Grondpad ry

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Joined: 2007/11/09
It's a combination of all things mentioned that no experiment can simulate. ..........I think. Kobus
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Wat ek bedoel is dat geen eksperiment met 'n "rigit body" bo op die bike kan werk nie. Kobus
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Charles Oertel's picture
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Ja, without getting into a philosophical debate about reductionist vs holistic science, while the experiment won't tell you everything, it will prove that the centre of gravity is higher when you stand compared to when you sit...

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Basset's picture
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Joined: 2007/10/01
Hoekom wil ons alles wetenskaplik verklaar? Daar is sekere dinge in die lewe wat is soos dit is en ons moet dit aanvaar. Al die hoe-ha oor centre of gravity en gewigs-oordrag na fiets ens ens is termes wat uitgedink is deur mense wat te bang is om te sê: "as jy staan in moeilike terrein kan jy makliker daardeur kom sonder om jou gaai af te val". Fullstop. Nou word enige twak uitgedink om dit vir iemand anders "wetenskaplik" te bewys. Bulbollie. Dis hoe dit is. Wetenskaplikes is in elk geval die ouens wat 'n Land Rover oil seal in die Challenger gebruik het. Hulle argument? Dit het gewerk soos 'n bom! I believe I can fly!
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Freakonaleash's picture
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Joined: 2007/06/19
Good one Basset! I RIDE THEREFORE I AM!
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Roy Linley, a tall man, was the first person to win the Roof of Africa on a motor cycle, a Yamaha 250. So what has this got to do with the above subject? Malcolm Smith, a small man and friend of Steve McQueen (Remember "On any Sunday") rode the Roof sitting down and won it. If it works for you - use it. Q E D Bum in the saddle, always.

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Neil Terry's picture
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Joined: 2007/07/01
Re centre of gravity: Centre of gravity is a noun in gramatical terms like cat, dog, man. Verbs do things: run, push, pull. The centre of gravity, being a noun, is just a name of a place; what acts is the attractive force of gravity acting upon the motorcycle and upon the man. Where the man connects with the bike, is where the potential energy of the man can act upon the bike. If the man were to be disconnected from the bike, he would no longer act upon the bike, no matter where the hell his centre of gravity now is. Now re-considering the issue; there are forces at play, and it is the inter-play of these forces, between the gyroscopic, vector forces from rocks, and the rider's body pushing downwards on seat, left peg, right peg, handle bars etc. which determine what happens. "Changing the centre of gravity" is actually changing the forces acting upon the bike e.g. in a race bike, a low centre of gravity can make it more nimble, as it can flick sideways more easily; again this is re-arranging the potential energy stored in the bike. But what do I know, Ek is net 'n karpenter.
die malletjie (not verified)
Neil May the forces always be with you!!! Do it today, maybe there is no tomorrow!
GeelKameel's picture
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Joined: 2007/06/21

Yes Neil, you are very correct - the standing/sitting/CoG issue is a case of forces at play, interacting. Different forces dominate in different situations, and the situation continuously changes as you and the bike moves forward, sideways, etc. We can debate it up and down ad infinitum. Fact of the matter is that it is a ''interplay of forces'' (static, dynamic, inertia, gyroscopic etc etc.) The CoG is but one (mathematical) characteristic that is usefull in the bigger picture. Standing is a proven way of controlling the bike in difficult terrain. Lets practice it more and debate it less. :)

------------------

Some food for thought from another thread...

(http://www.bmwmccc.org.za/whats-all-look-stuff-anyhow)

Centre of gravity is but one of many aspects involved in the sit vs stand issue.
Many instructors (and DVDs) seem to be using the wrong wording in their explanation re sit vs stand. In the process many many riders get confused.
Yes, I remember very well how Niel Terry once explained it - maybe poetic but perfectly correct!

What is much more important are the inertia forces and where/how they actually interact.
(inertia is the force experienced when a mass is accelerated or is being retarded).
Yes, mathematically the inertial forces act through the Centre of Gravity, but that is one of many aspects to consider in the issue regarding handling/response of a bike.

I made some pictures that could help, in a simple way, to show 
a) that total CoG is higher when standing 
b) the effect of inertia forces when rider is seated vs standing

c) Where forces act is not the same as where CofG act

 First pic: The total CofG (bike plus rider) is higher when standing.

Kobus

 

Second pic: Effort for sideways movement is dictated by the mass that needs to be moved and where it interacts. Seated: about 300kg needs to be moved, requires more effort, response is slower. When standing, less mass (~220kg) is moved, less effort is required and response is quicker.

sideways

 

And for good measure, a picture to show the points where forces (not CofG!) interact. This is where lots of confusion is caused. Forces and CofG are different entities and should be used in correct context.

pegs

:)

GeelKameel's picture
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Joined: 2007/06/21
Tyre pressure and heat. I had an interesting observation on our Wuppertal trip. I used 1.5 bar front and back on the dirt. After the last dirt we did 20km on tar before re-pumping the tyres. I did 120-130 all the way. When stopped, the front and back were 1.7bar, feeling very hot. After 20minutes waiting for my turn with the air hose, both tyres were still quite warm, but the pressure was down to 1.5bar. By the way, 1.5 bar was the right choice for the prevailing road conditions. Lots of (shallow) sand and plenty loose gravel. Bit soft for preventing damage from rocks + stones. I had one nasty hit, fortunately no puncture. We had one Dakar that suffered such a puncture.
Charles Oertel's picture
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Joined: 2007/04/14
Ja, when we drop pressure for off-road, the 1.5 bar is when the tyre is hot. If you set it to that when cold, the tyre warms up and the pressure increases, making the tyre a bit too hard. Also, on an overnight trip, take it easy when you start the next morning, because your tyre pressures are lower than when you stopped the night before. (Neil does that automatically because his joints need to warm up :).

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Neil Terry's picture
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Joined: 2007/07/01
Thanks Rony, may The Force be kind to us all!In the past it has on more than occation brought my COG much closer to Mother Earth than I intended. Charles your quip regarding my joints is`nt far off,my knees are stiff/sore possibly from 930+ skydives in the early eighties. PS. I posted to stir you ... just a little nibble I see. And Brenda putting on make up ????
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Joined: 2007/06/25
Charles, mother time is the cause of stiff knees & joints. Have you noticed how dirty that G S is in the photo? Bum in the saddle, always.

Think before you ink.

Trust is the most valuable asset.

I have the rest of my life to get old.

GeelKameel's picture
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Joined: 2007/06/21
Tyre pressure is important! I realised that yet again on a recent trip. Before the trip my rear tyre had a tiny bit of legal tread above the 2mm bumps within the grooves. I decided to keep the tyre for the trip (ek probeer maar altyd die laaste bietjie uitwurg!) I was shocked when I noticed the condition of my rear tyre after the trip - All tread in the centre was gone, including the wear pads! Reason? Too high speed for too long on tyres that were too soft for the speed. We deflated tyres after 100km and we then did about 300km on ~40% tar and 60% good dirt/gravel. I used 1.9bar back and front (I had a pillon (which adds 0.2bar to my standard 1.5bar for dirt plus another 0.2bar because I knew that the roads ahead were quite ''Green'') The pressure suited the conditions on some sections really perfectly, especially a long stretch of gravel road with plenty of loose pebbles. But most of the time we did about 120km/h. In retrospect this speed was fine for the roads, but I (we?) should have used much higher pressures. Or travel at much lower speed. I am sure the wear would have been much less.
Piet Cronje's picture
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Joined: 2013/04/11

Hi julle , ek luister lekker na al die raad en ervaring wat hier gedeel word, wens ek was daar om te kon rondrits op die gronpaaie in die mooiste KaapWink

Piet Cronje      email: cronje02@yahoo.com

"Do unto others as you would have done to you."

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Joined: 2013/05/22

Hi BM-ers

I would like to hear what the concensus is out there without starting a mud slinging match.

I had a 1150GS and sold it because the bike was way to intimidating for me. I would like to get back into biking, possibly more touring with the occasional Swartberg pass and similar.

Can the RT actuall do this without danger to myself and or the bike?

Give your brain a breather - ride a bike .. or fly!
Charles Oertel's picture
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Joined: 2007/04/14

On one intro to offroad pillion course I did, there was a fellow on an old RT.  He did a fair amount of dirt road riding - but the RT would not allow him to turn off the ABS.  This is a deal-breaker - it means you cannot do an emergency stop.  He failed the course as a result.

One way to stop being intimidated by your bike is to do some adventure bike training (advertisers on this site).  One of them has the slogan "Double your skills and halve your fears", with many trainees exclaiming that their bike now feels smaller to them.  I would get a GS - the 1200 is not as heavy as the 1150 - and the 650 is a lot smaller yet still punchy (but not as great for touring).  The 800 has its lovers and not so lovers.

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