Bike rack for 3 bikes for an expanda?

[Dreaves]

Hi All, Agree with Burnsy comments but I thought I would share my observations over the years as well. I drove light trucks with trailers for while carrying cars for dealerships. The trailer was single axle with a pintel hook hitch. I could tell if the car was sitting right on the trailer as the whole load would feel balanced with no sway or pitching. A couple of inches either way on the trailer did make a remarkable diifrence for me as the driver. In addition a while back i saw a tandem trailer behind a Commercial van. I guess there was no weight in the van but the trailer had gravel & bags of cement loaded more heavily behind the axle. It looked a handful but saw the driver tap the brakes & the trailer flew out sideways spinning van & trailer on their wheels. Luckily no roll over.

I live in Central Qld & the amount of Caranvanners I see with incorrectly loaded vans is really scary. They go slow, the van swaying etc. It is a daily occurrence. I can tell you there is nothing worse than the seeing the highway closed for hours to see the ambulances or rescue helicopters take off & then see a caravan emptied out on the highway. I know speed & fatigue are factors but I believe incorrectly loaded rigs are a major contributor as well.

Cheers, Dreaves :wave:

 

[straydingo]

Ok, this is getting further away from physics that I still remember from high school and that was a while ago!!. So I won’t try to comment on more than I understand. I see your point of view, and might come around…if I fully understood it
(Yes Zero, we are looking into too much, but I think there is a degree of relevance when many of would like to load bikes and spare on the back. Plus I think we’re both just having a bit of fun)

However angular momentum IS effected by weight distribution as your example of the ice skater indicates. The ice skater with arms out has quite different properties in relation to angular motion, than the iceskater with arms in, but the COG is the same in each case. Therefore the position of the weight - even where it doesn't change the COG - affects handling.

The COG (or technically centre of mass) of the skater is the same yes, but the size of the outer radius is reduced which is where angular momentum comes into play.
In a spinning but horizontally stationary object where the COG is over the pivot point, yes the momentum of the spinning weight will create a greater outward expressed force effectively trying to throw the mass further from the COG. However the pivot point is on the vehicle (closer to it’s rear axle the better) and the COG of mass is 2.5-3m away from it, combined with the effect of the drag of the wheels, gravity and the horizontal velocity of the mass would counteract most effect wouldn’t it?

We can't add more mass and not change the COG. Its not physically possible, unless added equally in a sphere around the COG. So we still come back to adding excess mass to the back without counterbalancing will move the COG back closer to the axles, thus changing stability.
However, I do vaguely recall the rule of squaring the weight the further from the fulcrum (axles). So a 50kg box over the axel would have an effective weight of 50kg, 1m from the axle would become an effective weight of 100kg, and 2m from axle effectively 200kg. But even so, as that weight increases, the force required to initiate movement also increases proportionally. Thus the force of the swerve/gust of wind/etc to start the sway needs to be stronger……

Perhaps a better example would be undoing a van wheel nut...use a short spanner and you have buckley's - use a long one and you have a chance. Stick a lot of weight out the back of a van (even if balanced correctly back to front and not changing the COG), i suggest is like using the long spanner applying torque to the COG of the van to wrench it sideways in a turn (with centrifigal force being applied to the weight an torque/leverage aggravating the problem because of the distance from the COG.)

I think I see what your thought is- torque applied determines rate of change to the angular momentum from rest. Otherwise….over my head

:noidea:

 

[glee]

(Yes Zero, we are looking into too much, but I think there is a degree of relevance when many of would like to load bikes and spare on the back. Plus I think we’re both just having a bit of fun) :

Exactly SD. It is good to open the mind and sweep a few cobwebs away occasionally

The COG (or technically centre of mass) of the skater is the same yes, but the size of the outer radius is reduced which is where angular momentum comes into play.
In a spinning but horizontally stationary object where the COG is over the pivot point, yes the momentum of the spinning weight will create a greater outward expressed force effectively trying to throw the mass further from the COG. However the pivot point is on the vehicle (closer to it’s rear axle the better) and the COG of mass is 2.5-3m away from it, combined with the effect of the drag of the wheels, gravity and the horizontal velocity of the mass would counteract most effect wouldn’t it? :

I don't think we disagree much. (Except your point above about centrifual force tending to throw the weight away from centre of rotation is not a point I have argued - it is a good point though - it would tend to pull the car backwards and sideways in the opposite direction to the rear of the van, around the COG wouldn't it?).
In my case the difficulty is more a result of trying to express complex concepts, with rudimentary typing skills! My earlier posts agree with yours above, in that the impact on handling of having the load out the back is influenced by many factors including: the weight, power and traction of the towing vehicle, the towball weight, the speed etc and you have quite rightly added the position of the COG. But none of those things can possibly remove the effect of a lot of rear weight, which is subject to sideays force such as centrifugal force going around a curve, acting like the long lever on the wheel nut, and tending to rotate the rear of the van towards the outside of the curve. That is problem number 1. The remedial action would be to try to gently accelerate and drag the van back into a straight line (I'm no expert on this bit so correction is welcome). However assuming that is true, this is where my other point comes in - that is, trying to stop the van's rotation (when it has a lot of weight out the rear) is more difficult - like trying to stop the person spinning with dumbells in each outstretched hand, as opposed to a person without dumbells - "higher angular momentum".

We can't add more mass and not change the COG. Its not physically possible, unless added equally in a sphere around the COG. So we still come back to adding excess mass to the back without counterbalancing will move the COG back closer to the axles, thus changing stability.
However, I do vaguely recall the rule of squaring the weight the further from the fulcrum (axles). So a 50kg box over the axel would have an effective weight of 50kg, 1m from the axle would become an effective weight of 100kg, and 2m from axle effectively 200kg. But even so, as that weight increases, the force required to initiate movement also increases proportionally. Thus the force of the swerve/gust of wind/etc to start the sway needs to be stronger…… :

With respect SD I think I do disagree with the above little bit, although its not a big issue: the guys and gals on this forum pride themselves on being able to add heaps of mass without changing the fore/aft position of the COG. The position of the COG relative to the axle determines the towball weight doesn't it? Assuming we all keep towball weights approximately constant, then we must be keeping the COG approximately in the same place, longitudinally anyway (I think everyone here knows how to do this. Either the rear of your car is bottoming out under the load if COG too far forward, or the jockey wheel is floating up past your head when you try to hook up). Your point about inertia is also a good one. I mentioned this in an earlier post - less likely to move but more dangerous if it does. Like towing a 10 tonne van - not much will throw it off course but if it happens - goodbye.

We haven't actually estimated the weight of 3 bikes, a spare whee,l and lot of steel for folding arms and reinforcement. Those things could weigh a couple of hundred kilos couldn't they? Lets say the COG is 1 metre in front of the axle and the bike carrier and spare wheel are 2-3 metres behind the axle. That is a 200kg weight acting on a 3-4 metre long lever! Lets not worry about front to rear balance - that is sorted with all the weight put up the front - but the sideways leverage that much weight can exert, sobers me up.... temporarily.

We might have done this to death by now...if so thanks for the discussion! I will finish how I started: I would love to stick a whole lot of stuff out the back because getting three adult bikes in the van (over the axle of course!) is a PITA! But personally - I will persevere. BTW I tow with a Subaru Liberty 3.0R - all 1500 solid kilos of aluminimum and plastic.....!