Triple tree.

[pancho]

  A curiosity question for no other reason than I just thought of it.
 If you change a triple tree on a 'conventional' frame by moving the fork legs further forward [which lengthens the wheelbase] and no change is made away from the original rake angle, what effect is that likely to have on cornering or straight line stability or anything else?
  cheers pancho.    By the way I had fun at Nepean practice on sat.

[brent j]

Using a triple tree with more offset will result in a decrease in trail.

Put simply, Trail is the amount the centre of the tyre's contact patch is behind a point where a line through the centre of the steering shaft would intersect the ground.

Less offset, more trail and vice-versa. Less trail usually means quicker steering and less stability conversely more trail, slower steering but more straight line stability. More trail can also mean more effort is required to turn the bars.

Think of a super market trolley. Trail is from the centre of the contact patch of the tyre to a vertical line through the pivot (vertical because the steering axis is vertical) If you make that distance shorter it will wobble and dart around, make it longer and it follows much better.

hth

Brent

[JC]

Pancho, as I understand it, depending on how much you change it, there's potentially a change in (at least) 3 diff areas: steering, cornering, & straight-line stability. But to complicate matters, there's  overlap & some counter-effects.

With more offset, you change trail (reduced), wheelbase (lengthened), weight distribution (less on the front) & mass moment of inertia of front end about the steering axis (it increases cos forks & front wheel are further from the axis)

Steering:   Brent summed this up pretty well with the changes in trail (ie reduced) w more offset, tho  I would dispute the claim (often made) that it gives "quicker" steering. It feels quicker cos its lighter on account of having less self-centring effect from less trail, & cos of less wt on the front wheel from changed wt distribution. But quickness is a function of rake not trail.

But one of the counter-effects is that the increased mass moment of inertia from more offset can make it slighly heavier feeling to turn the bars. (or at least it reduces the lighter feeling mentioned above) It can also give it more head-shake.

You may also find the front end now washes-out cos of less wt on front wheel giving less traction. One of the 70s cures for Husky wash-out was to reduce the offset.

Cornering: Depending on how much you increase the offset, you may find it doesn't corner in the tight stuff quite as well cos of longer wheelbase

Straight-line stability: Less trail gives less stability (less self-centring effect), but more wheelbase gives more stability. Another one of the counter-efects.

[pancho]

 Interesting stuff, thanks for the input fellers.
Although I am aware of caster angle, effective trail, rake angle etc, it occured to me that there is another factor that I'm a bit fuzzy on, that is, provided that the trail has been returned to original by altering the steering head angle or repositioning the axle on the fork leg, when the machine is layed to one side [in a static state] the centre of gravity of the hole plot is further back from the front wheel which should turn the front wheel with more force toward the other side.
 Has any-one found this to occur and if so to what effect on handling?
    This stuff came into my head after a chat with some-one who wanted to fit a larger diameter front wheel but had insufficient clearance from components that can not be moved out of the way.
  cheers pancho.

[brent j]

To add a bit to the mysteries of rake and trail.

My XT500 is a mixture of parts, most have been decided on by trial and error. Believe me there was a lot of error involved.

Currently it has a PE swingarm, Yamaha wheels and Yamaha YZ triple clamps (38mm) with Suzuki PE forklegs.

On Friday a mate and I took the bike out to try some triple clamps with different offsets and different fork heights.
Due to having recently lowered the rear of the bike to stop it kicking under drive on bumps, I had slid the forks tubes about 75mm above the triple clamps to get back to about 27-28 degrees of rake. The forks have YZ250G damper rods so are quite long and have 11" of travel.

The first change was to drop the fork legs down til they were level with the top of the clamps. We rode it like this and then tried about 6 different heights but very quickly realised we could not improve it, only make it worse!
We ran out of time to change triples and as it turns out I forgot the required axle spacers anyway.

The bike looks like a chopper! Funny thing is it looks very much like a Cheney Yamaha 500. It has a rake of 30 degrees and 135mm of trail. Everything I know about steering says this thing should not turn a corner but it does.
I raced today and having no vintage class I raced in C grade and Vets. Now anyone who's seen me ride knows I'm not fast but I was turning inside people today and doing it with both feet up! There was simply no need to put a foot down on most corners. I was passing people by turning inside them! The front wheel would just stick and the bike would follow. I now realise how this bike used to push as I put power on but not now.
Straight line stability is fantastic and it will take any line the rider wants through a corner.

I'm still going to try the different triple clamp offsets but I have every detail of the current set up written down!

Lately I've been riding my TM125 as it's been more fun than the 500 was, not any more

[pancho]

Hmmm

[JC]

Very interesting Brent. Its sort of counter-intuitive isn't it.

30deg/135mm was reasonable for early/mid 70s bike, but for one w 11" travel...

Methinks weight distribution has more affect on steering than most realize. Perhaps you've just hit the sweet spot of wt dist for that bike.

Dropping the rear end will have moved the wt dist forward a little.

Dropping the front 3" (ie raising the fork tubes) on a 30deg rake reduces wheelbase by 1.5" at the front end which is a lot & moves the wt dist even further forward. Perhaps too far.

In what ways was it worse when you dropped the front end? (ie raised the forktubes)

[brent j]

This bike has been changed so many times in 9 years. It’s been high (which looks really fast) and it’s been low. I’ve tried triple clamps with different offsets and the frame has been deraked. I’ve also run 4 different length swingarms.
This only refers to the last lot of changes, not the last 9 years!

I dropped the rear of the bike until it stopped kicking under acceleration on square edged bumps. I figure the steeper swingarm angle was pulling the rear axle under the bike (anti squating) and causing the rear to stiffen up.
I was running the rake at about 27-28 degrees and thought it was quite good at turning. At this setting there was 75mm of fork leg above the triples, with the current forks.

To ensure there was a noticeable change we put the fork legs flush with the triple and tried it with care.
The bike turned into corners pretty much the same. At the point where you make the turn there was no longer any push when the power was applied. I can sum this up by saying that you now had the confidence to open the throttle and not have to wonder if the front would hold or not. With or without power the bike just simply turned. Exiting a corner it was easier to hold a tight line.
Corners could be taken tighter with confidence, on a really tight corner you can just turn the bars and the bike will follow. The steering feels lighter. The bike will slide much more predictably than before. Straight line stability is total.

We pulled the fork legs up 20mm and tried it. The preciseness in the steering had gone. When sliding it almost had a dead spot in the centre and you had to keep correcting. The ability to turn into a tight corner was not there and the front felt like it would push but didn’t.

We dropped the fork legs about 15mm down into the top clamp (not recommended!) the bike became very tail happy and would slide at any excuse. The steering became a bit slow it and wouldn’t hold the inside of a turn.
This Sunday I have two other sets of triple clamps to try, one with more and one with less offset. I’ll also be trying different fork heights, fork springs and another pair of shocks with the same springs but different damping. I have a friend who is a far better rider than me to help. I ride first but don’t tell him what I think. I don’t tell him what I’ve changed and then quiz him when he stops. So far we’ve reached the same conclusions.

This link (scroll down)
http://forums.mxtrax.co.uk/showthread.php?t=36080&page=4
has a pic of a Cheney Yamaha 500. I’ll try and get a pic of mine but the proportions are much the same. When I first saw this pic a few years ago I thought it looks like it wouldn’t turn but Mr Cheney knew a bit about making motorcycles handle

What have I learned from all this, that I know far less about motorcycles than I though I did.

[pancho]

  ME to!

[crash n bern]

You blokes might be interested in this thread.  The OP writes some interesting threads.  In this one he tilts his motor and drops the alingment of the countershaft sprocket to get a better center of gravity.

http://husaberg.org/forum/viewtopic.php?f=8&t=13268

[Curly3]

That's exactly the theory behind the current design of Speedway bikes, Crash n Burn.
I missed a lot of the development history of " Modern " Speedway and I'm not sure exactly when they went to " Laydown " engines but it was at least 15 years ago.
There are some interesting observations and theories on this thread, food for thought.
Steve

[pancho]

 One of the big things about all this is like was said earlier, WRITE IT DOWN!
 I used to write everything down once but these days I'm getting older and lazier.
  cheers  Main thing I'm getting older, better than the alternative!

[brent j]

One of the big things about all this is like was said earlier, WRITE IT DOWN!
 I used to write everything down once but these days I'm getting older and lazier.
  cheers  Main thing I'm getting older, better than the alternative!

Good point, when we changed my XT front end we both wrote down what we thought, separately.
And after every change we changed back to the previous best setting just to see if it was an improvement or just perception.

I took the bike out again on Friday and tried a set of XR triples with 3mm less offset than the Yamaha clamps fitted previously.

With the same rake the bike lost the precisness in corners, especially tight corners but was more stable (something it has in spades anyway) and it now oversteers from the back noticably in fast corners. It would make a great flat tracker.

I was going to change back to the Yamaha triples for a final comparison but simply ran out of time. I was a bit annoyed as I had to race the bike as it was on Sunday. I'm not saying I would have done any better but I'm sure it would have been easier to ride if it was back as it was.

[crash n bern]

That's exactly the theory behind the current design of Speedway bikes, Crash n Burn.

Which is what they've done with the new Husabergs.  I got to test ride one recently and was amazed at how it handled.  It definetley make an average rider good.  The bike was so effortless to ride.  Haven't been so impressed in a bikes handling since I jumped of my DT250a and got a test ride on a 250 Pursang when I was 14.

Brent, being a TT500 fan I'll be keeping a close eye on your findings.  Thanks for posting them.

[brent j]

Unfortunately I don’t think I’ll be riding again before July so won’t have any more updates for a while.
After our holiday I’ll take a week off and just play with the XT.

I’ll change back to the Yamaha triple clamps but will run it with the Honda clamps first to confirm the difference.

My current frame has been deraked, I could set up a standard frame and achieve the same swingarm angle and rake as the best set up I’ve found so far but the front would sit lower and the wheel base would be shorter. It would be interesting to try (and a lot of work) and set up another bike close to the same to see how the different weight distribution and wheelbase would change things.

All the things I've written about XT's in the past is now out of date and the bike has been a work in progress for many years now.

How do you make one handle well? I'll let you know when I find out but I can tell you how to make one handle badly!