A useless dyno question

For some reason I can’t seem to wrap my head around this. Suppose I put a car on a dyno. I attached a chain to the car to keep it from moving and the chain was attached to a scale to measure how hard it was pulling. How accurately would the pressure on the chain correspond to the load put on the dyno??

Look out! It’s the airplane-on-a-treadmill argument returning in disguise! :wink:

If the dyno is working correctly, all the torque generated is transferred to the cylinders, thus no force elsewhere. Any chains, chocks, or other restraining devices are purely for safety.

To use an industry term, “pretty fucking accurately.” The chain is the only thing holding the car back. The only other thing touching the car is the dyno rollers, so they alone must be exerting a force on the car that is exactly equal and opposite to that of the chain.

And a nitpick: you are interested in (and your scale will measure) the force on the chain (expressed in pounds or newtons) rather than pressure (expressed in pounds per square inch or newtons per square meter).

Thats what I thought. We were having a discussion about finding an optimum gear ratio and optimum type of rider to reach a top record breaking speed on a bicycle. The thought crossed my mind that if you could somehow guage how hard the cyclist was pulling you might be able to fine tune a cyclist/cycle combo using a dyno of sorts.

If the car’s tires are sitting between 2 rollers (which seems to be a fairly common design for a dynamometer), the front roller is helping to hold the car back. So the force on the chain would be less than the force registered by the dynamometer.

FYI, there are bicycle power meters that provide real-time measurements of the rider’s power output. I have a PowerTap hub on one of my bikes. There are other designs - some are built into the pedal, some are built into the crank.

So now we have posts asserting zero load on the chain, total load on the chain, and partial load on the chain.

I guess we’ve given the right answer for sure. If only we had a way to know which one it is. :smiley:

I vote for: A properly rigged dyno is absorbing 100% of the output of the car/bike/whatever and there is zero additional force on any chain. If there is any tension force on the chain, the dyno is lying to you.

The chain I believe is what is holding the bike, climing the front roller may take a small amount of the pull excerted.

The straps are most certainly seeing a lot of load. I’ve seen powerful cars rip the straps apart.

If the dyno rolls are pushing forward on the vehicle (as they must be, if the driveline is putting power to the wheel), then something has to be holding the vehicle back, lest it accelerate forward. That something is either HoneyBadgerDC’s chain, scr4’s bracing roller, or some combination of the two. By “properly rigged,” you must mean that the dyno is utilizing some anchoring system other than the HoneybadgerDC’s chain to secure the vehicle - in which case, yes, there won’t be any tension on the chain. My assumption is that the chain itself is the anchoring system.

scr4 is thinking of a configuration like this, in which case, yes, the tension on the chain would be less than the tractive force being applied by the dyno (since the rolls form a cradle that helps keep the vehicle in place).

I am more familiar with this sort of chassis dyno configuration, in which the vehicle is positioned so that each axle is directly above the axle of the corresponding dyno roll - and then the only thing keeping the vehicle from moving is the anchor system. Assuming that anchor system is nothing more than the chain, then the chain tension is exactly equal to the tractive force applied to the wheels by the dyno rollers.

I believe there must be some load on the chain.

Imagine the dyno wheels are stuck. As the wheels try to turn, the car will try to propel forward, and the chains will see maximum tension, as if the car was on pavement.

Now, imagine the car wheels are elevated completely off the ground and in contact with nothing but air. If the wheels spin, there is nothing propelling the car forward, and the chains see no tension. This would be true for a weightless and frictionless dyno roller.

However, dyno rollers are not weightless and frictionless. The car has to overcome the inertia of the rollers, friction of the rollers, as well as the inertia/friction of whatever torque measuring mechanism the rollers are attached to. All of these things make the rollers “less like air” and “more like pavement”. The sum of these “resisting” effects should be equal to the tension on the chain.

No, the dyno is measuring power, not force.

Power is work divided by time.

Work is force divided by distance.

If the force on the chains does not move anything, it is doing no work so it is consuming no power.

The car’s wheels apply a force to the rollers on the dyno. They are turning at a given RPM. The RPM represents distance divided by time (as determined by pi and the diameter of the wheels an/or rollers). By recording the force on the brake on the rollers and the RPM of the rollers, you can determine the power used in turning the rollers.

Of course, for every force, there has to be an equal and opposite counterforce or the car would move. Since the wheels of the car are applying a forward force on the car, there has to be something to apply an equal and opposite force pushing the car back; otherwise the car would move.

Here goes Chassis Dyno 101 - this IS a CHASSIS dynamometer - there are also directly measuring dynos that are attached directly to an engine in a rebuild shop.

Horsepower is a function of the rate (or RPM - engine or tires) of force (or torque) applied to accelerate a load. You could measure force on the chains and the wheel RPM and calculate power but in this condition the car is probably not delivering its true maximum power for a given speed. With sufficient power the tires will slip. This is NOT how a chassis dyno works.

The chains are for safety and pull the car down against the dyno rollers for positive tire engagement and hence do have tension.

There are two type fundmental types of dynos; inertial and load controlled. A load controlled chassis dyno where the the friction to turn the rollers is adjusted is somewhat rare. What is usually seen are inertia dynamometers where the tires are turning a heavy roller - lots of inertia - and the rate by which the roller is accelerated can be translated to the instantaneous power at a given RPM. It’s a scientifically accurate, cheap way to build a dyno but the continuous load model is generally thought of as a better engine tuning tool.

Thank you all. I pulled a stupid here. Time for a nap.

Times, not divided by.

WHAT ? if the net external force on the car, is seen as a force pushing the dyno back… then the car must be experiencing a net external force forward.

Turns out that a small bump up front is enough force to stop it moving anywhere, but if the car was sitting on a frictionless flat surface… it would need a chain.
The tension on the chain, if there was no air resistance, static friction in the front wheels, and the floor was perfectly flat, etc, would be the exactly the force on the dyno’s drum … and power is force * speed, so that would give the power result we want.

BUT, the car won’t sit flat, as it will move on the suspension, causing the chain tension to change, the friction of the front end is even higher when its stationary, as static friction is higher than dynamic, and the floor isn’t perfectly flat… So the chain tension isn’t very stable or accurate, in fact terribly inaccurate as the vehicle would be wobbling forward and backward, sending spikes of tension into the chain … (hugely higher force than the tyre is putting on the dyno … ), as F= = mA… when the car pulls the chain tight, the attempt to stop the car instantaneously would send a high force through the chain … of course depending on the flexibility of the chain mounting points and suspension and car body and so on. Too many variables, the chain is an inaccurate place to measure force and hence derive power.

I vote for this. There should be no force at all on the chain (and no need for it) since the rollers should absorbing all the power from the wheels.

Hold on. Isn’t this basically the same question as the plane on the treadmill, if the plane was being propelled by the wheels instead of the propeller?

If the dyno rollers spin freely from the tire rotation (and we discount friction), what force is left to act on the chain?

The rollers don’t spin freely. They are called “brake dynamometers” for a reason. If they spun freely, they couldn’t measure torque and would be pretty useless.

For those who think the valley between the two driven rollers is somehow relevant, the existence of single roller dynamometers should be enlightening.