Straight line stability

cbertozz

cbertozz

Hero member
Location
Australia
So having lengthened the rear shocks and pulled the forks up higher through the clamps for racing I now have a quite distinctive wobble at freeway speed. It?s not an issue on back roads but freeway feels a bit unnerving.

Conscious that playing with the geometry can be risky but thinking I should try just shortening the shocks a little first. Or should I fix both ends at once?
 
Should have done your homework first...

Jacking up the rear and lowering the front steepens head angle, shortens wheelbase and reduces trail, good for quick direction changes but no good for straight line stability.

No way to have the best of both worlds with our clunkers without delving into major surgery.  Steering head angle, fork yoke offset and swingarm length can be changed to achieve something close to what you're looking for.  Other option is to buy a RSV/CBR/FZR/GSXR etc, you get it all inclusive.

piet
 
I also have on occasions a weave, I am not sure why. It was apparent a lot during the Tassie ride but that was shocking windy. My bike can be affected following big trucks and that's aerodynamics at work. I need to do a little fairing mod I hope improves that. My forks are through from memory 25mm and I liked that effect. When I added the longer rear shocks I made a top mount for the standard length shocks that I used to see what would happen. It ended up longer than I really wanted and at 30mm making the seat hight way to high but it did make the steering way to fast for me so I ended up jacking the rear 15mm. It's great for the typical close to Sydney tight stuff. I haven't found the need for steering damper so it's still reasonably stable even on bumpy stuff. What is important is wheel alignment, I like to check swingarm centre to rear-wheel centre. I use a couple of bits of 2x1 bolted together with nails in the end and I use it as a compass or divider to check that distance is the same both sides.gets past the mufflers when measuring this. Tyre pressures have caused this to me as well, my last tyre BT016 would weave if the front was low, much better at 37psi. What's there now, T30 likes lower pressures,34psi.



 
Very important also not to be holding on tight and let the bike move a bit, which can be hard going fast or into headwinds without a good fairing.
 
shagged front fork springs do this as it drops the front end changing the geometry in all the ways that Piet describes.
CLEM
 
sfcpiet said:
Should have done your homework first...

Jacking up the rear and lowering the front steepens head angle, shortens wheelbase and reduces trail, good for quick direction changes but no good for straight line stability.

No way to have the best of both worlds with our clunkers without delving into major surgery.  Steering head angle, fork yoke offset and swingarm length can be changed to achieve something close to what you're looking for.  Other option is to buy a RSV/CBR/FZR/GSXR etc, you get it all inclusive.

piet
Click to expand...

Ummm to be fair I did my homework Piet, the operative words were ?for racing? and that was your advice to raise the bike if you?d like to dig out your post  :LOL: :LOL: Now, after a couple of years, the bike is no longer racing so it?s time to rehabilitate.

Anyway, glad to see a fairly unanimous agreement. I also agree Tippie, I hold very lightly when I can but in strong gusty winds that can be a bit challenging. I?m going to start by lowering the rear a bit at a time and see how we go as I note in the old Laverdamania suspension articles it suggest the front should be dropped anyway.
 
To be clear when I said my bike can weave here is what I meant. I made a note of this heading home from the last Eastern Creek race meet after watching the Redax bikes. So that's a good 50 to 70ks of expressway to my place. It was windy and I was doing my usual 117 or so just above the traffic speed and passing a lot of big trucks, boring ride I want over as soon as possible. That's when I noticed this weaving, behind big trucks and on a windy day. Hardly surprising really. I do want to level the lower part of the SS bevel fairing as it's tilted down at the rear and I think that's not so good at cutting the wind smoothly. It's on the list of stuff to do
 
I think you're right to start everything the whole package back towards something resembling standard, Carlo. With the improvements you;ve made in suspension it should realistically be a much better package than a std triple.

Once you make (fairly) radical changes each part of the equation plays its part to varying degrees. I had the opposite problem when I went from 18s to 17s and a modernish front end with about half the offset of the std triple clamps. A 29 degree rake with 25-30mm offset was giving me absurd trail figures and the steering, while stable, was not right. The new setup on my SFQ is 25-26 degrees of rake (big job!!), 30mm offset, fully adjustable and rigid USD forks with long caps, and triple-length shocks, to retain ride height and cornering clearance with the 17s, a longer (lighter) swingarm to counteract the loss of 65mm of wheelbase ...

IMHO, with something like a triple (high CoG, looong unsupported forks, not particularly rigid frame) on the road you want stability first and modern flickability a long way back in second.
 
Vince said:
To be clear when I said my bike can weave here is what I meant. I made a note of this heading home from the last Eastern Creek race meet after watching the Redax bikes. So that's a good 50 to 70ks of expressway to my place. It was windy and I was doing my usual 117 or so just above the traffic speed and passing a lot of big trucks, boring ride I want over as soon as possible. That's when I noticed this weaving, behind big trucks and on a windy day. Hardly surprising really. I do want to level the lower part of the SS bevel fairing as it's tilted down at the rear and I think that's not so good at cutting the wind smoothly. It's on the list of stuff to do
Click to expand...

Yep wind is a factor, but an additional factor, the other feeling is still there. I will shorten the shocks before I see you guys at the weekend.
 
It's always handy to do a full-scale scetch of your bike to see what its actual trail figure is. Drop a plumb bob to the ground and mark it and extend the fork line and mark that and the distance between is the trail figure. We did that for Crispins Space Frame expecting the usual 120mm plus for old school slow steering geometry, what we got was from memory around 70mm. That's modern GP fast steering geometry. You can track what this dimension is and any changes that come from jacking the rear or lowering the front.
 
Make sure the following factors do not mask the real issue (and sorry if this feels as if I question your skills or knowledge, it is not intended that way!! Only intention is to help and avoid something is overseen):

-rear suspension setting, especially rebound to soft
-your gear is still leather and not fabric (friend of mine had that when he went from leather to fabric...)
-wheels are balanced?
-Air-pressure right?

My bike was also raised at the rear and lowered at the front and it is still very stabil. But then I am very heavy and so the real riding hights might be different to yours.

HTH
 
Geometry aside, loose or worn head stem bearings can cause a wobble. The test is to ride in a straight line, lock your knees in the tank and let go of the hand grips. Wobble will be obvious if the bearings aren't right.
 
Lots of good advice there.  I would be interested to see the trail, will try and do that, in fact I seem to remember buying a program that could do that years ago. The basics are Ok, the tyres are good, tyre pressure checked daily, front was new recently and rear is still very evenly worn. Steering bearings were replaced not so very long ago, but I will check that the head is properly tightened although it feels good.

My definition of wobble some might call a vibration - its only just translating to a tiny bit of movement of that ilk in the bars not a proper wobble, and the wheels still feel planted. It's very very like heavy wind is causing it, sometimes when you have hands on the bars on a naked bike a little movement is inescapable, and its better when laid flat on the tank.

I dropped 10mm at the rear with the adjustment on the shocks today so will see how that pans out before I make any other changes. Test ride today, on a road I'm comfortable with I was riding the bike around fast sweeping turns in full attack mode, balls of the feet, very light touch on the bars and it felts good. Was steering more with the foot-pegs and body weight than the bars. For the first time I was able to really ride out of the village without fearing for my spine so it definitely helped there.

Now I just need to stop the bike imitating a British bike and dropping oil everywhere when it stops!
 
cbertozz said:
Now I just need to stop the bike imitating a British bike and dropping oil everywhere when it stops!
Click to expand...

That is not oil, it is Laverda swet. Race horses always swet when they got ridden properly... ::)

:D

to the steering bearings: good advice from Chuck, had forgotten that. Make sure though you do not overtighten the bearing. The right setting is achived when they are almost lose and just before you can move the triple-clamp up and down. really not tight, just not "clappering" anymore if you will. Everything tighter might feel similar to your description and the bike is not really turning in easily anymore.
 
+1 on the steering head bearings. If they have been adjusted too snug [poor mans steering damper] they will cause a weave due to picking up and transmitting road imperfections through the bars. It usually goes unnoticed on the track because you are focused on other things along with the death grip on the bars. Tire wear from track use [more wear on one side] can induce a vague feeling at certain speeds too.
 
In this instance it?s not the bearings being too tight - I know exactly what that feels like. But could be because they were a fracTion too loose.In fact I tightened a little as we have absolutely horrific roads here and the front which is stiff anyway has been taking a hammering. Felt better after tightening but haven?t been on a freeway to test.
 
The funny bit is you have only ridden the better smoother roads we do. I am pissing myself at the moment. Wait till you do the rough ones. And even those have had some work recently, they tared over the bumps and there still as rough as guts. Your in for a real shock if you get around Dungog Vacy area, they patch the patches. The local bumpiest are the Bells Line up around the Sydney end of Bell, Wisemans Ferry Rd past the Oaks turn off, Spencer along the Hawkberry River and the back way to Wollembi at the upper Putty Rd end. All teeth rattlers.
 
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