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Crash bars, safety bars, valve cover or engine guards on a BMW motorcycle twin

by Duane Ausherman

This page applies to BMW motorcycle twin models from the mid 50's through the mid 80's and maybe later.  

It is only natural to want to protect those vulnerable appearing heads sticking out on the BMW boxer.   Several manufacturers have offered their version of protection of valve cover guards.   The engine gaurd sold by BMW is a loop that bolts to the frame.   The loop lays in one plane.   It goes out around the exhaust pipe and protects from the front only.  

crashbar.jpg (18532 bytes)

I consider this one (and others like it) to be the most dangerous and it was sold by BMW.


This type has been very popular and as far as I know, were first sold in the USA by Flanders for the /2 series.

Here are some common characteristics of crash bars.  

1.   They stick out slightly farther than the valve cover, in the theory that it will protect it if the bike falls over.  

2.   Some require removal or loosening and swinging out of the way to adjust the valves.   See photos above.

3.   Some require dropping the exhaust system to mount them.   One doesn't want to remove this exhaust nut with aluminum on aluminum threads, any more often than necessary.  

4.   Most require mounting on the front motor mount bolt.  

5.   Most allow "hiway pegs" to be mounted.   Very dangerous.  

What is a "low side" and a "high side" crash?

"Low siding" is where the bike is leaning, looses traction and slides out.   If one slides until friction slows the bike to a stop, the result usually isn't drastic.   On the BMW, many riders have reported that they just pulled their leg out of the lower side and sat on top of the sliding bike.   Hitting trees, cars, curbs and many other things is where the damage occurs.   It is while sliding, the bike incurs a bump and some of the weight can be transferred back to the tires.   The tires can get enough traction to allow the motorcycle to "high side." High siding is where a sliding bike's tires gets traction and flips.   It is usually safer to just slide along and grind off the valve cover.   Any low side can turn into a high side.

The bike flips and crashes down on the pavement very hard.   If the first thing to hit is the head, as opposed to the front or rear wheel, the energy is transmitted to the case, and the case can distort or break.   If this energy is at an angle, then it is easy for the cylinder to snap off.   Even with a crash bar the crash is too great for it to be effective.   

More importantly, consider the rider.   The "high side" rider can be "launched" into space.   Some rather impressive heights and distances have been reported.   The likelihood of injury in a "high side" is much greater than in a "low side." High siding is where the damage to the bike and the injury to the rider is the greatest.   So what are the factors in a high side and how can we reduce the chances of it happening?

Let's first consider sliding along merrily and coming to an uneven pavement bump up of 1/2.  This bump is the edge between where two slabs of concrete have been poured.   If the valve cover takes the "hit," then consider that it is soft and it breaks and tears away easily.   The more that the "hit" is absorbed by a disintegrating valve cover the less energy is transferred to the tires and less chance of a high side.   The radius of the valve cover is about 3" and it can also sort of ride up and over the bump.  

The diameter of a crash bar is about one inch.   The radius is only 1/2.  A bump of 1/2" is the same as if it were 2" high.   The crash bar has hit a "brick wall" because of two reasons.   The bar is too small to go ever it and it is made of strong stuff that won't tear and break while absorbing energy.   The valve cover has a chance to ride over it and break away.   The valve cover will transfer less weight back to the tires.   Weight on the tires will allow them to do their job, get traction.   This is one case where hard rubber is an advantage, sticky rubber is very bad.  

Over the years I have seen several wrecks where the cylinder has been broken off.   In all wrecks I tried to ascertain all of the details possible.   In all, but one, of the "snapped off cylinder" wrecks, one thing was in common.   The bike first went down and slid.   It "hit" something and high sided and flipped over to the other side and snapped the cylinder off.   These bikes all had crash bars.   Crash bars cause cylinders to get snapped off and riders to sustain greater injury.  

The one exception was a bike that was hit broad side and the bike and rider launched.   The rider was killed and the bike caught on fire and suffered total ruin.   While the bike had no crash bar, it wouldn't have helped.  

Crash bars have also caused crashes in another way, though less often.   To protect the head, they must stick out farther.   In a serious lean the bar can touch the ground.   If it is a smooth touch then it's not so serious.   If it hits hard, then the rear tire can be levered up off of the pavement and maybe it becomes a high side crash.  

Sport riders understand this and elect to not have crash bars.   A conservative rider can, through no fault of his/her own, get into trouble and need that extra lean angle that is lost by crash bars.  

With the BMW boxer, one should consider that the very best crash bar was installed by the factory, THE VALVE COVER.   They are cheap, especially when one considers all the factors.

From Wikipedia

Crash bars (also called "safety bars," or "roll bars") are common equipment on cruiser-type bikes. They are designed to protect a rider's legs (and the motor) from injury in a rollover. Critics claim these only work if the accident doesn't throw the rider away from the motorcycle, or alternately, trap them under the bike. The Hurt Report concluded this regarding crash bars:

Crash bars are not an effective injury countermeasure; the reduction of injury to the ankle-foot is balanced by increase of injury to the thigh-upper leg, knee, and lower leg.


My note:  This study considered all motorcycles, not just BMW.  The BMW boxer is more likely to suffer damage caused by the crashbar(s) and should be avoided.


I am the first to admit that some accidents can use the crash bar for a benefit.   This is like saying a helmet can break your neck.   Statistically speaking, helmets are safe, and I have found that crash bars are dangerous on a BMW twin.  

I have no knowledge of their effectiveness on the newer engine configurations.   My experience is with the boxers from 1950 to the middle 80s.   I fully realize that different styles of crash bars have different dangers to consider.   In my opinion, all crash bars increase the risk of a crash, greater damage and injury in a crash.   The BMW factory style of boxer crash bar (flat loop) is probably the most dangerous.  

BMW requires dealers to carry all factory accessories.   So, as a dealer, how did I satisfy BMW and not violate my own standards?   It wasn't easy.   Any inquiring customer of crash bars was given this talk and allowed to decide.   We mounted almost no crash bars.   Our regular customers suffered none of the crashes that resulted in a snapped off cylinder.  

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This page was last edited: 12/28/2006 - copyright Duane Ausherman
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