Iceliner-Speed Weekend inSweden

[kawabjorn]

Hi

I'm Bjorn from Sweden. I have participated in the Speed Weekend on ice here in Sweden a couple of times. My last attempt was last weekend with my 125 cc Iceliner.
The goal is to join the 200 kph (124 MPH) on ice club with a 125  cc bike.

I have serious problem with bike stability. The bike tends to follow any track on the ice. I have tested with several fork angles and also trails. It would be nice with some input from experienced LSR bike builders.

Some facts about my bike:
- Engine and rear end from a 1991 Yamaha YZ125 MX bike.
- Front end from a mini MX bike
- Approx 18" welded steel tube frame extender between front and rear end. Bolted to rear end. 
- Rear wheel 19"
- Front wheel 14"
- Wheel base 1900 mm (75")
- Weight with driver approx. 190 kg (420 lbs)
- Weight distribution front/rear 48/52
- Variable fork angle 25-38 degrees
- Variable trail approx. 75-125 mm. (3-5") Works best (less bad) at approx. 100 mm trail.
- Seat height approx. 625 mm (25")
- Rear suspension, tested with both original MX damping and also with stiff (iron bar)
- The frame has some sideways flex. When I shake the bike from side to side holding in the handle bars I can feel some flex but not that much.

The bike is more or less undrivable. It is very unstable and extremely track sensitive regardless of speed.
Are the chassis parameters totally wrong?

Any input and ideas are very welcome.

Regards Bjorn

(Please note than english is not my native language and my use of word might be incorrect)

[rgdavid]

I would say the front wheel is much too small,

Rough ground bikes such as mx or grasstrack or speedway use minimum a 21" wheel,
Grasstrack and speedway use 23" wheel to keep straight on the rough,

The big wheel has a lot less rolling resistance,
There is a stronger giroscopic force as well, a lot harder to displace or turn sidewards a bigger wheel so its much more straight line stable.

A bigger wheel will not fall in to the holes so much,
Must be an exiting ride with that small wheel.

[RidgeRunner]

A big +1 on the front wheel dia. being too small.   Larger dia rolls over the rough spots easier, makes for a larger gyroscope and more stability.

Try to isolate and correct the chassis flex.  Are you running a steering damper of any kind?

Also check the rear wheel bearing conditions, rear swing arm bushing/bearing conditions, steering head bearing conditions and adjustment, front wheel bearing conditions.

      Ed

[Peter Jack]

You've identified your problems. As the above have stated, your front wheel should be full size. That will definitely help the problem.

The flexing frame is at least as bad. I would mock up the components with a larger front wheel in their present layout without the frame and then design a frame around them. The mass of bolted connections and probably stock frame components that lack rigidity are most likely your problem. A clean, more simple frame design will not only improve your performance but also make the bike much more easy to work on.

Good luck.

Pete

[kawabjorn]

Hi

Thanks for valuable input.

I had a very strong focus on low front end and low weight, seems to be wrong.
I will rebuild the frame for better stability and also increase front wheel size and also check all movable joints for play.
Yes, I do have a steering damper but it really did not help to much.

Will try to measure the flex and compare to one of my road bikes, just to get a feeling how big it is.

Thanks again.

Bjorn

[Malcolm UK]

Have you tried the bike on ordinary tyres on tarmac or concrete? I have only observed bikes with studs, so cannot be an expert, but surely they and the rutted ice would be the cause of potential handling problems? 

[I am more a car guy, but I worked with a streamliner m/c team in the 90's who found all sorts of 'curious handing' (read as nearly un-rideable) with solid rims on salt and when on a sandy beach. But on rubber and concrete/tarmac the rider set British outright records].

Take a look at the bikes used in the UK for the discipline of sprinting in the 80's and 90's - lots of youtube footage.

[wobblywalrus]

Imagine a line drawn between the centroids of the fork tubes.  The distance between this line and the steering stem axis is the fork offset.  A bike with too much offset handles like you say and is almost impossible to ride. 

[kawabjorn]

"Have you tried the bike on ordinary tyres on tarmac or concrete"
No, only on ice. Might be a good idea to start outside ice. The only problem is to find a decent road without traffic and law....
I´ll check out the UK sprinting videos.

"A bike with too much offset handles like you say and is almost impossible to ride."
My fork has 25 mm, 1" and I guess that's kind of normal.

Bjorn

[wobblywalrus]

An inch offset would be OK for a bike with a normal wheelbase.  Do you grip the bars hard or put a lot of your weight on them?  Can you hold them with a very light grip? 

[floydjer]

Hmmm...I for one, would be gripping those bars very tightly............

[Old Scrambler]

Bjorn.............No engineering background on my part...............just a few miles down the road........off-road.........and the salt-flats for experience. Your front wheel is too small. Your seat needs a butt-stop.  The bars are too wide and flat........and the knobbies should be exchanged for something like a trials or dirt-track tread.

A larger front wheel will increase the trail and help tractability. The butt-stop will keep your body planted to the bike so your hands can relax. Changing the bars to 'speedway' style (akin to hand-milking a cow) will get your arms and shoulders out of the wind. A more solid type of tread will allow for more screws and constant contact with the ice-surface. Higher air-pressure may also help.

None of that is going to cure a flexible chassis or poor wheel alignment. 

[John Burk]

Bjorn , just a little kingpin friction can make even a 4 wheel vehicle handle badly . With the front wheel suspended by a rope to eliminate ground contact friction and with the load you put on with your hands how freely does it steer ? Are the studs in your front tire placed symmetrically ?

[kawabjorn]

"Do you grip the bars hard or put a lot of your weight on them"
This might be something. The upper part of my body is more or less horizontal and I put a lot of weigh on the bars. I have no support at all for the upper parts besides the bars. Maybe this creates the feeling of instability?

"and the knobbies should be exchanged for something like a trials or dirt-track tread. "
Sorry for being ignorant, what are knobbies?

"just a little kingpin friction can make even a 4 wheel vehicle handle badly "
The steering friction is tested and found to be very low.

Many thanks for your input and helpful comments

Bjorn

[wobblywalrus]

Bjorn, that might be a big part of the problem.  All of my bikes including the desert racer are set up and finely balanced so I can take my hands off of the bars and they will track straight with me centered in the saddle and not leaning to the side.  Sometimes I ran the land speed bike with a pussy pad on the tank to hold me up so I had no weight on the bars.  I let the bike flex and weave and give just enough input to keep it centered on the track.  Gripping the bars hard sends the bike into a speed wobble.  Experimenting will tell you if this is a problem.

[kawabjorn]

Bjorn, that might be a big part of the problem.  All of my bikes including the desert racer are set up and finely balanced so I can take my hands off of the bars and they will track straight with me centered in the saddle and not leaning to the side.  Sometimes I ran the land speed bike with a pussy pad on the tank to hold me up so I had no weight on the bars.  I let the bike flex and weave and give just enough input to keep it centered on the track.  Gripping the bars hard sends the bike into a speed wobble.  Experimenting will tell you if this is a problem.

Thanks for sharing experience. Next version of the bike will have a pad.

Bjorn