Thinking Behind Sliding Carriage

If a top box, sitting on a sliding carriage, attains a slideways velocity and then bumps to a stop against stops further away from the centre of the fore/aft centre line of the bike then isn't that creating more powerful turning moments and therefore more instability than if the top box were rigidly attached at the centre line? Any experience of with/without sliding carriage out there? Just wondered.

 

I wondered the same
Usually something sliding about causes instability

 

It slides slowly and not beyond the balance point ! A rigid top box keeps trying to fall into the natural balance point , hang off the side of your bike at speed and shuffle/shift your body parts (knees , arms , head ) you'll feel a sweet spot ! This is where side shifting panniers and top box will have settled ! As there isn't any force's pushing them beyond there !

 

I have the sliding top box on my Deauville doesn't make any difference that I can tell only slides on a very small margin

 

Thanks Capt.......interesting. I just figure that a top box with contents, accelerated and then force stopped away from the centre line is going to have a bigger impact on stability than if held at the centre line without moving? F=ma and all that. But suspect I may be wrong and hoping that somebody will come along with the science behind it.

 

Rational behind the "sliding" feature is for reducing energy transferred to the rear of the bike during gusty winds with the top box installed

 

Thinking about it....
There should be no sideways movement at all because it’s a bike and not a car. The forces of the bike go from the rider DOWN into the bike then onto the centre of the tyres whilst cornering.
If it was a car the forces would go either left or right subject to opposing direction, so in theory if you had a water container on the back with a red line round the height of the level The water would stay to the line at all times.

 

This is from a trimuph 1200 web sight

 

Well call me a cynic but the rider is a darn sight bigger than the top box......fairings also catch lateral wind noticeably when overtaking trucks in wild weather.
There are various forces at work but for loose box contents Google ‘free surface moment’. It capsizes ships and particularly ro-ro ferries because they lack compartmentalisation on the main deck once water gets in. The phrase ‘loose cannon’ derives from the same problem of a couple of tons of cast iron punching holes in the side!
I return to the marketing idea that perception trumps reality and thereby justifies the price. But maybe it works!

 

The only time I notice the sliding rack on my Tiger is when flicking quickly from side to side, you can feel the rack/top box 'catch up' as you lean over, a bit disoncerting at first but I'm used to it now.
Personally think it's a solution for a problem that doesn't really exist....

 

Interesting comments folks thanks. I should think there is a side to side force on box and contents when negotiating the twisties. Presumably this sideways momentum comes to an abrupt halt when weaving the other way. I understand that the mass of the driver is far larger but not interested in that - just the science behind how a sliding carriage makes a bike more stable. Imagine this scenario: a 40 knot gust of wind hits you sideways on. The top box and contents tries to accelerate to the same speed and then rams against the stops. So you have an accelerated mass at a given speed trying to topple the bike over sideways at a point displaced from the fore/aft centre line of the bike. Perhaps this changed, effective lever angle reduces the turning moment (or whatever it is called)? I don't know.

 

If the C of G of any two wheeler wasn’t constantly weaving L to R you’d fall off or be going in circles.

https://en.m.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics

 

Thanks for the interesting link Callumity. However, it doesn't help me to understand how the effect of a now moving mass (which is moving independently of rider and bike), blown sideways that comes to rest against the stops away from the centre line of the bike improves stability? Or, perhaps it does and I do not understand it. The only way I can think to describe it is; imagine you are taking a right hand bend, a gust of wind blows the top box/contents from right to left and belts it sideways against the stops - is that better than having a fixed top box at the centre line of the bike?

 

Think of it as a shock absorber. Your steering inputs and unconscious body shifts have far greater impact on your direction of travel. If you really think about it ‘stability’ comes from balance rather than ‘stabilisers’ - even when you were three!

 

I've always thought that the sliding carriage was designed to allow a top box to oscillate at high speeds. I've done plenty of high speed miles with one and the bike has always been rock solid. Never glanced behing though to see what the top box was doing.

 

I agree, seems like a solution in search of a problem

 

I am a fairly experienced adventure-bike rider and I own one of the first Explorers that landed in South Africa.

The floating topbox bracket is nothing but a massive pain in the behind. It rattles on a paved road. Imagine what it does on a dirt road. And I ride dirt as often as I can - long distances and fairly high speeds.

After having owned two 1050 Tigers, the first thing I did after running in the Explorer, was to tie the sliding bracket down so it won't rattle. And every now and then I have to replace rubber spacers as they wear out.

I have experienced the dreaded weave in cross winds (at illegal speeds) and the moving topbox wouldn't have made a difference - just like the moving panniers didn't make a difference.

Maybe the system works on a sleeker bike like a Sprint, but on the Explorer it is just weird and irritating. The panniers don't bother me, but the rattling topbox assembly does.

 

Thanks Rehann.... that's very interesting.

 

All I know is a sliding carriage for my 800 Tiger cost $290AU, which made the bargain top box for $300AU on eBay a bit less of a bargain.
I firmly believe in bolting things on because wobbly bits fall off so I regard the sliding top box and moving panniers as an odd gimmick. Do they still do them on the 900?

 

Been out for a 25 miler today without top box fitted.......no sign of a rattle (Had three comments regarding rattle without top box). Will try with top box next; without a load in and with a load. A very boring report to follow as soon as the weather improves.