[_Mark_]

Fair enough Mark - but by your figures - how many hp would it take to shoot a 2200lb dragster 6" from a stop in 0.5 seconds running the gearing they run?

I submit it is measureable - and that maintaining that deflection on the tire on a rolling dyno will pick it up...

Wouldn't prima face' evidence be that others have done just that?

No. In fact, in the other Dyno thread you'll see that David Weaver states to increase pressure to 35 lbs to get rid of any sidewall flex. Flexing that sidewall is sacrificing power to increase traction and reduce the moment arm of the axle around the tire. As the tire's RPM increases, the moment arm can increase.

My point above was half the answer, Dean said the other half.

 

[Mister Bone]

Fair enough Mark - but by your figures - how many hp would it take to shoot a 2200lb dragster 6" from a stop in 0.5 seconds running the gearing they run?

I submit it is measureable - and that maintaining that deflection on the tire on a rolling dyno will pick it up...

Wouldn't prima face' evidence be that others have done just that?

Seeing how they 60 foot in under a second and reach over 100MPH before that mark it is one hell of a lot of power. :nod:

The tq sensors on Doug Kalittas (to rear dif) car has shown approx. 8000 HP. I forgot the tq numbers.

Shit I am straying here... :bonk:

 

[Mister Bone]

If there wasn't a lot of useful energy stored in the tire sidewall, they wouldn't need beadlock screws to keep them from slipping on the rim. You can either do it with screws, or internal pressure. The correct engineering term for this phenomenon is "rimpull."

OK, this is true. I was saying that about stored energy before.

But I got lost in what your point was...sorry but I am about brain dead right now.......too many things going thru my head.......must go back and re-read this.

 

[GTS Dean]

Dean - that makes perfect sense, I wasn't thinking of it in a decreasing-diameter sense,

No Clint, remember it's a decreasing RADIUS issue. The only (easily changeable) distance that affects output torque is the effective loaded radius of the tire.

 

[CS]

By the way Barry, that stored up energy in the tire isn't ever released as power measured on the dyno. That's because as that tire flexes, it remains flexed althrought the run; the power transferred the the wheels increases and increases throughout the run and the tension on the sidewall isn't released until after the power is disengaged at the end of the run.

That stored energy in the sidewall doesn't show up on the dyno.

 

[CS]

Dean - that makes perfect sense, I wasn't thinking of it in a decreasing-diameter sense,

No Clint, remember it's a decreasing RADIUS issue. The only (easily changeable) distance that affects output torque is the effective loaded radius of the tire.

Ahh, good point Dean. /images/graemlins/freak

 

[Mister Bone]

By the way Barry, that stored up energy in the tire isn't ever released as power measured on the dyno. That's because as that tire flexes, it remains flexed althrought the run; the power transferred the the wheels increases and increases throughout the run and the tension on the sidewall isn't released until after the power is disengaged at the end of the run.

That stored energy in the sidewall doesn't show up on the dyno.

Ah but yes it does.

Remember we are not talking race slicks so the amount of wind-up will be very small, but enough to have an effect.

God I wish I could explain this. :bonk:

 

[_Mark_]

Ah but yes it does.

Remember we are not talking race slicks so the amount of wind-up will be very small, but enough to have an effect.

God I wish I could explain this. :bonk:

You can explain it fine. What are you talking about, a Viper on a dyno or what?

 

[Mister Bone]

Ah but yes it does.

Remember we are not talking race slicks so the amount of wind-up will be very small, but enough to have an effect.

God I wish I could explain this. :bonk:

You can explain it fine. What are you talking about, a Viper on a dyno or what?

I forgot wtf I as talking about /images/graemlins/laughing.gif

Oh ok....Tire rim size in general and how it effects dyno readings.

Take a Viper, dyno with 16" wheels and 19" wheels and see which give the higher numbers. Both same OD, width, compound, tread design, and pressure.

 

[_Mark_]

Same OD (and weight) = same reading by laws of physics.

However, I think your point was that due to the greater sidewall flex of the 16" combo, the reading will be different. Assuming you pump up the 16" combo adequately, the flex will be neglibible and the difference will be lost in the noise of any series of dyno pulls. And if you did not pump up the 16" combo, the reading would be less, because energy is wasted in the tire flex instead of being transferred to the drum (there is no traction issue here like with the dragster from rest).

 

[SEric]

Holy crap, in the time it took me to type my post the levee broke on this thread.

To highlight:

A dyno measures TORQUE, not hp -

Changing drive ratio, overall tire diameter, etc, changes the Torque reading at the wheels, and thus the dyno gives a different reading.

Its simple physics people.

Mark, hate to disagree with you but you're wrong on a number of counts here. JPH was correct that the inertial dyno (like a Dynojet) measures the rate of acceleration (change in drum speed between discreet time samplings) of the drum and back-calculates torque from it. Torque is a static property that cannot be measured directly from a rotating device like a dyno drum. So although Hp seems unreal to many of us and we dismiss it as nothing more than a derived entity, in actual fact the dyno only measures power and reports torque as the *mathematical* afterthought.

Changing the drive ratio and tire diameter does have an affect on the force that your tire applies to the ground and hence to the dyno drum but I will reiterate that the dyno does factor this out in it's calculations of flywheel Hp and Tq. You said earlier that it couldn't be doing this because there's no way to enter drive ratio and tire diameter into the dyno program. My response is that you don't need to enter that crap because the dyno has a much more simple and elegant way of factoring out the torque multiplication due to gearing and tire and it does so using the ratio of engine rpm to drum rpm. If your crank were hooked directly to the drum axle with no gearing induced mechanical advantage they would rotate at the same speed, right? If your motor rotates the drum at half the engine's rpm then there's a mechanical advantage (torque multiplier) of 2:1 at work somewhere in the driveline, correct? The dyno doesn't give a shit whether that multiplier is due to tranny, diff and/or tire size. It simply knows to divide it's measured power/Tq numbers by 1/2 to get the flywheel values ... it's really a beautifully elegant system when you realize how it works.

As far as why we put the tranny in 4th (1:1) to run dyno pulls? It's not because 1:1 is necessary for the calculation to work, it's because in direct drive (1:1) you get less power loss through the clusters than you do when it's in some other gear. You could perfectly well do a pull in first gear or in 6th but the "rule of thumb" loss figures that we all apply would be wrong.

 

[Lawboy]

...engineers....

 

[CS]

Same OD (and weight) = same reading by laws of physics.

However, I think your point was that due to the greater sidewall flex of the 16" combo, the reading will be different. Assuming you pump up the 16" combo adequately, the flex will be neglibible and the difference will be lost in the noise of any series of dyno pulls. And if you did not pump up the 16" combo, the reading would be less, because energy is wasted in the tire flex instead of being transferred to the drum (there is no traction issue here like with the dragster from rest).

That's what I'm saying Jerome.

 

[_Mark_]

Holy crap, in the time it took me to type my post the levee broke on this thread.

To highlight:

A dyno measures TORQUE, not hp -

Changing drive ratio, overall tire diameter, etc, changes the Torque reading at the wheels, and thus the dyno gives a different reading.

Its simple physics people.

Mark, hate to disagree with you but you're wrong on a number of counts here. JPH was correct that the inertial dyno (like a Dynojet) measures the rate of acceleration (change in drum speed between discreet time samplings) of the drum and back-calculates torque from it. Torque is a static property that cannot be measured directly from a rotating device like a dyno drum. So although Hp seems unreal to many of us and we dismiss it as nothing more than a derived entity, in actual fact the dyno only measures power and reports torque as the *mathematical* afterthought.

Eric is right. I was under the impression that the dyno drum worked like a torque wrench does, but it does not. It does in fact measure horespower and extrapolate torque using the rpm sensor of the engine.

I stand humbly corrected.

Changing the drive ratio and tire diameter does have an affect on the force that your tire applies to the ground and hence to the dyno drum but I will reiterate that the dyno does factor this out in it's calculations of flywheel Hp and Tq. You said earlier that it couldn't be doing this because there's no way to enter drive ratio and tire diameter into the dyno program. My response is that you don't need to enter that crap because the dyno has a much more simple and elegant way of factoring out the torque multiplication due to gearing and tire and it does so using the ratio of engine rpm to drum rpm. If your crank were hooked directly to the drum axle with no gearing induced mechanical advantage they would rotate at the same speed, right? If your motor rotates the drum at half the engine's rpm then there's a mechanical advantage (torque multiplier) of 2:1 at work somewhere in the driveline, correct? The dyno doesn't give a shit whether that multiplier is due to tranny, diff and/or tire size. It simply knows to divide it's measured power/Tq numbers by 1/2 to get the flywheel values ... it's really a beautifully elegant system when you realize how it works.

As far as why we put the tranny in 4th (1:1) to run dyno pulls? It's not because 1:1 is necessary for the calculation to work, it's because in direct drive (1:1) you get less power loss through the clusters than you do when it's in some other gear. You could perfectly well do a pull in first gear or in 6th but the "rule of thumb" loss figures that we all apply would be wrong.

Changing the ratios of the above (gear ratio, tire size, etc) affects the curve for sure, but I can see that it should not affect the peak readings. However, gear swaps rarely result in similar dyno measurements, but I'm happy to chalk this up to different driveline loss and/or just typical dyno differences between trips.

So I humbly drop trou and offer up my lilly white ass for a public VA caning, and can only add this link with a good writeup of how the whole thing works:

http://www.bristoldyno.com/info/whatis.htm

(I'm stickin to my guns on the drag tires though!)

 

[Mister Bone]

Same OD (and weight) = same reading by laws of physics.

However, I think your point was that due to the greater sidewall flex of the 16" combo, the reading will be different. Assuming you pump up the 16" combo adequately, the flex will be neglibible and the difference will be lost in the noise of any series of dyno pulls. And if you did not pump up the 16" combo, the reading would be less, because energy is wasted in the tire flex instead of being transferred to the drum (there is no traction issue here like with the dragster from rest).

Well I don't have a dyno, rwd car, and two sets of rims/tires so I can't prove this. But I have seen it in person, I try not to talk out my ass, but well at times... /images/graemlins/laughing.gif

But how about I throw another kink into this /images/graemlins/laughing.gif

Rotational mass and its location between the 16's and say 19's again :doh:

Same stats as the above equation.

Discuss

 

[_Mark_]

So in the interest of taking this baby to a 4 pager...

I have no doubt Jerome that you have witnessed it (I was careful to say ("by laws of physics"). Rarely are two different tire sizes on two different rim sizes the same OD. Even the same size tire in two different brands isn't even the same OD.

And rotational mass could certainly affect the reading, as it will affect the change in rotational speed over time.

So yeah, measure with stock gts rims you'd get one reading, measure with lightweight bbs rims, you should get a different reading.

(Whoa, I gotta get to the airport...)

 

[Torquemonster]

Back from lunch - Japanese Sashimi - yummy

I know nobody cares - but what the hell.... BURPP that Jap beer wasn't bad either :cheers:

so what's the story?

Eric's cleared up the mystery of why a dyno doesn't need gear ratios factored in - hey thanks for that - I didn't know that and always wondered.. that was cool and makes perfect sense

what about why 16" tall tires make more power than 19" low profile ones with same overall diameter?

Many theories - some conflicting - all make good points.... the fact remains - higher readings are being recorded for the 16" tires... if the arguments against this happening are true - then what is causing this?

 

[Mister Bone]

I am going to let you guys take it from here. I need to find a better way to say what I need to say. And maybe dig up more info on this.

Too bad my friends shop has an engine dyno cell. No chassis dyno. Getting to the chassis dyno is expensive around here. And I no longer have a rwd car to use.

Carry on....

 

[SEric]

So I humbly drop trou and offer up my lilly white ass for a public VA caning, and can only add this link with a good writeup of how the whole thing works:
(I'm stickin to my guns on the drag tires though!)

You graciously fold in the face of my undeniable rightness, just like that? Turn in your Alley membership card and get out!

What's the first rule of the Alley? Right ... fight to the bitter end. What's the second rule of the Alley? Right again ... fight to the bitter end and if you're wrong, use more insults.

 

[KenH]

Good info, SE's explanation of the dyno finally made it click for me as well. Something that I still don't understand though is that if torque is a simple mathematical derivative of HP, then given the same HP curve for two different cars, will they also have the same torque curve? Seems like they would have to, but doesn't seem like they necessarily do in practice.