Geometry, rake, handling, high speed wobble & weave

[Bjorn]

Im looking for some advice regarding a problem I've been having with my XS for quite some time. I figured this forum has helped me out quite a lot of times. Maybe there are some people here that can shine some light on the handling/geometry problem i might have got.

Riding at higher speeds 100-140 km/h (60-90 mph) my bike start to feel more and more nervous. Hitting 140 km/h results in what feels as the start of a speed wobble. Very scary!
So riding on the highway is tricky. Cruising around 100-110 km/h is okish, but the slightest input in the handlebar makes the bike react dramatically. I want to get rid of this behavior.

I've done some research, and have already checked a couple of things. But i'm not there yet. Biggest thing to note here, my bike is not stock. The front end is of an Yamaha R6. It's shorter but ive take measures to counter this. I'll explain below.

My bike:

• 1980
• Stock main frame
• stock swing-arm
• non-stock rear wheel (120/90-18")
• Koni shocks 320mm
• R6 front fork (shorter than stock)
• custom triple clamps with step down to compensate for shorter fork.
• triple clamp offset 40mm
• stock front wheel 100/90-19"
• measured rake angle ±26°

Things I have checked:

• wheel run out
• wheel alignment
• tire pressure
• spokes
• swing-arm bearings
• wheel bearings
• head-stock bearings (no play, not to tight, not too loose)
• frame cracks
• new rear tire
• front tire is new
• played front preload setting (lowest - highest setting)

geometry/rake/trail
Im aware this has a lot of influence on the handling of the bike. I've used the following parameters and a calculator from Tony Foale website.

I gathered data online and found various results. However most agreed upon the following:

Stock XS:

• Rake angle: 27°
• offset: 49mm
• radius: 331mm
• ground trail: 113,8mm

My XS:

• Measured rake: 26,2°
• offset: 40mm
• radius: 331mm
• ground trail: 118,3mm

Based on these values, my bike should not be more nervous than stock right?

 

[Signal]

I think you are correct your geometry is what I consider would be within the envelope.
The only time I experienced similar problem the cause was to much weight on the back pack rack, redistributing the weight fixed the problem. Not enough weight on the front wheel.
Another possible cause is a seized shock or broken spring.

 

[Bjorn]

I'd like to figure out why im having this problem. When I see "trail values" of other bikes, they are sometimes much lower without issue. For example I have an Yamaha MT 07 with a trail value of 90mm! without steering damper, feels super stable at high speeds.

Here are my ideas, feel free to add.

• going back to the stock fork is not an option, sold it.
• changing the front tire. I have an Michelin classic road, used to have a Bridgestone BT45.
• measuring the weight balance front to rear
• add steering damper
  • good option, not to expensive either
  • however its not dealing with the root cause
• increase trail value:
  • increase rake angle by making a new top triple clamp that steps down even more.
  • reduce offset by making new set of triple clamps. Kinda expensive. And raises the question, what trail value should I even be aiming for?

 

[Signal]

I would start by making sure all the suspension is working smoothly.

 

[Bjorn]

Thanks for sharing Signal.

The forks have been rebuild last winter (new everything). The rear Koni's too. A local suspension shop gave me some advice on how to modify both fork and shocks in order to reduce damping. The damping was to high resulting in a very harsh ride. Front suspension seems good now, Koni's I rebuild (there is a topic on this forum). They are on the list to be replaced by a decent set like Ohlins by the suspension shop. But I haven't gotten around to do it.

The Koni's are not in perfect condition, but I don't believe they are malfunctioning

 

[Bjorn]

https://www.xs650.com/threads/winter-project-upgrading-time-r6-forks-and-more.56396/post-867211

Koni details

 

[Dunc]

A few things to look at that may cause this.
Are you using the original ball bearings in the steering head? Tapered roller bearings usually help.
Have you checked suspension sag correctly? If the rear springs are too hard on the Koni shocks it can make it flighty.
If the front is too soft it will shorten the trail/wheelbase causing nervousness.

 

[Bjorn]

Its not the original steering head bearings. They are different due to to the R6 fork. They are the ball bearing type. There is no play, and no feelable "dimples". The tightness I find hard to set. Right now its loose enough to let the bars fall side to side, with the wheel in the air and a small input against the bars. Maybe experiment with increasing the tightness?

regarding sag, i did check it before, and the suspension shop measured the spring rate. I believe im in the right ballpark with the sag regarding the front. I'm gonna check again! Because last week ive been experimenting with raising the front preload (all the way to the max). This was also done to increase the trail to see the effect.

additionally the suspension shop said the R6 springrate is higher with 7,6N/mm compared to the 4,4N/mm for the stock XS fork (based on their info). This question came from the quest to make the front fork more comfy. They said i could try a progressive spring. Dont know if any of this is related to my high speed wobble.

 

[gggGary]

tire size to rim width? tall tires make wobble worse especially on skinny rims. The stock tire rim combo is less than ideal.
Lower rear shocks? And maybe get your footpegs forward again. We often under value the effect our own body has as a motion damper.
A steering damper may help.
You might try changing to a low mount front fender or see if removing it changes anything.
The stock frame is a bit "whippy" at the neck.
Like Signal, the only time Madness got squirrely was with heavy high tail luggage.




mass above the red line tends to reduce stability, below it increases stability.

It's the way Erik thinks....

"mass centralization".
Exhaust and rear shock are low and centered.
I'm bad with that trunk up high and waay back.

When I camp the heavy stuff goes at the "passenger position" and panniers on all my bikes.

 

[AUTOMAN]

26° on a XS is aggressive
40MM offset? damn! lowered even
320 rears adds another .5 inch rear up

it is what it is..

 

[JOHNDADDY]

Would a fork brace help?
I've never run one, just the fender alone.

 

[gggGary]

While not identical this kind of illustrates the point, mass below the red line stabilizes, above it destabilizes.

 

[Mailman]

Three times , on different bikes, over the years, I’ve had a nervous or wobbly front end.

‘76 XS650….bad steering head bearings.
‘07 Triumph….defective front tire, a replacement cured it.
‘11 Suzuki V-Strom with a big rear trunk for commuting, going over 75mph upset the bike with that trunk on. The cure? Keep the bike under that speed.

 

[Bjorn]

Thanks for the input guys!!

Front tire: 100/90-19" (got a Bridgestone on its way to replace the Michelin i have now)

Rear tire: 120/90-18". Compared to the original SE tire, this has a bigger diameter of 673mm VS. 640mm. Lifting the back around 32mm (that's quite a lot now that I know...)

Regarding shorter rear shocks, this winter I borrowed a short set from a friend. Did some high speed testing: weird enough no increase in stability. (They were 20mm shorter)

I'm gonna switch my high fender for the OEM one to test! Don't have a fork brace lying around. But the steel fender could indicate if this is worth investigating.

Thanks for pointing out the centre of mass Gary. Even though my goal with this bike is always reducing the weight. Some of it is higher due to my exhaust for example. I got a set of scales coming in, gonna do 2 things:

- check weight distribution front to back.
- determine center of mass.

Another thing I thought of: add a bag of sand to the bike, see what that does.

Another thing I noticed during the last test: IF I stand on the pegs at semi high speed (100kmh) the bike feels much more unstable compared to sitting down....

 

[Kevin Werner]

Front wheel balance???

 

[Ratranger]

Mine would get a weave at 70+. I ended up adding front preload and while a harsher ride it's now stable to 90+mph. Different tires would probably help mine. I'm also set up with an agressive rake of 25* and less trail than stock. But everything has been changed on mine.

 

[Bjorn]

I always balance my wheels statically after changing a tire.

What tire size/models are you running Ratranger?

I always find the terms hard to distinquis. Whats the difference between a speed wobble and a weave?

 

[Ratranger]

I always balance my wheels statically after changing a tire.

What tire size/models are you running Ratranger?

I always find the terms hard to distinquis. Whats the difference between a speed wobble and a weave?

Wobble tends to be violent or fast. Weave is just a slight back and forth wander.

I'm running shinko 705 tires, 150/70/17 rear, 120/70/17 front. Zx6r big piston forks, fzr400 swingarm with fzr600 linkage and r6 shock. IIRC tires have balance beads in them. Honestly I need to change the tires just due to age. I might try dunlop gpr300, affordable and did well in the same sizes on a drz400sm.

 

[Charles13]

Maybe the stronger suspension components are taxing the frame and swingarm to the point that the wheels are coming out of line? I have an rz350 that I am restomodding with 17” wheels, r6 front end, Penske shock with an rg250 swingarm and I extensively strengthened the frame in the swingarm and steering head areas to combat this phenomenon

 

[dave_a]

Bjorn, why don't you weigh the bike's mass front & rear? Then hop on the bike and see where your weight goes. (Perhaps we can compare with a known good bike?)

Have you measured your suspension sag F&R with you on it?

Road racers say the 650 has a rear weight bias. It needs more weight forward for increased cornering. Longer shocks and or swing arm to accomplish that.