Can a car with lighter wheels resulting in a higher hp and/or torque on a dyno?

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In a short word no. But a lighter wheel and tire will accelerate and stop faster. It takes more energy to start and stop a heaver wheel and since the HP and torque of the car is fixed (at least during the test) it will take longer to bring it up to maximum speed and longer to stop it. The heaver wheel also stores more energy when it's in motion which is why it takes longer to stop it. Yikes I sound like an enginner don't I?..................mgg

 

I've thought about this one for awhile. Unfortunately I dont know much about how a dyno works.

However, that doesnt mean I wont b.s. for awhile

Ok I've heard about 2 types dynos 1) uses a set of heavy rollers 2) uses rollers with resistence (magneto? friction braking? I dunno). However in either case, what it seems to me is that what the dyno actually measures is the rotation of the rollers (angular velocity), or more importantly the change in rotation of the rollers (angular acceleration).

That being said, it makes sense that whatever you can do to make your car accelerate the rollers faster would result in higher HP/Torque readings. Since the rollers are in contact with your tires, what you really need to do is make your tires accelerate more quickly. Thus, lighter wheels and tires should influence the dyno results. q.e.d.

Ultimately, the true test is, do you accelerate faster on the street? I think all here would agree that with lighter wheels/tires, you will. All else being equal, this difference in accel would have to show up on the dyno in the form of hp/torque increase. (EDIT- I've already thought of a reason why this last comment is bogus but I'll stand behind my earlier statement (way waaay behind))

ok well, thats my b.s. for the day. I'm sure someone more knowledgable will call me on it.

Chuck

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Motorin' ... right now!

 

With lighter wheels/tires, you definitely have more HP/TQ at the wheels since the lighter weight allows for a more efficient transfer of power from the crank, so, I would say that a dyno would indeed show more HP/tq at the wheels.



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SSR Comps 16X6.5 With Hoosier A3S04's(80lbs less than stock!)
Koni SA's (firm rear, soft front)
SSR Comps 16X6.5 with 215/45 Azenis
Alta Exhaust (20lbs),
Nology Hotwires,
Denso Irridium plugs,
Unifilter, 2004 SCCA Solo II GS #85 Mini Cooper S

 

>>Can a car with lighter wheels resulting in a higher hp and/or torque on a dyno?

The dyno will measure a car's power in HP and torque under simulated conditions. Given that level of power you may go faster (reduce track or autocross times) if you have the right suspension, tires, or skill to drive.

Any lightness you can have from the car in general and especially rotational weight loss will help with acceleration and deceleration. It will not help you with constant velocity or top speed.

Read this old thread-
https://www.northamericanmotoring.co...mp;topic=19082

I mentioned:
From Car and driver's article on the John Cooper works kit in an MCS:
http://www.caranddriver.com/article....rticle_id=7279
"The last Cooper S we tested (July 2002) took seven seconds to reach 60 mph. The JCW package drops that time to 6.4 seconds. Our stock Cooper S ran the quarter-mile in 15.5 seconds at 90 mph. The JCW car turned the quarter in 14.7 seconds at 95 mph. "

That means comparing the JCW MCS to the stock MCS the additional 37 HP and 22 lb-ft of torque results in lowering the quarter mile time by 0.8 seconds. Therefore for every 4.6 HP there is a 0.1 second drop in the quarter mile.
And for every 25 pounds of rotational weight lost that would roughly equal a 4.6 HP gain assuming the benefits are linear as HP is increased.
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Although you cannot measure that on a dyno, you can see a drop in 0-60 times with lighter wheels.
Most of the gains are in the lower speeds from 0-70 mph as air resistance starts to play a major role at higher speeds.

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Aloha Hawaii

 

>>And for every 25 pounds of rotational weight lost that would roughly equal a 4.6 HP gain assuming the benefits are linear as HP is increased.
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Could you explain how you came up with these numbers?

 

here are enough numbers to hurt your head. the physics student's numbers linked in this thread are also correct

http://66.220.28.159/forum/showthrea...269&page=1

flyboy2160

 

>>>>And for every 25 pounds of rotational weight lost that would roughly equal a 4.6 HP gain assuming the benefits are linear as HP is increased.
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It's just a rough estimate.

>>Could you explain how you came up with these numbers?
As I already mentioned:
Read this old thread-
https://www.northamericanmotoring.co...mp;topic=19082
There is more explanation there.

 

>>Can a car with lighter wheels resulting in a higher hp and/or torque on a dyno?
>>
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Dave,

Yes it will change your readings on a dyno. Gear ratios will change your number as well.

 

Thanks for the info!

 

If you are using a Mustang dyno, it wont matter. It measures HP without the wheels on the car.
In the real world, lighter wheels and tires = faster.

 

Dynos measure torque at a FIXED RPM; the operator accelerates the engine to 3000 RPM (for instance) and holds it there at full throttle by applying a load to the dyno rollers. Therefore mass of the wheels or the car doesn't come into it.

It is only when measuring acceleration on the road where wheel mass and car mass come into play.

Cheers,
Dave

 

>>Dynos measure torque at a FIXED RPM; the operator accelerates the engine to 3000 RPM (for instance) and holds it there at full throttle by applying a load to the dyno rollers. Therefore mass of the wheels or the car doesn't come into it.
>>
>>It is only when measuring acceleration on the road where wheel mass and car mass come into play.
>>
>>Cheers,
>>Dave


Wheel mass & car mass can't be grouped together like that since the wheels are a rotating mass and the car is a static mass. On a roller-type dyno, a certain amount of power is being used to rotate the wheels which in turn are driving the power-measuing rollers.

Heavier wheels require more power to be turned, so less power will be read by the rollers.

 

Sports Car Revolution on Speed Channel dynoed their Project RSX stock and after the brake upgrade. The brakes were lighter but larger, and they LOST 5 hp. In the words of Linda Richman 'Discuss amongst yourselves'

 

>> The brakes were lighter but larger, and they LOST 5 hp. In the words of Linda Richman 'Discuss amongst yourselves'

for acceleration considerations, NOT handling considerations, what matters is NOT the weight, but the rotational inertia. a lighter overall disk COULD have a larger rotational inertia if its diameter were larger. the rotational inertia is very sensitive to the radius of the mass, since the inertia varies with the square of the radius.

for example, if 2 disks weighed the same, but one was 9" in diameter and one was 13" in diameter, the larger one has (13/9)^2 = 2.09!!!!!! times the rotational inertia.

so even if the 13" were lighter, it could still have more rotaional inertia than the smaller but heavier disk.

ain't no free lunches in physics,
flyboy2160

edit: above is a little too simplistic. it should read "NOT just the weight, but also the rotational inertia...) and this should be added
"the improvement in straight ahead accel. due to the lower weight could be offset by its increased rotational inertia."

 

Dynos don't measure horsepower. Horsepower is calculated from the torque, which is the ONLY thing a dynamometer can measure. Go ahead...make my day and argue with me
The other small factor you need to figure into top speed or quarter mile speed is that AIR resistance CUBES with speed........so in the end.......the only thing that matters is torque and whether you have it down low (acceleration) or at the top (ability to overcome wind resistance at high speeds).......torque.......can't live without it.......

 

>>Dynos don't measure horsepower. Horsepower is calculated from the torque, which is the ONLY thing a dynamometer can measure. Go ahead...make my day and argue with me

OK, I'll argue with you.

you say "Horsepower is calculated from the torque, which is the ONLY thing a dynamometer can measure."

From that, I think you mean that a dynamometer ONLY ONLY ONLY measures torque.

Right?

Am I reading that correctly?

And you want to argue about it right?

Just checking. Maybe one more time huh? for the lay people in the audience who might be temted to take your bullying seriously.

ONLY torque. Check..

Well, you are 100% wrong.

WHAT!!! I claim PRO is wrong!!! How could that be!!!

Well, it also measures RPM. Without that, you cannot ever ever ever compute horsepower.

Sorry, as you are so fond of saying, "More homework for you!" See the RPM in the equation?

(To everyone not familiar with PROs rants on off topic: Politics, I am REALLY yankin' his chain. He's really a sweet guy. I'm just copying his rant-style (And not doing very well at it either) so this is just like a teasing-joke not a personal attack)

>>The other small factor you need to figure into top speed or quarter mile speed is that AIR resistance CUBES with speed.

Oh NO, you got this one wrong also!!!

It is not velocity CUBED, but velocity SQUARED!!! Ah hahahaha.

Ref

Man, batting ZERO for two in one bullheaded post!

 

a few wacky comments in this thread.

The Mustang dyno I have used is driven by the wheels of the car on its rollers.

On the Dynojet as well.

Both work by putting the car in 4th gear, then running it through its rpm range to redline against the dyno load, then stopping. That constitutes a single pull.

One ingredient generally missing is accurate simulation of airflow through the intercooler at typical road speeds, which can substantially contribute more hp; therefore the dyno readings are skewed low compared to road reality

All chassis dynos measure at the wheels, giving a lower figure than the engine crank hp, which is fine, and more appropriate anyway. My personal gripe is that many dynos will also print out a "corrected" reading which is higher. The problem being that most owners don't realize this and confuse their hp claims, wheel and crank. Dyno computers also make corrections for ambient conditons like temperature and barometric pressure; in the US, the SAE correction will give a slightly lower reading than the European DIN correction and both are lower than Standard, no correction. On a recent dynojet run, for example, my car read 212whp SAE, 216whp DIN and 218whp, "Standard", all the same run, just different readouts by pressing a funtion key.

I have seen Helix dyno maybe 10 or 15 minis, stock and otherwise, but have never observed differences in baseline due to wheel configuration, even though some have the pig iron 50lbs stockers and some 35lb lightweights. Eric, care to comment?

 

To clarify a few things. Anything that is involved in the rotational mass of the vehicle will affect the dyno readings. The mass of the vehicle or other non-moving components will not affect the reading. It is not only the weight of the rotational masses but also their diameter. It takes different amounts of power to rotate two objects with the same mass, but different diameters with different weight distributions. This means everything from weights of valves, driveshaft, crankcase oil, wheels, tires, some brake components...... Now granted we may not see much fluctuation between various components sometimes. Why?
Because the dyno's themselves fluctuate sometimes 5 or 6 hp between runs. With that much fluctuation, you will probably not see a difference between a few pounds on the wheels, because they may not even make that much of a difference.

This is true for all chasis dyno's (i.e. Mustang, dynojet...) since the chasis dyno's are measuring torque based on the contact of rollers to the wheels. The only dyno that is not affected by this is an engine dyno, which requires the removal of the engine, which I don't think anyone on this board is using after each of their upgrades.

As jlm said, airflow is critical to measuring accurate results. Leaving a hood open with a fan blowing on it, is not an accurate way of measuring your power on a dyno. The gas flow dynamics are not being recreated as they would on the road. Depending on how you flow air on a car can wildly change the horsepower readings on a car. The key is to simulate normal driving situations as close as possible. I have not seen any public dyno equipment housed in a wind tunnel, so we as the general public may never get true results, but we should still strive for as close as accurate as possible.

Also along the same lines as jlm states. Corrected results don't mean as much as some people are placing stock in the numbers. Temperature and pressure cannot be fully calculated out and equalized. Again, why? Because cars are affected differently by these two components. For example, cars with forced induction and intercoolers are affected much more by temperature and pressure than a similiar normally aspirated engine. These factors cannot be calculated out, because the dyno is using a generic set of tables to equalize the numbers out.

 

Smaller wheel and tire, in mho will show a few more hp on the dyno, but all that is happening is that you've decreased the % change between crank and wheel hp...just like adding a lightwheight fly wheel.

 

The original question has basically been answered by the last three posts, here are a couple of comments.

Dyno shops are not equipped to replicate the variable wind speed of an on-street acceleration run. Therefore we have to simulate, as closely as possible, the conditions that the car sees on the road. We have found that intake air temperature (in particular peak intake temp), and not air flow, is the most critical simulator of road horsepower. We do on-street datalogging for various atmospheric conditions, and then replicate those conditions in the shop. Typically, we have to open the hood and move air across the intercooler to match engine temps (ECT, IAT, PEAK IAT) to road conditions. Before we did this (read: the first 150-200 MINI runs we did), we got skewed results. Early on, I was probably posting dyno results that were not accurate. We have done close to 600 runs on MINIs now, and I feel that we have a pretty good system.

The Mustang dyno is a load type dyno, meaning that it puts a variable load on the wheels depending on the weight of the car, and horsepower of the motor. This more accurately simulates road conditions, car inertia, and aerodynamic drag. Dynojets can't do that.

We never set the computer to automatically "correct" for atmospheric conditions. That adds another variable which can confuse the customer. If we are testing the output of a specific part, we will do before and after dyno runs in similar conditions, on the same car. If the conditions are significantly different on between the runs, we can correct manually afterwards.

When we are nailing our temps, i.e. getting good consistency run-to-run, our HP and TQ numbers are amazingly consistent: plus/minus .5 hp.

We have never isolated just a wheel change on the dyno, so I can't confirm the theoretically correct assumption that lighter wheels would make slightly higher numbers on the dyno.

I've said it before, and I'll say it again: dynos are excellent testing devices and poor advertising devices. There are so many ways for a dyno technician to show a horspower gain that really isn't there. Even here at Helix, we have had individuals bring parts in for dyno testing that have made no power whatsoever, and then see them posts false claims on the web. The bottom line is caveat emptor. If guys like me are making dyno claims for parts they are selling, take it with a grain of salt.

 

Inertia dynos such as a Dynojet do NOT measure torque. A dynojet operates by recording the speed of a set of rollers of known mass as they are being accelerated by the vehicle.

The kinetic energy is calculated from the speed of the rollers and the vehicle's mass
The change in kinetic energy over the change in time is power
Power is power

The dyno calculates torque by plugging the RPM into the Tq = HP * 5252 / RPM equation

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Load dynos such as Mustang DO measure torque by applying a known load from either a water brake or electric motor to oppose the efforts of the engine to twist the rollers. Most load dynos can also operate in "intertia dyno mode" without applying a counteractive force on the rollers.

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There is no way of measuring power or torque at the crank without removing the engine and testing it on an engine dyno. For some reason, European dyno shops like to do this by measuring how fast their rollers slow down after the car stops accelerating. This is completely bogus and should not be taken seriously by anyone.

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As to the original question, yes wheel and tire mass and distribution of that mass will affect acceleration and therefore results from an inertia dyno. The greatest effect is seen in 1st gear, followed by 2nd gear, etc. This test showed the difference was pretty small even with huge differences in wheel weights:

https://www.northamericanmotoring.co...mp;topic=24982

 

Erik,

You are right on the purpose of dyno's. The best testing is done under exactly the same conditions, or as close as you can get to them. Compensating for temperature, pressure... differences by a calculation is not as accurate as performing the tests in a similiar environment. Also, the effects of wheel size, weights.... are not theoretical. I could sit here and calculate out all the effects with real numbers. Unfortunately I don't have the time.

Andy,

The only difference I have with your statements is that the effects of weights, wheel size... affect the readings on either an inertial or a load dyno. You statement makes it seem that it only affects an inertia dyno.

Again, a lot of these effects may or may not be measureable on the dyno, because the accuracy of the dyno is variable based on a lot of conditions, so the small numbers may barely register a difference.

 

If a load dyno is used in pure load mode where it holds the roller speed constant, the mass of wheels and tires won't matter at all. If it is used in partial or full inertia mode, the mass of the wheels and tires may make a difference.

 

>>If a load dyno is used in pure load mode where it holds the roller speed constant, the mass of wheels and tires won't matter at all. If it is used in partial or full inertia mode, the mass of the wheels and tires may make a difference.

Under the Conservation of Energy Law, energy can neither be created or destroyed, it is only transferred.

Even under the constant speed of load mode, there is acceleration occuring. Since the wheels, tires, driveshaft, flywheel.... are circular, the angular velocity requires acceleration. That means energy is constantly being applied to maintain the angular velocity. Since the distribution of weight across the cylinder (for a lack of better classification), affects the amount of force required to maintain a constant velocity it will affect the amount of force that is transferred to the roller. If two wheels with two different sets of tires with different weights are accelerated and maintained a constant velocity, the wheel with the heavier tire will take more energy to both accelerate and maintain a constant speed, since a constant speed does require acceleration, because energy is lost through friction, heat, sound....

Therefore it will have an affect, albeit a lot less than on a inertial dyno.