Basic bump load value?

[sbrats]

I am looking for basic bump (G) value - If I would use, say 3g at the wheel - what effect does dampening have on a object mounted to the chassis/body - would this mean that 3G would be to high a value?

- which opens up the question a bit to... if bump is the highest value then use this value to envelope for all directions or is there a better way of looking at the loading of an item that is along for the ride.

I wish to build a hitch mounted carrier for a bike. I could buy one of course - but where is the fun in that.

 

[NormPeterson]

A long time ago I welded up my own bike rack out of 1" x 14 ga steel tubing. Basically a plane rectangle with a single diagonal brace plus the requisite hooks for loading the bikes onto. Mine attached with a steel rod in shear through a hole drilled in the steel bumper and two clips per "leg" to hold it on, with a pair of "regular bicycle rack" fabric straps going to clips that grabbed the forward edge of the trunk lid for fore/aft constraint at the top of the rack. We used to load it up with three to five BMX bicycles on race days, all climb in and go off without worrying about a thing.


Norm

 

[GregLocock]

When you run over a bump does your weight double? No

Therefore you (and the sprung mass of the car) actually see less than 1g dynamically in bump

BUT

better safe than sorry. I personally would overdesign this by a fair bit, mountain bikes spread all over the road are embarrassing and expensive.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Bribyk]

3 to 3.5g's at the WHEEL is a reasonable estimate from my limited research but, like Greg said, if the car and passengers experience more than 1g they will be airborne.

You've also got vehicle pitch to worry about as the vehicle traverses bumps one axle at a time (maybe someone here has a typical pitch value for a passenger car); which may cause more than 1g acceleration on the end of your lever arm (bike rack) protruding from the rear of the vehicle.

A safety factor of 4 on the load sounds reasonable to me, if you're a decent welder.