Archive of BMW Motorrad Club Cape (2006 - 2019)

Jammer man Leon - as ek dit nou weer lees klink dit baie erger as wat ek bedoel het. Jy ken beslis jou fisika, en al wat my omkrap is die praat van 'n laer swaartepunt wanner dit eintlik die teenoorgestelde is. Ek stem saam dat dit nodig is om 'n verduideliking van hoe opstaan help aan leerlinge te gee, maar ek glo die waarheid le nader aan die ontkoppeling storie as die swaartepunt een. Ek will graag 'n paar prente met mooi verduidelikinge opsit, waar die volle fisika bespreek word, maar ook 'n "so what does this mean for me" gedeelte waar 'n gewone ou kan lees van (bv.) 'teen meer as 42 km/hr sit so wanner jy draai' en 'as jy stadig draai maak so' ens. (Ek sal sommer gou my oorspronklike kommentaar 'n bietjie verbeter ;-) groete Charles

Charles, jy is vergewe, maar ook net oor ek 'n sms van BBC af gekry het. Kom ons praat gou oor die centre of gravity (COG). Die fiets het sy eie COG en jy as die ryer het jou eie COG aangesien dit 2 verskillede voorwerpe is. Saam het julle ook 'n COG en as jy staan gaan daardie gesamentlike COG hoër wees (indien jy deel van die fiets bly) as wanneer jy sit. As ons egter staan probeer ons onsself van die fiets "losmaak" so vêr as moontlik sodat ons glad nie inmeng met die "kragte" wat op die fiets inwerk nie. Swaartekrag het egter steeds 'n invloed op die ryer en die gewig van die ryer word deur die pegs na die fiets oorgedra. Die ryer se gewig word dus deel van die fiets deur die pegs, maar die ryer beweeg afsonderlik. Kom ons neem die voorbeeld : Neem jou motorfiets, sit 1 sakkie sement (50kg) op elke peg. Waar is die COG van die pakaas ?? Neem nou die selfde 2 sakkies sement, sit hulle bo-op mekaar en dan op die fiets se saal. Waar is die COG van hierdie pakaas ??? Duidelik is die COG hoër is nie tweede geval ! Dieselfde met die ryer as hy staan of sit : As hy sit plaas hy sy gewig oor op die saal se hoogte en hy is deel van die pakaas. As hy egter staan plaas hy sy gewig oor op die pegs en hy probeer om nie deel van die pakaas te wees nie. Hoop dit maak sin in leke taal.

Ah Leon Nou het ek die punt van die probleem beet. Jy is heeltemal reg oor die COG wat laer is as die sakke sement op die pegs rus. Maar, dit is omdat die sakke sement self laer is (hulle is nader aan die grond). 'n Mens se gewig is nie '''in''' sy voete nie - sy COG is seker in sy maag (''in some more than in others'' ;-) ). Om dit nou met die sakke sement te verontewig: in die eerste geval, sit die sakke sement op 'n platform net bo die sitplek, wat deur gewigtelose stokke aan elke peg vasgeheg is. Dit is meer waarskynlik hoe die COG van die ryer lyk wanneer hy staan. In die tweede geval, sit die sakke '''op''' die sitplek. In watter geval is die gesamentlike COG laer nou? Sien jy wat ek bedoel? Ek glo die 'losmaak' van die ryer van sy fiets se beweginge, en ook die vermindering van positiewe ''feedback'' van die ryer na die stuur is wat die meeste tel. I still want to do an experiment where you hold a bike upright, with a rider, and shake the bike from side to side with him sitting and standing. I believe that when he sits, a sudden jerk of the bike to the left causes him to pull the right handlebar which in a riding situation would countersteer his bike further down on the left (i.e. positive feedback). When the rider stands, he stays upright when the bike goes down on the left, causing the '''left''' handlebar to be pulled towards him and thus countersteering the bike to come up on the left (thus negative feedback) and correcting the motion automatically. What do you think?

Ons kan baie tegnies raak op hierdie onderwerp, en in die proses word baie semi-waar en semi-onwaar fisika voorgehou as die finale feite. Bv, as die sakkie sement op die pegs le^ is die swaartepunt wel lekker laag, maar in werklikheid is 'n mens soos die sakkies sement wat op twee lang pale op die pegs staan (maw die sakkies sement is selfs hoe"r as die sakkie sement wat op die saal le^) En dan kom traagheid ("inertia"), traagheidsmomente en versnelling ook nog by...

Dit alles is tegniese dinge --- en ver van jou gedagtes wanneer jy wind in jou gesig voel en stof in die spieels sien --- staan is lekker, en jy kan die bike beter beheer, en jy en die bike "gesels" met bene en gewigverplasing.

Belangrik is dat mens se bene "sag" is en jou lyf se gewig (massa!) isoleer van die bike s'n. En jy kan die bike en jou eie massamiddelpunt relatief tot mekaar skuif, tot voordeel van julle albei. Met sit is jy en die bike een groot massa, met min buigsaamheid. En jy skud jou longe uit op rowwe pad. Dis nie verniet nie dat ons geleer word om te staan-ry. Dis 'n beproefde tegniek en kom al baie jare. Die tegniese redes/verklaring is miskien interessant, maar die resultaat is wat regtig belangrik is. (soos daai advertensie gese^ het: ...it's not whats inside, it is how you get it out...)

Vir meer oor CofG en swaartepunt ens, sien hierdie skakel:

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

 

Baie interessante beskrywings en verduidelikings. Plaas asseblief die sketse! Sonder om af te dwaal - net om bietjie kleur aan die besprekings te gee - 'n dame se swaartepunt is - weens haar liggamsbou - laer as 'n man s'n. Sy behoort dus oor die algemeen beter balans op 'n motorfiets te he. Maar help nou asseblief ons mans verder! (Dit was aangenaam om vir Leon en Charles by die pillionkursus te ontmoet, al kon ek nie die kursus meedoen nie)

[quote=JohanM] Sonder om af te dwaal - net om bietjie kleur aan die besprekings te gee - 'n dame se swaartepunt is - weens haar liggamsbou - laer as 'n man s'n. Dit is wel wetenskaplik bewys, maar geld net vir die met klein tieties. 'n Man kan sy swaartepunt verplaas. Al gehoor van "jou verstand sit in jou g@t" of "jou brein het alweer gesak tot in jou albasters". I believe I can fly!

Julle mis my punt. As ek staan op die fiets is ek nie deel daarvan nie. Dis net my gewig wat deel is daarvan. En dit word oorgeplaas deur die pegs. So die sakkies sement is op die pegs en nie op die hoër platform nie.

Leon, I'll make a little experiment so you can see. I have a toy R75S. I will suspend a wire from it below with a weight that can be adjusted until it balances on its own. Then, I will put two little sinkers or something like that (nuts) # '''on the pegs (or maybe the pots)''' and move the weight until it just balances. # '''on the seat''', and adjust the weight until it balances. # '''at the end of two little wires attached to the pegs''' so that the nuts are just above the seat, and the wires are like legs, and balance it again. I bet you anything you like, the balancing weight needs to be the lowest for point 3 (to counterbalance a higher COG of the bike and nuts). Then point 2 (because the nuts are at seat level). For position 1 the nuts are very low and the counterbalancing weight can be the highest. But first I must get supper.

After a lifetime of racing and riding dirt I have been watching this with great interest. I agree with parts and disagree with others, for instance 1)Loosening yourself from the bike is a folly. 2)The variables you will encounter out in the dirt is endless and unpredictable. 3)Weighting the footpegs to control grip and rearwheel steer is of the utmost importance. 4)Standing adds suspension. And so much more I RIDE THEREFORE I AM!

I met a trial rider who did the West Coast sand standing, but holding the bike tight with his knees. Not one fall in 3 days. I think the important thing is to avoid positive feedback to the steering via your arms. One way of achieving this is to grip the bike with your legs, and keep your arms loose so that you can control the steering independently of how the bike is shaking you. But, for non-motoX, non-trial occasional sport riders like most of us, probably the most effective way (which is why it is official BMW rider academy policy) is to '''stand up, look up and open up'''.

It's a combination of all things mentioned that no experiment can simulate. ..........I think. Kobus

Wat ek bedoel is dat geen eksperiment met 'n "rigit body" bo op die bike kan werk nie. Kobus

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...

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!

Good one Basset! I RIDE THEREFORE I AM!

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.

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.

Neil May the forces always be with you!!! Do it today, maybe there is no tomorrow!

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. :)

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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.

 

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.

 

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.

:)

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.

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 :).

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 ????

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.

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.

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 Kaap

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?

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.