# Kart Dyno Talk

Huh? Wait the dyno was heading with you to the big race?

Seperate event: who’s got the most hp?

Out of curiousity, why would a builder/team choose to bring a dyno to a race? What services would be used?

Yeah figured it would be something fun.

They could use it to validate an engine is performing as expected, test changes between sessions where perhaps track time is scant. Break in after a rebuild. Troubleshoot running issues

It could just as easily open a can of worms though

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So basically a dyno uses weight to create resistance to simulate the stresses of actual driving?

why does a huge weight need to be involved? Other than to make it a pita to take to street races.

Maybe it has to do with the weight being the thing that’s being turned as opposed to axle. Like the weight is a constant of some sort. Perhaps we are measuring the force required to move a weight of a known mass?

“First, we must define what a dynamometer is. Essentially, a dyno is a machine that measures engine torque. Pretty simple, right? Well, it can be. Measuring torque output is done by placing a load on the engine or drivetrain of a vehicle and evaluating how the engine reacts. For engine dynos, the load is often produced by connecting the back of the crankshaft to an external water pump. Chassis dynos—probably what you think of when someone says “dyno”—generate engine load less directly: They place the vehicle’s driven wheels on weighted rollers that resist the tires’ motion via an electrical or a hydraulic system. Whatever the method the dyno setup uses to put stress on the driveline, the goal is to measure torque, and that is when the math comes in.”

It’s not so much the weight for the trailer, but the width of the unit between the roller part plus the flywheel. It’s over 6ft wide. Engine inertia dynos are pretty common in karting and could easily fit in a trailer

It doesn’t have to be, only for inertia dynos where the measurement is done via recording the acceleration of a known mass (Technically Moment Of Inertia) and then doing some math. The MOI is chosen based on the power output and the length that you would like your runs to be. Generally 6-7 seconds is considered decent. Too low and the reading will be off because what you are measuring is closer to the inertia of the engine itself, so it will read artificially high. Longer runs are not really a concern other than heat build up in the engine.

However there are other type of dynos that do not use inertia rather they exert a brake load on the engine/axle and the force is recorded via a strain gauge which essentially gives you a torque reading. This can be done with a hydraulic pump, water brake, regular brake caliper or a retarder\eddy brake of some sort. Each has their advantages\disadvantages.

Worth noting that not all roller dynos are inertia dynos, many are braked with retarders,

/edit wasn’t sure where to put this… so placed it under “equipment and tools” for now since it applies to all kinds of engines.

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Thanks for that! Yeah researching this it appears that some dyno uses water resistance instead.

Something I remember from my sports car racing days is running the motor for the CRX I occasionally drove on a water resistance dyno. One of the members of the local SCCA region built everything from MG Midget motors to sprint car motors and used a water dyno to test them all. IIRC, the sprint car motor pulls could empty a 1500 gal water tank in less than 30 seconds. Those runs sounded like Thor’s Hammer…

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The hydro Dyno proved too complex for James

That’s weird normally the water is recycled, if it can be kept cooled cool enough.

My dyno came as an intertia but I can (and likely will) retro fit with an eddy brake. I’d like to run simulated laps and the controller I bought supports it.

Another thought I had would be to keep some of the inertia and use the brake to simulate air resistance.

What heats the water? Simply the act of moving it fiercely or is it getting heated by exhaust gasses?

If the water is being recycled, all of the energy produced by the engine goes into the water as heat via viscous losses. If the water is being dumped, you lose a bit of energy to the kinetic energy of the water as well as the heat. Think of it this way - if your engine produces 1hp, the different dynos are just different ways to absorb 100% of that energy. Once they absorb the energy, you must dispose of it or store it. Inertial dynos simply store that energy as rotational kinetic energy. Brake-based systems (whether water, hydraulic, eddy, etc) create massive amounts of heat. You can combine systems to either allow finer control (like by adding a brake to an inertial dyno) or make managing heat easier or both.

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Thanks for the explanation.

In this case, there was a stream/ditch behind the shop and the water was pumped into the ditch. He used a well for water supply so the cost to refill the tank was minimal.

I understand what you’re saying, which is why I made the caveat of temps being under control.

Oh, I was referring to Dom’s question. Everything you said was technically sound from what I could tell, and I noticed that specific caveat was very appropriate.

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Yeah I realized that after I typed it

My Dad’s motor builder did the exact same thing. It negated all the heat control issues between runs, but require a little bit of down time between runs to refill the storage tank. Pretty nifty setup. Would be hard to make it portable given the size, but a downsized version for a Kart could be if you had a water source nearby.

I’m gonna have to look this up on the internet to see video of dyno in action.

I’ve seen the traditional one where the vehicle drivetrain turns a roller, I haven’t seen the water ones at work.

They really are not a lot different, just a water brake as the brake. Here’s what a water brake looks like:

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Following…

The dynos connected to a large motor with a resistive load running as a generator are the ones I’m most familiar with. The numbers have to compensated for the load temperature, but that’s pretty straight forward and pretty compact. Run time is infinite as long as you can keep the load in operating range.

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Its a fairly basic concept. The Water Brake is an impeller that essentially pumps the water. The energy required to pump the water is known and the rate measured tells you the energy spent. Water has a fairly constant viscosity throughout most of the temperature in its liquid form so, its better than a non-polar molecule that changes viscosity more with temp.

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