Build a rolling road (Inertia Dynamometer)

[ajlovering12345]

Hi.

Here is a post I put on the speadfreaks site about building an Interia type dynamometer

Build you own rolling road

The biggest problem for us speed guys is getting a nice flat place that is long enough to run the car up near its full speed. It takes me ages to get to the track and when I’m there it usually rains. It’s more of a problem if you have an IC engine as you need to make a number of runs to tune the car properly so a decent speed run is a hit and miss affair

To help resolve this I am going to build an inertial dynamometer for the speed car. This is the rolling road type dynamometer. The principle is that it takes a certain amount of HP to accelerate a mass at a given rate. So the HP of the engine is directly linked to how fast and how quickly the back wheels can spin a heavy drum.

What’s needed is drum like roller that you can put the wheels onto and then rev the car and gather data on how fast the drum is rotating and how quickly it changes revs. The problem is “How big do you need the drum?”

This is not an easy question to answer. I want to try to get the car to accelerate like it would in the real world so it should take about 10 seconds to spin the drum up to full speed from start to full throttle. If I’m using a single engine in the car and want to get about 115mph I can calculate the drum to be 350mm wide by 60mm dia and made of steel. Now if I use this roller on a car with 3 engines it only takes 2.5 seconds to get to full speed. I need a roller that’s 80mm dia to get a 10 second run. It will still measure the HP ok but I need to see how the car reacts with the engines at full revs for the whole 10 seconds so need a different roller for a 3 engine car

If the roller is mounted on bearings and the cars back wheels placed on the roller and secured so it doesn’t take off down the road you can then jam the throttle and measure how long it takes to accelerate the roller. What you need is to take hundreds of measurements a second and this can be done with a simple car logging device like the Eagle Tree system. You can then plot a graph to show the HP and any given time and it’s a very useful tool to help tune the engines and adjust the gear change points.

If anybody wants to make an inertia dynamometer I have all the calcs on a spreadsheet so you can work out the roller size for your car

Cheers

[ducati777]

You should also put a variable vane fan on the drum. RC airplanes have tons of options here, then you could dial in wind resistance. Just spinning a drum up won't tell you much.

[ajlovering12345]

Non the contrary. The weight of the drum imparts an intertial resistance and so needs torque to accelerate the drum. This torque times the revs gives you the HP required and so gives you the HP at the wheels.

This is the very same as a rolling road dyno they use on race cars and bikes

Cheers

[wqqdster]

I'm picking up the bearings for mine today....got the steel Monday. A friend is going to turn the ends down for 3/4 inch bearings. I think I'll slip a piece of radiator hose over them to improve the traction between the rollers and tires. I know this will change the overall wieght/leverage equation but as long as it takes a few seconds to accelerate the rollers it'll be a huge help.

[Yaksaddle]

Please post some pictures when you are done! I would love to see it.

In the meantime 1/10 scale electric on-road calculations please:)

[ducati777]

Yea if your only interesting in HP....

Now that you know at 37500 rpm your motor made .357 hp, what does it tell you?

Throwing a fan on it would let you simulate a run, and turn it into a true rolling road, and not just a dyno.

[wqqdster]

Ducati.....the problem I'm trying to solve is getting the engines tuned together in order to be able to even MAKE a speed run. I need that mass resistance to let me see what's happening during acceleration.

The fan/prop added would maybe give you a more accurate idea of what the top speed would be but you'd have to know what the Cd of your car is at that speed and then figure out what size prop would put that same amount of drag on the rollers. Cd is also exponential so small changes in speed make big changes in the Cd. Just putting the car on the ground and running it will give you the top speed.

I built a test stand with a big prop on it and tuned the engines in to max rpm but without the mass resistance they reved up very quickly to that max. However, when they were reinstalled in the car they ran badly, not being tuned to a long pull.

The dyno is more of a comparative tool, anyway. Your example of a .375 HP engine is good in that if you make a change and now you only have .362 HP the car isn't going to go as fast, however fast that is. You can make tuning adjustments to accelerate that mass quicker which is more HP and regardless of what the Cd is, if it makes more horsepower its gonna go faster.

[nordkapp_nutter]

can u send me the spread sheet?

cheers!

[ravoll]

HAs anyone progressed further with this idea?Have been tinkering with dyno designs for R/C cars for about a year now.Have all the hardware and have done all the math.Need asssistance with software if anyone is still interested.