvious. The start of the track is a big player in the ET, but a small player in the mph. The end of the track is a big player in the mph, but a small player the ET.

So for the casual T-Bricks member who wants to get a HP value, you don't have to buy slicks, or wish you had a limited slip differential. You don't really need to heat the tires in the waterbox, or launch with huge power braking. As long as people get their shift rpm right and don't miss a gear, even a rookie will get the appropriate trap speed for their vehicle.. but honing the perfect ET. requires being rude to a clutch, buying steeper gears or slicks.... hey, we're trying to make this recreational.

OPTIMIZING SPEED

If your goal is to get a good trap speed, what are your options? More power, of course - and less weight is obvious (but it will come out in the power calculations as no increase in power). Shift rpm chosen (auto or manual) and the time it takes you to shift (with a manual) are probably the most important tools you've got. Try different shift points to maximize your trap speed. Reduce rolling resistance by pumping up all tires to their rated pressure. Some people think that running lower pressure might help the traction in the rear, though. Of course more traction will help et, but with most street tires, running street tires within 5 psi of rated pressure will provide you with maximum traction in the first place.


REACTION TIME

The ET clocks don't start until you've actually moved around 8 inches (this is called the rollout)... so don't worry about trying to leave right on the green light. You could wait 5 seconds after the light turned green, and still get a 19.50 timeslip in our truck example above. Your timeslip does show a separate calculated time, the "Reaction Time", which in this case would be 5 seconds. That is the time from the light turning green until you rolled out of the starting zone. It's not a big thing for our discussion here.

THE LAUNCH

For the most part, a decrease in ET is accompanied by an increase in trap speed, but don't go overboard on the launch in your zest to rule the world. Just try to get smartly underway without spinning the tires much at all. Traction levels usually drop a solid 0.10 g when the tires start spinning.

THE HP FORMULA

Here's the formula to use to calculate HP:

Net HP = Weight in pounds* (Speed in MPH/228.4)^3

As an example, Car & Driver tested the 744 Turbo in their June 1990 issue. The car weighed 3,081 lb. without the driver.. the 'race weight' was 3,231 lb. The car ran a 15.7 second quarter at 86 mph. Let's plug it in to the formula:

HP = 3231 * (86/228.4)^3
HP = 172 Net

Volvo rated this at 162 Net. We come out a little high. Or does Volvo underrate a little? I'll say this - I've used this formula for years and that's how the 228.4 was honed - actual experience from cars that had actual power curves - and when I use it on Volvos it tends to always come out a few percent higher than the factory rating. This could simply be that Volvo underrates just a little.

Still, for such a simple formula and such a simple test, it's surprising how accurate this can be. And the best thing is - there's no arguing the numbers on a timeslip. There are always differences between a DynoJet and an Eddy Current Dyno, or G-Tech numbers, but every setup is done by someone different and subject to error. The quarter mile is arguably the best comparison a diversely located group like Turbobricks will ever have. The only real difference to argue about is the altitude of the track! You can compare ET and mph all day and have a good discussion.

HANDY RULE OF THUMB

Once you have a baseline, you should probably use a rule of thumb that each additional 6 HP will give you another mph. That's for a 3200 lb car that runs 88 mph. If you want the real formula for different weights or speeds, here it is:

HP for another mph above "X" speed: = Wt * (((X+1)^3-X^3) / (228.4^3))

For instance a 89 mph quarter vs. an 88 mph quarter for a 3200 lb car:

HP delta = 3200 * ((89^3-88^3) / 228.4^3))

HP delta = 6.3 HP

Once you're going 110 in the quarter, it would take an additional 10 HP to go 111 mph in the 3200 lb car.
 
Second half...

60 FOOT TIME

This is the standard measurement tool to evaluate your launch. It's the time that it took you to travel the first 60 feet of the track. Naturally, patterns emerge again after looking at lots of runs and of course these correlate best to time, not mph. Typically, most everyone's 60' time will be between 14% and 16% of their quarter mile time. If it's under 13% or over 17%, this was not your best pass.

1/8 MILE VS. 1/4 MILE

After monitoring tons of good passes, patterns emerge. Typically, the mph at the quarter is around 1.26 times of the mph at the eighth, and the time at the quarter is around 1.55 times the time at the eighth. You can use these values if you only have a 1/8 mile track and get a real good idea of the theoretical 1/4 mile.

IS MY ET TO SPEED RATIO REASONABLE?

One fact of the quarter mile is; no matter how slow or fast your car is, the mph multiplied by the ET will pretty much be the same number every time. Before the NHRA changed the way that speed is measured in 1989, the product of speed and time was around 1400. Let's calculate some easy examples of this. A 14.00 et usually resulted in a trap speed very near 100 mph. A 10.00 et meant around 140 mph. A 200 mph pass usually takes around 7.00 seconds. These are still good rules of thumb to remember, but now the product is more like 1380 for us - The example from Car and Driver above comes out at 1350. (The reason for this shift is explained below). Remember, most everyone focuses on ET so much that they'll even optimize a car for slower mph if it gets them a better ET. (Rear end gearing is one way to do this). Those guys tend to have a product closer to 1300.

RESPECT MORE SPEED - A LOT. EVEN 3 MPH.

If you look at the formula again, you'll note how trap speed shows up as the cube root of power to weight. That's critical to understanding how fast one car is over another. Let's say your car does a 90 mph quarter and the guy who raced you in the other lane ran 71 mph. After the race, he wanders over to you to say the 'race was close'. Your reply: "I could have towed you and still smote you". (This might not be the best way to make friends, but yes, it is TRUE if the cars weigh the same.)

Do the math. (90/71) cubed is 2.04. Yes, the 90 mph car has 2.04 times the power to weight of the slower car. It has 2.04 times the acceleration of the slower car. It's just that the track is a fixed length, and in accelerating to higher speeds, you use up the track quicker. You accelerated to 90 in about 20% less time than he had to accelerate to 71, right?

Bottom line; Down where most of us run, a 3 mph difference between two cars is NOT a race. It was a clear win. There's a full 10% difference between these cars.

SOME MAGAZINES SHOW THE CONSTANT AS 230.5 OR 234.0. WHERE DID YOU GET 228.4?

Some people try to correct to different things. Like Gross HP instead of Net. But most commonly, these other constants that you'll see in magazines were originally published before 1989 when the NHRA changed their lights, and the 'new' journalist doesn't realize the formula should change accordingly. Here's what I mean; previous to 1989, there were three timing lights at the end of the track; one AT the end of the quarter mile, and one 66 feet before, and one 66 feet after. The middle light was used to calculate the et of the run, and the time to travel the 132 feet at the end of the track was used to calculate the trap speed. This gave the average speed at the end of the track, but you can see what this lead to. Most of the racers stayed on the gas for an additional 66 feet past the quarter to get a consistent speed to evaluate their setup. The track's 'shut down area' of course is a fixed length, but the pro racers were starting to hit 300 mph plus by the end. In an attempt to get these guys off the gas 66 feet earlier and 'make' the cars appear slower, the NHRA stopped using the last light around August of 1989. Today, the trap speed is calculated between the light at the quarter mile and the one 66 feet before. So any timeslip after 1989 is really giving the average speed 33 feet from the finish, which is pretty close to one percent slower than before. The old constant of 230.5 became 228.4 to compensate.

CORRECTING FOR ALTITUDE

If we were dealing with non-turbo cars, this would be easy and we'd publish a formula. But with pressurized cars, the correction factor for altitude depends on the boost you run.

For instance, Sea Level air pressure is 14.7 psi. If you go to a track in Boise, Idaho (2850 feet above sea level) the air pressure is now around 13.25 psi. That's 90.1% of sea level pressure. If the temperature doesn't change and you have an normally aspirated car, your power output will now be 90.1% of what it used to be, so I'd tell you to correct by multiplying your calculated HP by an extra 10.9% (1/.901, or 1.109).

However, (and this is the beauty of turbo cars!!) Let's say you were running 10 psi of boost in the first place. So at sea level, your car was really getting 24.7 psi (14.7 + 10). Now you leave the wastegate at 10 psi and race at Boise. Your manifold pressure is now 23.25 psi (13.25 + 10). Note that YOUR power isn't down as much.. it's down to 94.1% of what it is at sea level. So you should correct with an extra 6.2% (1/.941, or 1.062).

If you wish to calculate your own correction factor, here is a handy table of elevation (feet above sea level) vs. standard day atmospheric pressure (psi):

0 14.70
500 14.43
1000 14.18
1500 13.92
2000 13.67
2500 13.42
3000 13.17
3500 12.92
4000 12.69
4500 12.45
5000 12.23
5500 12.00
6000 11.78
6500 11.56
7000 11.34
7500 11.13
8000 10.91
8500 10.71
9000 10.51
9500 10.30
10000 10.11

Yes, the detail oriented will notice that I'm ignoring lots of small effects of higher pressure ratios in the compressor, lower density air across the intercooler and even the fact that there's less wind drag at higher altitudes, and they're right. However, the overall concepts above still hold true.

You'll have to play with the numbers a little... :thumbsup:
 
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