Engineering

CAD Design

Chassis Table

Design Criteria and Overview of Steering System.

Ultra-4 off-road racing puts extreme demand on steering systems. These steering systems must be capable of turning 37 inch tall tires not only on pavement, but also when bound up in the rocks on the race course. Suspension geometry also plays a major role in steering design. We designed the front suspension to assure that the axle stays perfectly centered with the chassis when viewed from the front as the suspension travels. This means traditional steering boxes and drag link style steering systems found on most commuter vehicles and trucks will not work. This is because drag link style steering requires the axle to travel in the same arc (when viewed from the front) as the drag link connecting the steering knuckle to the steering gear box. If the axle and drag link travel in different paths the tires will turn as the axle moves up and down this is known as bump steer. With strength and suspension geometry in mind we opted to use a fully hydraulic steering system.

Full hydraulic steering (full hydro) eliminates the need for steering geometry to be designed into the suspension and provides superior power to turn the wheels.

The system consist of:  (Figure 1):

  • A double ended ram mounted to the axle
  • Hydraulic lines
  • Pump
  • Orbital steering valve
  • Reservoir
  • Cooler

Figure 1: Full hydro steering system overview.

Static Friction of Tires on Dry Pavement

Figure 2: Tire on dry pavement.

The force to turn the tires was calculated using the following equations:

  • Fs=μ(N)
  • For dry pavement μ=0.9
  • Fs=0.9 (4500lbs/4tires)
  • Fs = 1012.5lbs
  • Fr(10in)=Fs(6in) (This is the moment about the ball joint on the steering knuckle)
  • Fr=607.5lbs×2tires
  • Fr=1215lbs

*Fs is static friction force, μ=0.9 is the static friction coefficient for dry pavement.  N is the natural force exerted on the tires (weight of the car divided by 4 tires), Fr is force of the ram.*

As seen from the above equations the ram is required to supply 1,215 lbs of force just to turn the tires on dry pavement.

Pump Selection and Calculations

Next calculations were performed to select the correct pump. We opted to use the CBR 15.1 pump from PSC Steering (Figure 3) with the following specs:

  • 1800psi
  • 4.5gpm @ 3000rpm
  • Vane style
  • Driven off engine accessory drive
  • HP required Q(gpm)×P(psi)/1714
  • Q=4.5gpm @ 3000rpm
  • P=1800psi
  • 4.73 HP

*Q is the flow rate, and P is pressure*

Figure 3: PSC Hydraulic Pump.

Hydraulic Ram Specs

The hydraulic ram we selected had very little information when we acquired it, so all specs had to be measured and calculated.

  • 2.5” I.D.
  • Rod: 1.25”
  • Manufacture: unknown
  • Volume of stroke: 31.76 in^3
  • (Volume piston – Volume of shaft)
  • F = p(Ap-Ar)
  • F = 1800psi(3.683in^2)
  • Force (theoretical) = 6,629.4 lbs

*F is the force, p is pressure, Ap is the area of the piston, and Ar is the area of the rod.*

Theoretical force is approximately 5.5 times the force required to turn the tires on dry pavement. Even with 30% loss in overall efficiency, 4,640.7 lbs will still turn the tires and push good size rocks around.

Not only does the steering system need to be powerful, it must also turn quickly in order to keep up with the fast pace of racing. To ensure the steering system would not limit us when reaching speeds of 75+ mph in the open desert, ram velocity calculations were conducted.

  • Ram Velocity, v=Qin/(Ap-Ar)
  • v=(1039.5in^3)/(4.91in^2-1.28in^2 )
  • v=286.36ipm
  • 23.86fpm
  • 0.4fps

* v is the ram velocity, Q is the flow rate, Ap is the area of the piston, and Ar is the area of the ram*

At 0.4ft per second the ram will steer the wheels with plenty of velocity to assure responsive steering at high and low speeds.

Hose specs

One of the advantages of fully hydraulic steering is the ease of placement due to the flexible hydraulic lines. These lines must be strong enough to be safe under racing conditions, not create flow restrictions or pressure drops, and be large enough to keep the fluid moving under 20 fps to avoid internal damage. The selected line size and specs are as follows:

  • #8 single wire braid
  • O.D. 49/64
  • I.D. 13/32
  • Shear stress @1800psi = 2,796.38 psi  F.S. (P*A) = Shear stress
  • F.S. = 6 for 1000-2500psi
  • Fluid Velocity = Q/A = [4.5gpm(231in^3)]/0.13in^2= 8019.5 in/min
  • 133.65 in/sec
  • 11.134 ft/sec

*O.D. is outer diameter, I.D. is inner diameter, F.S. is the factor of safety, P is pressure, A is the area, and Q is the flow rate.*

Orbital Valve

The orbital valves controls the direction of flow to the ram and as a result turns the vehicle. A simplified schematic of how they work is below (Figure 4-7).

Selecting the orbital turn ratio is a simple calculation “The displacement of the valve divided into the steering cylinder volume will determine the required turns of the steering wheel to turn the steering axle full lock to lock.” -PSC

  • (31.76ci)/7.3ci = 4.35 turns
  • (31.76ci)/9.7ci = 3.27 turns
  • (31.76ci)/11.3ci = 2.81 turns

We opted for the (31.76ci)/9.7ci  orbital for a 3.27 turn lock to lock steering ratio.

Figure 4: Orbital Valve

Figure 5: The fluid is directed back to the reservoir.

 

Figure 6: Right turn configuration.

 

Figure 7: Left turn configuration.