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Who wouldn't want their own hub dyno! Let's see if we can do it on the cheap (<$1000 for the pair)

We want to be able to tune steady state (fixed RPM/RAMP speed) so we need a way to dump 100+ KW of heat and we need to do it on a budget. Eddy brakes and water brakes are out. We're going to use an off the shelf commodore rotor and brake caliper.

Some math (so we're not going into this blind):

Rotor weight: 8.2kgs (VE Commodore) each (18lbs)

Energy required to heat steel (Cast iron needs a little more but we don't know the exact alloy so we'll go with steel) : Watts = 0.05 x Lbs of Steel x ΔT (in °F) / hr

We'll say ambient temp is 30deg and 150deg is as hot as we want to get our rotor (170deg is in the danger area for bearing grease so we'll want to keep it cooler than that). That's a Delta T of 120deg C, or 248deg F.

Plugging the numbers in says we can dump 223watts of energy into the rotor for 1hr until we exceed our maximum rotor temp. Looking at it another way, we can dump 100KW into the rotor for 8.0352 seconds until we hit that same temperature. Using two rotors that gives us 16 seconds of steady state dissipation at 100kw - without cooling.

Real world? a 15 second dyno pull on two hubs on a NA engine would be considered safe at up to around 200kw peak power, without water cooling. A shorter pull, water cooling, larger rotor mass, will all increase the capacity of the dyno. Rotors are actively cooled too which will dump a significant amount of heat. The above math does not take this into account.

Proof of concept? A 1.6L VW motor (40kW) dissipated into a single 4.5KG nissan pulsar rotor. With water cooling, indefinite disspation was possible and the rotor did not exceed 100deg C. The rotor had no issues at 5000rpm though the CV boot expanded like a balloon and caused a bit of vibration. A well aligned rigid shaft would have been preferred.

Next up, designing the dyno!

I like where this is going.
Well thought out and a working model already.

There are a couple of design obstacles - I'm no mechanical engineer but with the help of google I'll take a crack at them.

1) Supporting the weight of the vehicle - bearing selection, bearing loads (radial, axial, torsional) - coupling the car's hub to the dyno's hub.
2) Electronic braking of a hydraulic system
3) Measuring Torque and RPM to calculate HP/KW

To keep it as simple as possible, as much of this dyno should be off the shelf. For that reason, I've decided on these hubs as the heart of the dyno:
1 ton capacity, 120mm PCD stud pattern. 50mm stub, 200mm long. $135 shipped for the pair.

The 50mm stub will hopefully fit nicely into a 50mm self centering pillow block bearing: $98 shipped for the pair. Self centering is important to absorb any misalignment/tow in/out/camber etc. To add braking to the hubs, I'll add a pair of VE rotors and calipers: $50 for the pair from FB marketplace. This is the first problem I may need to fix up on the lathe - I didn't consider the trailer hub diameter and if the rotor would fit over it. If it doesn't, I'll need to turn a few mm off it. Buying a disc braked trailer hub kit ($350) would solve this but the disc was small, not ventilated and used a small caliper. I decided against it for this reason. The stub axle's will arrive in a few days and I'll see if it's a problem or not.

For the pillow block, I figure I'll only need one per stub thanks to the car's hub being able to support itself. Any camber should be compensated in the pillow block.

A cheaper possible solution - You can separate the VE hub from the knuckle - you could possibly make up a plate and shaft to replicate the stub axle, saving $130 on the build and using a perhaps more robust bearing than a chinese trailer bearing. This may be the way ahead if I produce more of these (yup, that's the plan!)

Next up, Hydraulics!

I like the sound of the VE set-up. Those Chinese stubs look cheap.

Hydraulics:

On the engine dyno I used an ebay $15 pitbike rear brake master cylinder: Given the size difference between the caliper cylinder and the master cylinder, there was more than enough mechanical advantage to stall the full 40KW using only hand weight direct on the master input shaft. The VE caliper has two slaves and are larger. Hopefully there is enough volume in the master to actuate it but its not so much a volume thing with the brakes, its nearly all pressure. I'm sure it'll be fine. I have two of these on the way ($40 shipped for the pair).

To get the pressure to the caliper, A $15 10mm 500mm long banjo brake line is used. These were $30 shipped for the pair.

I have been looking into using an ABS pump to generate and regulate the pressure to the calipers. Any forum anecdotes I have found say an ABS pump doesn't generate pressure, it modulates the supplied pressure to the wheel, preventing braking. Looking up the schematics inside these pumps suggests that isn't necessarily the case and thanks to stability control and downhill brake assist, they most certainly do generate their own pressure. See attached:
My only problem with this method is the valves are not PWM controllable, rather on/off only at a max of around 10hz. While this will work, the load cell measuring torque won't see a nice linear analog style load and the resolution of one valve 'click' may be too course of a resolution and bog the engine down. I've grabbed the VE's ABS module to test anyway. This seems like the easiest Electronic to Hydraulic control method I've found. I figure if I can slow the motor down enough, a single valve click may be small enough. Adding check valves and pressure compensated flow control valves will blow the budget.

The backup, and probably the way I'll go is using a linear actuator like this: 600kgs force, 5mm/second may be a little slow but with that kind of force I could add a lever to speed that up, or use a 300kg actuator at 14mm/s. I tested the car this afternoon with the motor off on the highway to remove any vac from the booster to see how much force and travel was needed to pull the car up. It was almost zero travel and all force so I figure 5mm/second will be fine as long as it can translate the force. A pair of these shipped is $76.

'quipt4it wrote:I like the sound of the VE set-up. Those Chinese stubs look cheap.

my thoughts exactly! haha. I had a look under my trailer and looks pretty much exactly like this. I guess the 1t static load would have a huge overhead for dynamic loads (speed bumps, pot holes) so I *fingers crossed* think a steady 500kgs between the pair should be safe. I'll definitely look deeper into the VE option on version 2

Those look exactly like trailer wheel hub assemblies.

Brake rotors get a lot hotter than the numbers you mentioned above, it's the ability to shed heat which allows the rotors to safely get a lot hotter without overheating the bearings. Can always use a high temp grease to increase your thermal load limit too. Even a couple of cheap chinese hairdryers pointed into the centre of the brake rotors to force cool air out via the centre of the ventilated rotors will allow you to dump a lot more heat or increase the load time limit.

immortality wrote:Those look exactly like trailer wheel hub assemblies.

Forgot to say in the post, they are in fact trailer stub axles off eBay.

That's a good idea about the grease too! Thanks! The hotter the rotor, the more effective the ventilated cooling would become so it would give me even more overhead.

I'll mount some IR temp sensors to monitor the rotor temp to be safe. With forced cooling and decent grease like you suggest I wouldn't be surprised if we triple the capacity or at the least permit very long duration steady state loading.

I might try one of those eBay 'electric turbos' as a simple high flow cooling fan

Hub to Hub coupling?!!?

I was thinking laser cut flanges for both hub side (5x120pcd) and vehicle side (4x100pcd), heavy wall pipe to join them. Alignment could be a little tricky but I think possible. Then I went searching and found what the real dyno's use:
They're not readily available so I was thinking...

Why not use disk rotors, flipped, and the discs drilled precisely so they bolt together.

Any thoughts on this? Is there an easier way? Is there a company that'll CNC or index drill rotors?

You would need new rotors so the disk faces are true (or machine old ones). would need a decent machinist as you really want this stuff to all run true.