Shortening an FXR swingarm

[MrMarko]

Would have to have a closer look at it but, I thought the retaining protrusion on the swingarm would hit the back of the slot on the carrier it moved further forward than it was designed. Anyway no big deal.

Unfortunately the old Mt Wellington club hack was stolen a few months back. Henk Zeeven has fronted up with a new one.

Yeah it will hit thats why i was saying make the slots a bit longer

Ahhhh bloodey BASTARDS. i bought it as an insurance write off origionally, only had fairing damage. thats a shame.

[richban]

The biggest problem and also the best part of the FXR chassis was the flex. They never feel very precise when pushing them to the limit. But on the plus when the front starts to tuck it gives you like an hours notice.

I would look at flex rather than making it hi side better. my 2 cents.

[Wil_K]

That's a short cut, yes. It can work well enough too.
To get weight forward on the basically stock chassis the easiest way is to move you...Clipons with a forward extension, a seat hump - taped as a trial.
My suggestion of adjustable steering head would be worth pursuing. have a look on the chassis thread for the RS125 geometry and compare. Measure, adjust and try.

Thanks again for the info.

[Rick 52]

Worked very well for me, 25mm shorted and adjusted the forks to compensate steering, Rods FXR had 50mm taken and he loved it .

[Wil_K]

The biggest problem and also the best part of the FXR chassis was the flex. They never feel very precise when pushing them to the limit. But on the plus when the front starts to tuck it gives you like an hours notice.

I would look at flex rather than making it hi side better. my 2 cents.

Interesting. I am pretty uninformed on motorcycle geometry in general, but really know nothing about chassis flex. Very keen to learn and try out different ideas. I am more interested in having the sharpest handling FXR than the most powerful.

What aspect of flex would you say is the weak point on an FXR. Torsional, lateral?

[Wil_K]

Worked very well for me, 25mm shorted and adjusted the forks to compensate steering, Rods FXR had 50mm taken and he loved it .

Wow 50mm is a lot. Was that all from between the pivot and the shock? Do you know if he moved the shock mount?

[Michael Moore]

Small light bikes with short wheelbases can be sensitive to changes in rider position, especially with a larger rider. Here are some notes I wrote up about measurements/calcs I did for me on a Honda CB160 vintage racer, which may be a slightly shorter WB than your FXR150. The AHRMA 160 racers with stock chassis are generally doing everything they can to move the rider forward -- short tanks instead of period-style breadloaf tanks, forward offset clip-ons, etc. There are a few people who've gone to lengthened swing arms and they seem to like that.

My slow-rider opinion is that very short WB may be fine if you've got a diminuitive 110lbf rider. For anyone of some size it may be counterproductive. Also, I'd rather be on a bike where I fit properly as I don't enjoy having leg or hip cramps in the middle of a race.

I'd suggest you do some similar calculations/measurements and see what your starting numbers are before chopping up parts.
------------------------

Mon Apr 27, 2009 7:44 pm
#12379 April 27, 2009

I got an email asking me about my weight distribution comment so after
I got the 160 unloaded I found a level spot on the garage floor and
some boards to make a spacer the same height as my digital freight
scale and did some weighing:

221 lbf -- for a 6' tall rider in full gear (Shoei RF1000, Alpinestar Super
Tech R boots, Held gloves, and Helimot leathers, back pad and chest
pad) (197 lbf stripped stripped weight this morning). That's 24 lbf for
protective equipment, but I don't think I'll go the Rollie Free route with
swimming trunks as the riding gear.

246 lbf -- CB160 with approx 2/3 fuel load, stock tank and fenders
50" wheelbase

For a different project I'd done a CoG spreadsheet that let me plot
horizontal/vertical Cartesian coords for the mass center of each part on
a bike which would then be summed for an overal CoG number. I
modifed that to do just horizontal weight distribution and plugged in the
above info. I've rounded slightly on some of the following numbers to
make them easier to deal with.

Bike only weight distribution: 48.4%F, 51.6%R

Rider in full tuck scooted up against the back of the fuel tank (a position
which had my eyes several inches in front of the front number plate so I
can't imagine riding like that):
47%F, 53%R

Rider scooted backwards against seat back (which is 33.5" behind the
center of the steering stem nut):
42.8%F, 57.2%R

So when you add the rider in a typical tuck the bike becomes
significantly tail heavy

With a 54" wheelbase, bike CoG and rider kept at the same distance
from the front axle (this could be done by moving the engine a little
forward to offset moving the mass of the rear wheel back, basically
what I did on the 216)

Rider in full tuck scooted up against the back of the fuel tank :
51%F, 49%R

Rider scooted backwards against seat back:
47%F, 53%R

A longer wheelbase would still have the bike be a bit tail-heavy, but with
the rider's butt moved back the overall weight distribution would be
essentially the same as the standard wheelbase bike with the rider in a
maximum forward riding position which is unlikely to ever be seen in
real life except momentarily during a trip over the handlebars.

If the above are recalced with a 20 lbf lighter bike but the bike only CoG
being kept constant (so the weight is evenly pulled off the bike) the
front/rear distribution will shift very slightly to the rear (under .3%)

cheers,
Michael

[TALLIS]

I would definitely agree that felling comfortable on the bike is key, so you can ride the bike, not the other way round. I also believe an adjustable headstock is more beneficial than shortening the swingarm, but no harm in trying, you might as well change the shock linkage system as well, my old fxr had that. To be honest, was next to no difference in feel, and we'll all know how well Adlam jnr can pedal his stock framed fxr.

[Tard]

Hey Wil,

As we all know, there's always different theories on how best to achieve 'something' in your case its to improve turn-in and handling in the tight turns better?

I'm no engineer, or fast either however; tend to agree with quite a few different points-of-view so far

My 2c...
I ride a short wheelbase bike - so on this point agree with Michael Moore. I'm not particularly light and the shorter the bike, the more sensitive it is to weight distribution changes...
Coincidently I've just changed to a slightly different bike. Overall wheelbase is the same, frame is stiffer (agree with Rich there) and steering head angle on the new bike is 'slacker' - this was intentional as the old bike felt very 'twitchy.'
New bike feels more stable BUT...initially I was sitting further back which meant less weight over the front wheel...so the combination of getting weight over the front of the bike and the head angle seems to be a key factor - I think that's why we don't see too many choppers being raced

If you've got the means/skills/time/patience, go for it - whatever that may end up being - I think that's one aspect of what 'F4/F5' is all about...and then go hard...until you 'find the limit'

[andrew a]

To be honest, was next to no difference in feel, and we'll all know how well Adlam jnr can pedal his stock framed fxr.[/QUOTE]

I got told it depends if you like a high side or a low side. Long swing arm=more likely low side Short= more likely high side. The wheel axle holders are cast and I would look at removing them for weight saving. I do get annoyed when Adlam jnr beats me on a standard frame FXR and I have the fast motor!

[MrMarko]

If you just want turn in and weight over the front drop the forks through i do on all my bikes.

The Honda dominator was a comfy tourer stock sure but it wouldnt turn in worth a damn, dropped the forks 38mm and it goes around a corner now, just need some bar risers because i'm becoming a sore old man.

bloodey sportsbikes.

[MrMarko]

I got told it depends if you like a high side or a low side. Long swing arm=more likely low side Short= more likely high side. The wheel axle holders are cast and I would look at removing them for weight saving. I do get annoyed when Adlam jnr beats me on a standard frame FXR and I have the fast motor!

Count the teeth on his sprockets

[chrisc]

Worked very well for me, 25mm shorted and adjusted the forks to compensate steering, Rods FXR had 50mm taken and he loved it .

I wonder if he would still like it after riding his TZ125 framer.

[Rick 52]

Wow 50mm is a lot. Was that all from between the pivot and the shock? Do you know if he moved the shock mount?

Yes between the Pivot and shock, I used a 25mm hole saw and cut as close as possible to the pivot tube as possible then welded the pivot tube into the back of the new hole, with a long rod through the pivot tube and the axle slots you can measure for centers and alignment .

Standard FXR Wheel base is longer than a RS wheelbase and they are a little long for a Kart circuit .