RR hub vs wheel
Discussion
Hi,
In some other forums I keep seeing people stating that power measure at the wheels on a rolling road should be different to that measured at the hubs, I wanted to know are they talking poo? As far as I am concerned power at the wheel and hub should be the same
Am I right in thinking this or does the power reading really change from hub to wheel? I understand it will at the fly wheel... Any elaboration would be excellent
In some other forums I keep seeing people stating that power measure at the wheels on a rolling road should be different to that measured at the hubs, I wanted to know are they talking poo? As far as I am concerned power at the wheel and hub should be the same
Am I right in thinking this or does the power reading really change from hub to wheel? I understand it will at the fly wheel... Any elaboration would be excellent
Edited by RKDE on Tuesday 27th October 16:01
This case is similar with mine about ideal size of wheel that gives best grip without too much lessening comfort because tyres change to the smaller ones. This might be something related to power optimization on RR hub is transferred to the wheel. According to the resource, plusing one will give the best result, means if the original rims are 15", change it to be 16" would be the best choice.
What needs to be considered here as most readers are looking for some correlation of wheel HP to engine HP.
The following quoted phrases are from 2 recent articles about chassis dyno testing.
From Car and Driver
"There's no really accurate way to get engine horsepower from a chassis dyno," says Matt Harwood, marketing coordinator for Mustang Dynamometer, a major supplier of chassis dynos in Twinsburg, Ohio. Some tuners use the so-called 15/20 rule, which assumes a 15-percent driveline loss for manual transmissions and 20 percent for automatics. But, says Harwood, "I've seen losses as high as 35 percent." So unless it's printed in the brochure or was measured on a true engine dyno or by a tuner with tons of experience with your particular brand of car, any flywheel horsepower number quoted by a hot rodder under the shade tree is most likely just a calculated guess."
And from Hotrod Magazine:
The Truth: Are Chassis Dynos Legit?
What you need to know about getting your car tested
"There is really no way to measure, predict, or otherwise determine engine flywheel power from a chassis-dyno test with any repeatable certainly."
I would like to add this:
A major misconception is that flywheel HP and flywheel torque can be measured from a chassis dyno. The ONLY thing a chassis dyno measures is wheel torque and engine RPM that is generated while the vehicle is either accelerated under full throttle in one gear (inertia dyno) or the dyno loads the drive wheels (or axles) while trying to maintain a constant RPM (load dyno). HP is calculated from torque measurement at various RPM increments and the two are presented in a graphical format.
There are many things between the engine and the drive wheels that will also cause a loss and this is referred to as "driveline loss". Most people try to back into flywheel torque and HP by wheel torque measurement and then "convert" to wheel HP and then add in an assumed driveline loss. The driveline loss was usually expressed in terms of percentage loss, which is absolutely incorrect!
Many if not all dyno shops use a % of loss for the drivetrain that can vary between 15% to 25%. This type of calculation can be very misleading and produce crank HP numbers that are not correct!. I recently saw a person who had done a dyno test of his ’91 M5 and he reportedly go 245 HP at the wheels. He then backed into crank HP by saying that a stock M5 should have 310 HP at the crank so his powertrain loss was 21%. He then went on to rebuild the engine that increased the displacement (stroker) to 3.9L. and did another dyno test he got 365 at the wheels. Using his logic and 21% drivetrain loss he reported that he had 462 HP at the crank.
Now why would he have a loss of 65 HP in the drivetrain with the stock engine and a loss of 97 HP after he rebuilt the engine. Same transmission, same driveshaft, same differential, same brakes, same tires. If he were correct in the first case of a loss of 65 HP in the drivetrain (310HP crank HP to 245 wheel HP) then he would have the same HP loss after installing the stroke engine. Measured wheel HP of 365 plus drivetrain loss of 65 HP equals 430 crank HP. Big difference between 430 HP and 465 HP.
One thing that can be said, wheel HP is what is measured and that is the number that should be fairly close to reality, any other number using a calculation method is only a guess. Using % loss for drivetrain is NOT an accurate method! Based on dyno testing that I have done and witnessed, firstly using an engine dyno and then a wheel dyno with the same engine with a known and measured crank HP showed that most BMW big car powertrain loss will be in the 50 to 70 HP loss range. Why the variance? Manuals have less loss. Gear ratios effect loss to some degree albeit small. Lubricant have an effect, thicker lubricants have higher loss, colder lubricant have higher loss, brake drag varies to a small degree, if a wheel dyno rather then a hub dyno is used a slight bit of tire friction will cause a small additional loss etc.
To equate this to a reality:
First scenario:
A stock 1991 850Ci 6 speed is on the chassis dyno and it measures torque that calculates out to 234 HP at the wheels. If we were to take BMW published engine HP numbers (flywheel HP) that would be 300.
OK, 300 into the driveline and 234 out of the driveline would equal 78% output or the driveline would loss about 22%.
Second scenario:
The engine is replaced in the same vehicle with a 1994 850 CSI stock engine and then dyno’d. The wheel numbers measure out to 315 rear wheel HP. Using the above driveline loss of 22% would calculate out to 404 flywheel HP, right? That is what everyone does! Must be right!
Totally wrong! IF of loss is calculated in terms of percentage, in the first case the loss is 65 HP, in the second case the loss is 89 HP. Same car just a different engine. The driveline wasn’t change at all, why would the driveline loss when expressed in HP go up just be cause we changed the engine?
It doesn’t, This is where the mistake is made. The loss of the driveline is a constant and will not change with an increase or decrease of input HP.
The loss of the 1991 850Ci 6 speed is typically 65 HP, automatics have a little less loss at around 60-62 HP on average.
If, in the second scenario, the measured wheel HP of 315 HP was added to the driveline loss of 65 HP the Flywheel HP estimate would be 380, right where it should be.
Again, when you use a wheel or hub dyno, the only thing you can say for sure is what has been measured at the wheels and that is wheel torque and calculated HP, crank torque and HP is purely an estimate or speculation.
The dyno at 1320MINI measures in WHP then works out drivetrain loss during coast down to find out the flywheel BHP. Jove's car measured the same in WHP at 1320 as it did Thor. Remember this quote: "So unless it's printed in the brochure or was measured on a true engine dyno or by a tuner with tons of experience with your particular brand of car..."... Over 150 MINI's measured at 1320 would be termed experience, especially as it's backed up by Santa Pod MPH figures when the cars are weighed.
The formula has been used since the 60's: hp = (mph / 230)to the power of 3 x weight (lbs)
Hope this clears this up for you Richard.
The following quoted phrases are from 2 recent articles about chassis dyno testing.
From Car and Driver
"There's no really accurate way to get engine horsepower from a chassis dyno," says Matt Harwood, marketing coordinator for Mustang Dynamometer, a major supplier of chassis dynos in Twinsburg, Ohio. Some tuners use the so-called 15/20 rule, which assumes a 15-percent driveline loss for manual transmissions and 20 percent for automatics. But, says Harwood, "I've seen losses as high as 35 percent." So unless it's printed in the brochure or was measured on a true engine dyno or by a tuner with tons of experience with your particular brand of car, any flywheel horsepower number quoted by a hot rodder under the shade tree is most likely just a calculated guess."
And from Hotrod Magazine:
The Truth: Are Chassis Dynos Legit?
What you need to know about getting your car tested
"There is really no way to measure, predict, or otherwise determine engine flywheel power from a chassis-dyno test with any repeatable certainly."
I would like to add this:
A major misconception is that flywheel HP and flywheel torque can be measured from a chassis dyno. The ONLY thing a chassis dyno measures is wheel torque and engine RPM that is generated while the vehicle is either accelerated under full throttle in one gear (inertia dyno) or the dyno loads the drive wheels (or axles) while trying to maintain a constant RPM (load dyno). HP is calculated from torque measurement at various RPM increments and the two are presented in a graphical format.
There are many things between the engine and the drive wheels that will also cause a loss and this is referred to as "driveline loss". Most people try to back into flywheel torque and HP by wheel torque measurement and then "convert" to wheel HP and then add in an assumed driveline loss. The driveline loss was usually expressed in terms of percentage loss, which is absolutely incorrect!
Many if not all dyno shops use a % of loss for the drivetrain that can vary between 15% to 25%. This type of calculation can be very misleading and produce crank HP numbers that are not correct!. I recently saw a person who had done a dyno test of his ’91 M5 and he reportedly go 245 HP at the wheels. He then backed into crank HP by saying that a stock M5 should have 310 HP at the crank so his powertrain loss was 21%. He then went on to rebuild the engine that increased the displacement (stroker) to 3.9L. and did another dyno test he got 365 at the wheels. Using his logic and 21% drivetrain loss he reported that he had 462 HP at the crank.
Now why would he have a loss of 65 HP in the drivetrain with the stock engine and a loss of 97 HP after he rebuilt the engine. Same transmission, same driveshaft, same differential, same brakes, same tires. If he were correct in the first case of a loss of 65 HP in the drivetrain (310HP crank HP to 245 wheel HP) then he would have the same HP loss after installing the stroke engine. Measured wheel HP of 365 plus drivetrain loss of 65 HP equals 430 crank HP. Big difference between 430 HP and 465 HP.
One thing that can be said, wheel HP is what is measured and that is the number that should be fairly close to reality, any other number using a calculation method is only a guess. Using % loss for drivetrain is NOT an accurate method! Based on dyno testing that I have done and witnessed, firstly using an engine dyno and then a wheel dyno with the same engine with a known and measured crank HP showed that most BMW big car powertrain loss will be in the 50 to 70 HP loss range. Why the variance? Manuals have less loss. Gear ratios effect loss to some degree albeit small. Lubricant have an effect, thicker lubricants have higher loss, colder lubricant have higher loss, brake drag varies to a small degree, if a wheel dyno rather then a hub dyno is used a slight bit of tire friction will cause a small additional loss etc.
To equate this to a reality:
First scenario:
A stock 1991 850Ci 6 speed is on the chassis dyno and it measures torque that calculates out to 234 HP at the wheels. If we were to take BMW published engine HP numbers (flywheel HP) that would be 300.
OK, 300 into the driveline and 234 out of the driveline would equal 78% output or the driveline would loss about 22%.
Second scenario:
The engine is replaced in the same vehicle with a 1994 850 CSI stock engine and then dyno’d. The wheel numbers measure out to 315 rear wheel HP. Using the above driveline loss of 22% would calculate out to 404 flywheel HP, right? That is what everyone does! Must be right!
Totally wrong! IF of loss is calculated in terms of percentage, in the first case the loss is 65 HP, in the second case the loss is 89 HP. Same car just a different engine. The driveline wasn’t change at all, why would the driveline loss when expressed in HP go up just be cause we changed the engine?
It doesn’t, This is where the mistake is made. The loss of the driveline is a constant and will not change with an increase or decrease of input HP.
The loss of the 1991 850Ci 6 speed is typically 65 HP, automatics have a little less loss at around 60-62 HP on average.
If, in the second scenario, the measured wheel HP of 315 HP was added to the driveline loss of 65 HP the Flywheel HP estimate would be 380, right where it should be.
Again, when you use a wheel or hub dyno, the only thing you can say for sure is what has been measured at the wheels and that is wheel torque and calculated HP, crank torque and HP is purely an estimate or speculation.
The dyno at 1320MINI measures in WHP then works out drivetrain loss during coast down to find out the flywheel BHP. Jove's car measured the same in WHP at 1320 as it did Thor. Remember this quote: "So unless it's printed in the brochure or was measured on a true engine dyno or by a tuner with tons of experience with your particular brand of car..."... Over 150 MINI's measured at 1320 would be termed experience, especially as it's backed up by Santa Pod MPH figures when the cars are weighed.
The formula has been used since the 60's: hp = (mph / 230)to the power of 3 x weight (lbs)
Hope this clears this up for you Richard.
Edited by BigShow on Wednesday 28th October 13:11
Cheers for the response bigshow and I have come to the conclusion that I have worded this wrong.
So if I start over I think you may see what I meant. I am for a starting point not considering an engine dyno just a stock car on a hub dyno vs a wheel dyno. Now if we consider the torque is differnt due to the size of the wheel to the hub thats power X radius of the wheel give or take. Now most dynos do this as part of the calculation. I know you are quite knowledgeable with RR so you will see that the wheel is taken in to account. by taking the wheel out of the equation which the dyno does you then see power at the centre or otherwise known as the hub, dynos in general can then predict power at they fly etc.
Now since a hub dyno has no wheels it just provides the reading given at the hub no power is given/taken away it is shall we say as it is raw data.
So I think that if say dyno X is accounting for the wheel size and the the hub dyno has no wheel then the power reading should be the same?
I come to this conclusion and let me suggest why I think they should be the same.
Power leaving the engine is the same with or without wheels, agreed? now we already know that the car needs to turn the wheels this is an external mass not attached to a hub dyno. We should consider that the dyno is a brake hence the Bhp, we are using a brake to measure the hp of the engine. From the graph we see two lines, one which denotes torque and the other Bhp now I know torque values differ with the wheels on but the bhp is the force required by a rolling road dyno or hub dyno to restrain the torque of the engine.
So surely measured hp should be the same as the rolling road dyno calculates out the wheel to provide a power at the hubs or at least all the ones I have used virtually remove the wheel to show power at the hub. I can use two local companies one a rolling road and the other a hub dyno and I see the same power, I see the same power because I am in the same car with the same engine and the dynos calculate to show the differences, which is why when I go to the rolling road they take pressures and wheel size.
Does this make sense or is my thinking too far off line? I think I have made this confusing by not making my point clear. Hence the question here,
I state that power is power like a long bar, the torque can be increased if the bar is made longer but the power I am putting in is the same so if we where reading this 1m down a bar we could say that the torque at the end devided by the length is the power put in. If the bar was twice then length and the torque put in is the same the power would show up at then end as a greater force granted but we could take the length and devided it to provide the input which would be the same as the 1m bar. So should we not expect a dyno on a hub or at the wheels to be the same read out.
I understand that the fly and everything between the crank and the hub will effect the results and we should consider that?
Also There is variables such as loss through the tires or pressure or the fact its not strapped down as is required
What do you think? what are your experiences?
So if I start over I think you may see what I meant. I am for a starting point not considering an engine dyno just a stock car on a hub dyno vs a wheel dyno. Now if we consider the torque is differnt due to the size of the wheel to the hub thats power X radius of the wheel give or take. Now most dynos do this as part of the calculation. I know you are quite knowledgeable with RR so you will see that the wheel is taken in to account. by taking the wheel out of the equation which the dyno does you then see power at the centre or otherwise known as the hub, dynos in general can then predict power at they fly etc.
Now since a hub dyno has no wheels it just provides the reading given at the hub no power is given/taken away it is shall we say as it is raw data.
So I think that if say dyno X is accounting for the wheel size and the the hub dyno has no wheel then the power reading should be the same?
I come to this conclusion and let me suggest why I think they should be the same.
Power leaving the engine is the same with or without wheels, agreed? now we already know that the car needs to turn the wheels this is an external mass not attached to a hub dyno. We should consider that the dyno is a brake hence the Bhp, we are using a brake to measure the hp of the engine. From the graph we see two lines, one which denotes torque and the other Bhp now I know torque values differ with the wheels on but the bhp is the force required by a rolling road dyno or hub dyno to restrain the torque of the engine.
So surely measured hp should be the same as the rolling road dyno calculates out the wheel to provide a power at the hubs or at least all the ones I have used virtually remove the wheel to show power at the hub. I can use two local companies one a rolling road and the other a hub dyno and I see the same power, I see the same power because I am in the same car with the same engine and the dynos calculate to show the differences, which is why when I go to the rolling road they take pressures and wheel size.
Does this make sense or is my thinking too far off line? I think I have made this confusing by not making my point clear. Hence the question here,
I state that power is power like a long bar, the torque can be increased if the bar is made longer but the power I am putting in is the same so if we where reading this 1m down a bar we could say that the torque at the end devided by the length is the power put in. If the bar was twice then length and the torque put in is the same the power would show up at then end as a greater force granted but we could take the length and devided it to provide the input which would be the same as the 1m bar. So should we not expect a dyno on a hub or at the wheels to be the same read out.
I understand that the fly and everything between the crank and the hub will effect the results and we should consider that?
Also There is variables such as loss through the tires or pressure or the fact its not strapped down as is required
What do you think? what are your experiences?
Edited by RKDE on Wednesday 28th October 16:34
stevieturbo said:
Really though. Who cares ?
Its a tool for tuning. As long as it can give before and after whilst tuning, thats all it needs to do.
Spot on..Its a tool for tuning. As long as it can give before and after whilst tuning, thats all it needs to do.
Everyone really should take any figures with a pinch of salt.
As Stevie says - what is important is the before and after figures.. assuming someone is tuning (changing the map, changing components etc) on your car.
why do people try and use Chassis dynos to figure out engine BHP?? its like useing a kitchen nife to try and cut down a tree! lol
one would assume rolling roads and great at telling you what power the 'car' is putting down AT THE WHEELS!!
one would assume a hub dyno are great at telling you what power the 'car' is making AT THE HUBS!!
one would assume an engine dyno are great at telling you what power the 'engine' is making!!
Chris.
one would assume rolling roads and great at telling you what power the 'car' is putting down AT THE WHEELS!!
one would assume a hub dyno are great at telling you what power the 'car' is making AT THE HUBS!!
one would assume an engine dyno are great at telling you what power the 'engine' is making!!
Chris.
I go with Stevie. It's a tuning tool and it's key purpose is for you to ascertain what is going on with the engine.
Keep your tyre pressures the same and use the same tyre/wheel combination.
I saw an engine dyno going for about £150 a few years ago and it would have been nice in the shed, however, it's only something to set the thing up on.
I'd almost say that the output is irrelevant, you want an engine brake so that you can optimise your settings. The fancy graph is only pub-w
k material IMO.
Keep your tyre pressures the same and use the same tyre/wheel combination.
I saw an engine dyno going for about £150 a few years ago and it would have been nice in the shed, however, it's only something to set the thing up on.
I'd almost say that the output is irrelevant, you want an engine brake so that you can optimise your settings. The fancy graph is only pub-w
k material IMO. Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff