BMW K bikes (Bricks)


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1Back to top Go down    Wheel sizes and their dynamics on Thu Aug 24, 2017 11:59 pm

floyd

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Hi krew,

wondering if anyone can point me to a resource, or chime in with their expertise, regarding the effect of wheel diameter on performance and handling dynamics.

I understand the effect of front wheel diameter on steering speed and ability to take bumps...but Im curious about the effects of rear wheel diameter.

It seems curious to me that the K1 and 16v RS had a 17" front and an 18" rear rather than 17" at both ends. what was the reason for this?


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K100 with lots of K1100 bits - mongrel of a thing...
    

2Back to top Go down    Re: Wheel sizes and their dynamics on Fri Aug 25, 2017 3:51 am

92KK 84WW Olaf

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Smaller front wheel lowers centre of gravity and transfers weight forward. The bike will dip into corners more easily and make it easier to get around corners, like altering understeer and oversteer in a car but itwill also reduce ground clearance on corners. The same result to some effect can also be achieved by increasing rear wheel diameter but without affecting ground clearance.

The manufacturing accountants' input is probably that a larger rear wheel raises gearing and is probably a cheaper manufacturing route than tooling up for and making a very limited run of higher geared final drive or gearbox units [though changing a ratio or two isn't a huge problem] which would also be needed for a more powerful bike with a higher top speed.

For most of us in normal road riding we would manage with either I should think.


__________________________________________________
1992 K100LT 0193214 101,000 miles
1984 K100RT 0022575 Baja Red bought 36,000 now 89,150 miles
1997 K1100LT 58,645 now 60,500miles
    

3Back to top Go down    Re: Wheel sizes and their dynamics on Fri Aug 25, 2017 6:11 am

BobT

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@92KK 84WW Olaf wrote:Smaller front wheel lowers centre of gravity and transfers weight forward. The bike will dip into corners more easily and make it easier to get around corners, like altering understeer and oversteer in a car but itwill also reduce ground clearance on corners. The same result to some effect can also be achieved by increasing rear wheel diameter but without affecting ground clearance.
Changing a front wheel on a given machine might lower the front end by half an inch, but I challenge anyone who thinks that they can feel that very slight difference in steering geometry. If that was the case then try dropping the forks through the top yoke by 13mm and see if you can feel that.
The heros and posers will say that it makes a huge difference and that they can fell it, but try it for yourself.

    

4Back to top Go down    Re: Wheel sizes and their dynamics on Fri Aug 25, 2017 7:13 am

Dai

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@BobT wrote:
@92KK 84WW Olaf wrote:Smaller front wheel lowers centre of gravity and transfers weight forward. The bike will dip into corners more easily and make it easier to get around corners, like altering understeer and oversteer in a car but itwill also reduce ground clearance on corners. The same result to some effect can also be achieved by increasing rear wheel diameter but without affecting ground clearance.
Changing a front wheel on a given machine might lower the front end by half an inch, but I challenge anyone who thinks that they can feel that very slight difference in steering geometry. If that was the case then try dropping the forks through the top yoke by 13mm and see if you can feel that.
The heros and posers will say that it makes a huge difference and that they can fell it, but try it for yourself.
Bob - you never tried riding a LeMans IV MkI and Mk2 back-to-back. That 16" front wheel on the MkI really, really, really screwed up the steering characteristics of the Guzzi. It just flopped in everywhere - it was truly horrible. It was the only Guzzi I never wrote about because I couldn't find anything nice to say about it; the factory obviously agreed because they reverted to an 18" wheel for the second run.


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'83 K100 upgraded to K100RS spec
Others...
'78 Moto Guzzi 850-T3, '79 Moto Guzzi 850-T3 California,'93 Moto Guzzi 1100ie California,
'03 Suzuki Blandit GSF600SK3 (NFS any more because wifey has claimed it)
    

5Back to top Go down    Re: Wheel sizes and their dynamics on Fri Aug 25, 2017 7:35 am

92KK 84WW Olaf

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On road riding with a touring configuration its not going to make a big difference but I didfind it does become noticeable when you are travelling with gear on a trip.

All the stickers say not to exceed 120kmh with panniers and kit on, but little changes become noticeable at that speed,although a slightly off tyre pressure will be much more noticeable.

I have used 100/90, 110/80 and 110/90 front tyres, prefer the 110/90 but it raises the front end by 9mm. It may well be subjective but I don't notice a big change in handling but feels nicer on the road. On the other hand instead of 130/90 rear I use 140/80 which drops the rear all of 5mm along with a further drop on the suspension. All taken together means I can get my feet on the ground better making the K more manouverable in a car park etc.. It's not a sports bike and not ridden like one and in those conditions as BobT says not so noticeable.


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1992 K100LT 0193214 101,000 miles
1984 K100RT 0022575 Baja Red bought 36,000 now 89,150 miles
1997 K1100LT 58,645 now 60,500miles
    

6Back to top Go down    Re: Wheel sizes and their dynamics on Fri Aug 25, 2017 7:36 am

jbt

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Handling is no just a matter of wheel diameter.
The angle of the steering axle, the value of the trail, the lenght of the wheelbase are also important factors.
About the wheels, if you consider 2 wheels of the same weight, the smaller one will be more reactive to turn because of gyroscopic efforts that are less than a bigger diameter.
But a big diameter wheel is more stable, and gives more confort because it bumps less on road holes than small wheels. That's why off road bikes have an 21 inch wheel, to climb easier on obstacles.
The width of tyre and rim are also important: the larger they are, the heavier is the handling.
On modern road bikes, the large tyres are pretty stupid: they just are designed to imitate race bikes, but to keep an acceptable handling they oblige to have a reduced direction angle, with a high risk of death wobbles, and you have to put knees on the tar if you want to make it turn.
Yet...


Check on the video that big tyres and sport geometry are not always an avantage, specially in curves under 100 mph...

    

7Back to top Go down    Re: Wheel sizes and their dynamics on Fri Aug 25, 2017 8:01 am

92KK 84WW Olaf

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And to underscore that.....look at bicycle tyres, the fastest track tyres are the thinnest going. Admittedly with a lot of power you need more tyre to get it on the road, but bicycle handling is brilliant.

Oh, BMW do make bicycles so not entirely off topic.


__________________________________________________
1992 K100LT 0193214 101,000 miles
1984 K100RT 0022575 Baja Red bought 36,000 now 89,150 miles
1997 K1100LT 58,645 now 60,500miles
    

8Back to top Go down    Re: Wheel sizes and their dynamics on Fri Aug 25, 2017 8:59 am

charlie99

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@floyd wrote:Hi krew,

wondering if anyone can point me to a resource, or chime in with their expertise, regarding the effect of wheel diameter on performance and handling dynamics.

I understand the effect of front wheel diameter on steering speed and ability to take bumps...but Im curious about the effects of rear wheel diameter.

It seems curious to me that the K1 and 16v RS had a 17" front and an 18" rear rather than 17" at both ends. what was the reason for this?

I really don't understand this at all Floyd

the k1 and 16 valve jobies with big rear rims have an 18 inch front and an 18 inch rear ,... where did you get the idea that they put an 17 on the front ?

all the above comments are true ...but did anyone think about it on the subject ofk bikes ?

perhaps the lower profile rear 18 inch rubber gave a much more tractable feel (and it does justthat, in spades )...all well and good for road bikes , and the convention keeps going on ..

you might have to consider that radial type tyres ofthe lower profile variety were not "at that time " extremely common at all, nor were really good shockie designs with rebound damping for a start .

I guess for us older folks .we do remember the changes in technologies ... the radial tuned suspension theologies in cars ..and anything on the road for that matter .

but now with technologies of advanced compounds of rubber and stuff ...there are some amazing results ...but consider that there was no way of releasing a 200 hp bike to the open market at that time . but for racing, we would all be riding around on bloody "bantam bsa" type thingys (hmm royal enfield 500 single carries a bit of sway here) ...thank heavens for "Ariel " Vincent " and many more "outside the box" thinkers

interesting thread ..I wonder where it will take us ?


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cheezy grin whilst riding, kinda bloke ....oh the joy !!!! ...... ( brick aviator )

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%O
    

9Back to top Go down    Re: Wheel sizes and their dynamics on Fri Aug 25, 2017 10:22 am

RicK G

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The K100 2V has a 18 front and 17 rear
the K1 K100RS 16V and K1100RS have a 17 front and 18 rear
The K1100LT has a 18 front and 17 rear
I think that the factory went with those sizes because they were popular sizes at the time and slowly evolved to using wider rims and then 17 front and rear with the production of the K1200 models
Many bikes of the era of the K100 had the 18 front and 17 rear which is what my Z1300 Kawasaki has along with most of the big bikes of the day.


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If everything seems under control then you aint goin fast enough:- Mario Andretti
Bikes 1986 K100RT, 1993 K1100 LT, 1994 K1100 LT, 1993 K75 RT, 1996 K75RT, 1986 K75 GS, 1979 Z1300 Kawasaki X 2 & 1976 SR 500 Yamaha for now
    

10Back to top Go down    Re: Wheel sizes and their dynamics on Fri Aug 25, 2017 12:34 pm

BobT

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@RicK G wrote:the K1 K100RS 16V and K1100RS have a 17 front and 18 rear
I'll second that because mine is exactly that.

    

11Back to top Go down    Re: Wheel sizes and their dynamics on Fri Aug 25, 2017 8:52 pm

AL-58

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I know that dropping my front end by 13mm makes a difference that you can feel. I prefer a nimble, agile bike over a bike that is stable at all costs. Same reason I'd never put a shorter shock on the rear to lower the seat height, it slows the turnability of the bike way too much.

A smaller front wheel doesn't just lower the front (which it might not do anyway if the fork length has been altered at the same time to adjust for the height difference). A smaller front wheel turns into a corner more easily as it has less gyroscopic effect which makes the wheel want to stay where it is.

All that said, My K100RT with its narrow 18" front and 17" rear is much easier to ride around town than the RS4V with its wider radials and 17"F wheel which seems to steer like a truck at low speeds.

Al


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'87 K100RS/HRD sidecar (1100 motor)sc25
'92 K100RS-16v (Paint it Black)

'87 K100RT with Paralever backend

"When I'm too old and too foolish to handle a sidecar I'll buy a Sportsbike"

    

12Back to top Go down    Re: Wheel sizes and their dynamics on Sat Aug 26, 2017 12:18 am

floyd

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Hi Charlie,
36 31 2 310 708 CAST RIM, YELLOW FRONT - 3,50X17
36 31 2 311 272 CAST RIM SILVER FRONT - 3,50X17

As stated above, fitted to K1, k100rs 16v 👍

    

13Back to top Go down    Re: Wheel sizes and their dynamics on Sat Aug 26, 2017 2:37 am

charlie99

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as arrived off a k1 rear ...now silver
there may have been more options and I'm sure trapper etc could contribute
not sure about the front though ..





Last edited by charlie99 on Sat Aug 26, 2017 2:41 am; edited 1 time in total


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cheezy grin whilst riding, kinda bloke ....oh the joy !!!! ...... ( brick aviator )

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%O
    

14Back to top Go down    Re: Wheel sizes and their dynamics on Sat Aug 26, 2017 2:41 am

floyd

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Yeah, 4.5x18 rear and 3.5x17 front...

I'm considering the 4.5x18 rear mod to go with my k11 18" front... Or k1200 17's...

Going to put my beast back into RS mode now that I do all my off roading on Yammy XT



Last edited by floyd on Sat Aug 26, 2017 2:49 am; edited 2 times in total


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K100 with lots of K1100 bits - mongrel of a thing...
    

15Back to top Go down    Re: Wheel sizes and their dynamics on Sat Aug 26, 2017 2:44 am

charlie99

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yeah the xt is better for the dirt ....um easier to pick up at least ....depending on how you load it up
water and fuel held high can be a backbreaker

good luck


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cheezy grin whilst riding, kinda bloke ....oh the joy !!!! ...... ( brick aviator )

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%O
    

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