Sunday, July 12, 2009

Two Second Rule Inadequate, the proof

Last May, we blogged about the 2-second rule quoting and referring to a bunch of studies from the Quebec which did some nice research on motorcycle braking, coming up with new numbers for reaction time and average deceleration force which made Survival Skills look a bit optimistic.

Prompted by our discussion with Steve Garets of TEAM OREGON, who train much more conservative sight and anticipated path distances than standard, I dusted off my old physics 101 notes and did some calculations. Using the Promocycle observed average decelerative force of -0.774G and their 95-percentile reaction time of 0.62 seconds, I came up with numbers like 2 seconds from 25 mph at 50 feet, 3 seconds from 50 mph at 153 feet and 4 seconds from 65 mph for 242 feet, this being the average performances for experience riders on mechanically sound bikes in perfect traction conditions.

The full calculations and tables of stopping times and distances are here. Our revised notes on the bike time/space window are here.

As a result, we are following TEAM OREGON's lead for immediate path and sight distance rules, and add our own numbers for following distances.

Following Distance: At least 3 seconds over 25 MPH, at least 4 seconds over 55 MPH, at least 5 seconds over 75 MPH, at least an additional 25% in the wet or on loose surfaces, and an additional 10 percent on downhill grades.

Immediate Path: 10 seconds (the MSF standard is 4 seconds)

Sight Distance/Anticipated Path: 20 seconds (MSF recommends 12 seconds)

The good news is that these are average numbers for over a hundred tests run by experienced riders. If you could train up to a deceleration performance of -1.09 (the best Promocycle test) and were covering the brakes to reduce your reaction time to 0.47 seconds, your stopping time for 60 MPH would be reduced from an expected 4.15 seconds to a shade under 3 seconds, and your stopping distance from 210 feet to 152 feet. But if you performed at the worse Promocycle example for deceleration (just under -0.4G) and weren't covering the brake, 60 to 0 might take 7.5 seconds and 355 feet. Surely not good enough to keep you out of trouble in an emergency. Note that the very best performance in the Promocycle tests still have the bike stopping from 60 mph in three seconds. Two seconds looke very unrealistic even at this modest highway speed.

It seems to us that the difference between the worst and best performances is a matter of training and then practice. Good emergency braking skills seem like a skill worth acquiring.

The full Bikesafer reports are at time/space window and stopping distance calculations

Incidentally, Promocycle proved a benefit for ABS in improving braking performance significantly, even under good traction conditions where ABS benefits are smaller.

We plan a series on available training to put us on the good end of this braking skills spectrum, starting next week.


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