New Dipping
Sled enhances laboratory crash simulation
Abstract
A new test system developed by
Millbrook could significantly improve occupant protection in SUVs
and save more than €2million per vehicle programme
Case Study
A new test technique developed by Millbrook
could significantly improve the crashworthiness of one of the most
popular classes of vehicle. Developed with a leading European manufacturer
of SUVs (Sport Utility Vehicles), the new technique increases the
accuracy with which front and rear impacts can be simulated on a
laboratory test sled. Millbrook’s first customer for the new
tests estimates that it could save up to €2 million in late
design changes.
“Our client found that a number of critical interactions between
the occupants and the restraint systems, particularly the ride down
performance of the steering column, could be compromised by the
cabin pitching forward during the impact,” explains Millbrook’s
crashworthiness manager Phil Glyn-Davies. “This dipping motion
was not simulated by the test sled, so design refinement was only
possible after whole vehicle crash testing. As this is typically
at the end of a development programme, optimisation through late
design changes was extremely expensive.”
The problem is caused by the design characteristics of many SUVs
and other off-road vehicles which have high ground clearance at
the front to allow good ramp access. To achieve this, the chassis
rails are high at the front then ‘swan neck’ down to
allow a lower cabin floor that improves cabin space and occupant
access. When the rails are impacted at the front, the swan neck
causes them to bend like a hinge at the base of the ‘A’
pillars, tilting the cockpit down. The team found that due to this
affect, laboratory sleds (which only simulate movements in the horizontal
plane) could underestimate peak chest acceleration values by as
much as 20 percent.
“Typical vertical movement at the A pillar may only be 100
mm in a 56 kph front impact, but the resulting changes in angles
can have a substantial affect on the ability of the steering column
to collapse as designed and consequently also on the positioning
of the airbag,” explains Glyn-Davies. “Many of the other
interactions between occupants and the vehicle are also affected,
including contact with the pedals, seatbelts and knee bolsters and
the occupants’ movement in the seat.”
Millbrook’s solution is to mount the test piece on a new dipping
carriage, which is mounted on the sled carriage via a clevis pin
at the rear. The front of the dipping carriage is supported at the
appropriate height and angle by aluminium honeycomb blocks. During
the impact, the moment of forces at the pivot creates a downward
force on the dipping carriage, crushing the honeycomb blocks to
allow a controlled downward movement. Up to 120 mm of vertical pitch
down is possible at the A pillar position.
Calibration of the system is achieved by adjusting the size, density
and position of the blocks and by adjusting the height of the combined
centre of gravity of the dipping sled and test piece. This gives
Millbrook’s engineers the ability to tune the onset time,
rate and maximum vertical displacement to ensure a close correlation
with real-world impacts. The vehicle cockpit can be positioned at
different distances from the pivot to change the ratio of A and
B pillar movement.
Correlation with real-world results has proved to be exceptional.
“We believe this is not only the simplest, most cost-effective
solution to this problem, but also the most accurate,” concludes
Glyn-Davies. “It will save vehicle manufacturers millions
of Euros, allow them to bring their vehicles to market more quickly
and contributing to further improvements in occupant protection.”
To meet its customer’s urgent requirement, Millbrook designed,
built and validated the new Dipping Sled within very tight time
constraints. The system is now available to other customers of Millbrook’s
Crashworthiness Laboratories and patents have been applied for.
Enquiries
Business Development, Millbrook Proving Ground Ltd,
Bedford, MK45 2JQ, United Kingdom
Tel: +44 (0)1525 408408
Fax: +44 (0)1525 408468
Email: info@millbrook.co.uk
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