Just curious what you think about Ray’s most recent newspaper column in which he replied to a guy asking which was harder on the suspension, hitting a bump or an equal-sized pothole. I like Ray’s reply at the gut level, but doesn’t the question need to be more specific? The force on a damper is about proportional to the speed at which the axle moves relative to the car, so doesn’t the shape of the bump or pothole matter most?
Suspension engineer for 20+ years soooo…
My answer is that a 6 inch bump is far worse than a 6 inch pothole. Why? You are correct that shock damping loads are proportional to the speed of the wheel but… and it is a big BUT…
With a single bump, 6 inches of compression (upwards) travel is well into the bump stops on most cars so the loads generated are both shock and bump loads combined. The bump stop loads are on the order of 5 to 10 times more than the shock loads. A 6 inch bump at 25 mph may damage a tire, the tire and wheel or the tire, wheel and suspension depending on the car.
With a 6 inch pothole, the exit side of the pothole looks exactly like the 6 inch bump BUT the entry of that pothole allows the tire to drop 6 inches at first followed a bit by the body dropping and then compress 6 inches on the exit thus missing the bump stop entirely. Remember I said the bump stop loads were much more than the shock loads. Many cars will also hit the rebound (extension) stops when the tire drops into the pothole since many cars don’t have 6 inches of rebound travel.
A 6 inch bump or pothole is massive. A 6 inch bump is a “curb strike” test that would be assumed to damage the car. Preferred result is to bend something rather than tear the suspension off the car. Typical manufacturer’s test potholes are 4 inches deep with a long gentle entry and have a 45 degree angle on the backside so the tire isn’t guaranteed to be damaged. That makes it halfway between and bump and a pothole.
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Thanks Mustangman. Yeah, hitting the bump stops would make a huge difference. I imagine that driving speed would matter too…going fast enough would make the car’s vertical motion negligible, and just the opposite for slow speed. The other factor I wonder about is the extra leverage when the suspension is extended down into the pothole (the moment arm is longer). The forces aren’t just coaxial with the shock, so would the torque cause additional wear? If so, that’s a point in favor of less effect from a bump than a pothole. Thoughts?
Not sure what moment arm you think is longer, plesse explain that.
Any bump or pothole has a significant vertical and horizontal component. Transverse isn’t zero but it isn’t particularly large so bump or pothole, any wear or damage is insignificant. Lateral loads come from hard cornering. Add bumps or potholes to that and now we have significant wear and tear.
Suspensions (the whole car, actually) are typically sinusoidally cycled on test rigs at +/- 2Gs load fore aft and laterally for a significant number of cycles to test the suspension parts and the attachments.
As for the path, depends totally on the suspension design. 2 of my cars have the bump stop on the shock or strut. One is seperate. All 3 have suspensions designed to allow rearward motion upon impact.
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Thanks again, it sounds like the difference in wear is negligible. As for the moment arm, I just meant that a wheel extended down from its neutral position has a longer moment arm when subjected to a horizontal force. The suspension supports have to resist that moment, but that can’t be a real issue.
I’m not an engineer nor smart enough to be one… But I do know I have seen a vehicle hit something on the road big and heavy in a 20-30 mph zone and just about rip the front end out from under the vehicle, I have seen what l pot holes do to tires and wheels… I will take a 6" deep pothole over a 6" vertical bump (both not tapered) any day of the week…
Something else to consider is the diameter of the pothole and size of the tire, a tire can only drop so far into a 6" deep 12" diameter pothole and is not the same as hitting a 6" vertical curb…
So how big of a diameter does the 6" pothole have to be for a given 26" tall tire to hit at the same part/height of the tire as a 6" bump??? And a truck/suv is going to have a much bigger diameter tire so that would have to make the pothole even bigger right??
I may not be able to do the math on paper on this but I know how to use a tape measure, and I just took a tape measure and checked my 205/55r16 tire which is about 25" tall/diameter and measured up 6" and then across and it looks like a 6" deep pothole would have to be about 20" in diameter for the tire to be able to drop far enough to hit the 6" deep mark on my tire, so the taller the tire the bigger the diameter of the pothole has to be, and that is just sitting in the hole not driving over it at 25 mph, so it would have to be a little bigger at 25mph cause the suspension has to have time to drop, right?? Where as a 6" bump is just there and the tire size is the only variable??
Either one can do damage, so pay attention to the road and where you are going and try missing both safely… lol
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Agree, equal size, I’ll pick the pothole. Take it down to the size of a brick - as a bump it could bend a rim or worse, as a ‘pothole’ you might not even notice it. I think Ray’s answer was based more on what we actually see: potholes can get huge, with sharp edges, while there are few bumps of similar dimensions.
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