Automotive Vehicle Parking Brake Test on a Slope [How Convert to a Flat Ground test?]

[physicsnoob204]

TL;DR Summary

Slope Test to Flat Ground Test

Vehicle Mass : 3 tonnes

Manufacturer's instruction is to test vehicle's parking brake effectiveness is to engage parking brake on 30.96 degrees slope. If it holds, it is effective.

I do not have 30.96 degrees slope & I want to replicate the same test on flat ground. I was thinking of accelerating to a X rpm with my parking brake on.

Based on my free body diagram, the Parking Brake Force on 30.96 degrees slope is the same as the Acceleration Force on Flat Ground and is calculated to be 15140N. [Fg sin (30.96)]

But I can't correlate RPM and Force.. and I'm not sure what other variables are needed (Torque? HP?)

*Assuming no friction/resistance

 

[Baluncore]

Welcome to PF.

Tan(30.96°) = 0.6000 which is a rise of 6 in 10.

Have you considered making a set of short vehicle ramps, each with that 6 in 10 slope.
You might be able to place the ramps, then drive up the slope and park.
Alternatively, park the vehicle on the flat, jack it up and put a short ramp under each wheel until all wheels are supported on that slope.

There is a complication, the weight distribution to the various axles is determined by the slope and the centre of mass of the vehicle.
That may not be a problem with a dynamic stop where a pendulum hangs with the 6 in 10 slope during the deceleration. Forward or reverse stop?

Does the parking brake need to operate when the vehicle is facing uphill or downhill?

 

[physicsnoob204]

Welcome to PF.

Tan(30.96°) = 0.6000 which is a rise of 6 in 10.

Have you considered making a set of short vehicle ramps, each with that 6 in 10 slope.
You might be able to place the ramps, then drive up the slope and park.
Alternatively, park the vehicle on the flat, jack it up and put a short ramp under each wheel until all wheels are supported on that slope.

There is a complication, the weight distribution to the various axles is determined by the slope and the centre of mass of the vehicle.
That may not be a problem with a dynamic stop where a pendulum hangs with the 6 in 10 slope during the deceleration. Forward or reverse stop?

Does the parking brake need to operate when the vehicle is facing uphill or downhill?

Thanks for the prompt reply! Yes it is a 60% slope.

Unfortunately, our company has some considerations and short ramp would not be feasible as well. Generating a RPM is the way forward (if even possible) but I do need to present the calculations.

The parking brake needs to operate when the vehicle is facing uphill and downhill.

1. Currently, I've put a sample size of 20 vehicles on a 60% Slope. Without Parking Brake, they all took an average of 1160 rpm to stay on the slope.

2. With the same 20 vehicles on flat ground, I've fully engaged the Parking Brake and they all took an average of 1157 rpm before "inching forward abit".

Although this proves that my FBD is somewhat true that the RPM on 60% slope to hold the vehicle is the RPM required to "break" the Parking Brake Force on Flat ground, I had a hard time co-relating RPM and Force.

 

[Baluncore]

You did not specify an automatic gearbox with a torque converter. If a torque converter is used, 1160 RPM would be specified as converter slip. It would be very difficult to correlate RPM with torque, given oil grade and temperature variation. We need more information on the vehicle and driveline.
How many axles? 4WD? and does the parking brake operate on all wheels?

 

[256bits]

Have you considered making a set of short vehicle ramps, each with that 6 in 10 slope.

Would it not make more sense for a garage to use this as the criteria to determine if the brakes are adequate, using a ramp with this slope, than trying to find a road with the specific slope.
The manufacturer's instructions probably spell that out so any service centre can perform the test, and not just those with hills around the area.

 

[Baluncore]

Would it not make more sense for a garage to use this as the criteria to determine if the brakes are adequate, using a ramp with this slope, than trying to find a road with the specific slope.

I would just put the vehicle on a tilt tray truck, then raise it to the required angle.

 

[Baluncore]

2. With the same 20 vehicles on flat ground, I've fully engaged the Parking Brake and they all took an average of 1157 rpm before "inching forward abit".

When the vehicle is stationary with ALL brakes firmly applied, stall RPM was, and probably still is, a test of torque converter health. How would you know if you were testing the brakes or the torque converter?

You appear to be operating the torque converter in Stall mode?
https://en.wikipedia.org/wiki/Torque_converter#Operational_phases
What is the stall ratio and torque?

 

[physicsnoob204]

You did not specify an automatic gearbox with a torque converter. If a torque converter is used, 1160 RPM would be specified as converter slip. It would be very difficult to correlate RPM with torque, given oil grade and temperature variation. We need more information on the vehicle and driveline.
How many axles? 4WD? and does the parking brake operate on all wheels?

I'm still gathering all the information you need.

The vehicle is a very old tracked vehicle, and much like a customized excavator/bulldozer. The parking brake is a transmission brake.

Yes I agree! But for some considerations, flat ground is our only solution for our situation.

 

[Lnewqban]

1. Currently, I've put a sample size of 20 vehicles on a 60% Slope. Without Parking Brake, they all took an average of 1160 rpm to stay on the slope.

Welcome!
Are these vehicles exactly like the one in question?
Could you use same slope for a vehicle exactly like yours?
Do you have a practical way of applying (and measuring) torque directly to the transmission shaft near the brake?

 

[berkeman]

I would just put the vehicle on a tilt tray truck, then raise it to the required angle.

Great idea! I wonder if that 6-in-10 tilt is possible with most tilt-bed towtrucks. Also, the beds are usually metal, so I wonder if there would be enough traction for the truck's parking brake to hold it in the nose-down position...

https://www.jsausa.com/wp-content/uploads/2019/03/Tilt-Bed-Tow-Truck.jpg

 

[Baluncore]

The vehicle is a very old tracked vehicle, and much like a customized excavator/bulldozer. The parking brake is a transmission brake.

A wooden bed would be needed if the tracks were steel, with rough timber placed across the ramp.

A tip-truck tray would be an alternative for testing a small 3 tonne tracked vehicle. It will reach a steeper angle than a slide-tray, and if installed as a fixed test rig, would not need the truck.

 

[jrmichler]

A tip truck tray has one possible problem. It may not have enough friction to keep the vehicle from sliding down with the brakes locked.

short ramp would not be feasible as well

A tip truck tray is a short ramp. You might ask them to rethink building a short ramp. Have a contractor pour a concrete slab at the required angle, and specify a rough broomed surface. It might be possible to pour concrete at that angle by using a stiff (low slump) mixture. Or pour a reinforced flat slab, then lift up one end and push dirt underneath. A 3 ton vehicle is about the same weight as many American pickup trucks, and 4" of concrete supports them. A 4" thick slab 8 feet wide by 20 feet long will weigh about 4 tons, so one end can be easily picked up with typical construction equipment.

Even simpler: Is it practical to make a pile of dirt at the required angle? A medium size bulldozer could make such a pile in a couple of hours.

 

[russ_watters]

Does the vehicle have a hard point for towing? you could attach a cable and ratchet, and I assume there's a spring scale that can be bought or made for this.

This idea of using engine torque is a complete non-starter.