Shortening an FXR swingarm

[Wil_K]

Hi all,

I am interested in shortening the swingarm on my FXR. Has anyone here done this modification that can offer some advice?

Mainly I want to know:
1) How much can it be shortened by? I don't want to take too much off and ruin the handing. I don't mind moving the shock mount back if needed to keep the suspension ratio and ride height similar to stock.
2) Are the handling improvements noticeable and significant enough to reduce lap times?
3) Does anyone have a spare swingarm they want to sell?

Cheers

[Askor]

Just ride the damn thing. The time, effort, and money you spend attempting to shorten the swingarm would be better spent on more track time.

More laps = more better

[Wil_K]

I am an engineer and I enjoy making improvement to the bike as much as riding it. Cutting and welding the swingarm isn't going to cost me a cent. I am a quick rider and get as much track time as I can, never miss a practice or race day. The four weeks between meets can go to improving the bike.

[Grumph]

I am an engineer and I enjoy making improvement to the bike as much as riding it. Cutting and welding the swingarm isn't going to cost me a cent. I am a quick rider and get as much track time as I can, never miss a practice or race day. The four weeks between meets can go to improving the bike.

While the above is admirable, think about what you want to do a little longer...
Shortening the arm puts more weight on the rear wheel - taking it off the front...
The class leaders in NZ using FXR engines are undoubtedly the GPR boys - and those frames have more front end weight bias than OE - not less.
And a more radical steering geometry to go with the weight bias.
If you want a project for your FXR, look at doing an adjustable geometry steering head bearing setup, the rewards are potentially much better.

[chrisc]

Wil, chat to Rick Ford. He did it on the FXR he used to have.

[chrisc]

Come on guys, shortening a swing arm and trying it out will mean he learns something. Instead of saying don't do it, maybe share some insight into what might happen so he can give it a shot. It's only a FXR.

Wil, I have a spare FXR swing arm I don't need that you can modify.

[Bert]

Nice words Grump.

A long time ago I managed to shoehorn a TF100 into a FXR frame (prior to the rule change allowing FXRs).
I chased around handling for a long time until I found some fork modification suggestions. I found that getting the weight forward and getting those slicks working (closer to their intended operational loading the temperature yielded the best results).
Drop the forks through 30mm, add 8mm of preload, 10 weight oil. Stage2 changes to the damping holes (this is actually covered in the FXR forum).
but I am slow...:weep:

[Henk]

Wil

I have two spare swing arms if you want to have a go at chopping one up.
On the plus side if you modify one and hate it swapping it back is a fifteen minute job as long as you have a spare chain the right length on hand.

[Wil_K]

Grumph:
Thanks for your input. That is just the sort of information I was hoping to extract from the experienced folk. So if the swingarm is shortened, then would a solution to the rear wheel weight transfer be to increase the rear ride height to shift more weight to the front wheel and at the same time making the steering geometry more aggressive?

Chris:
Thanks for the swingarm, I will most likely take you up on that offer. Will get in touch via FB if I do. Yes I have heard of Rick Ford having done it, and know Terrene's FXR had a shortened swingarm. I had a look on Saturday and it didn't look like much length was removed. It was carted off by its new Wellington owners before I could get the tape measure out. She said she could feel the difference compared to a stock FXR, didn't get a chance to talk to Rod about it.

Bert:
When you say drop the forks 30mm do you mean 30mm from the triple clamp to the top of the fork tube? FXR manual says 50mm, but this seems like too much. Most FXRs i have seen don't have it that high. Mine is 45mm. I have cartridge emulators fitted. Too early to tell if they have been a worthwhile investment. Have done one meeting since fitting them with the recommended baseline settings. Front end feels good, I had no problems with the front over the weekend. Rear was sliding lots, especially on the Saturday with the cooler track. I would like to be able to turn the bike tighter and faster without feeling like I am close to losing control which led me to the whole shorten the wheelbase thing.

[MrMarko]

I would simply slot out the axle adjusment slot further first... get it as far forward as you can without the middle of the tyre hitting the swingarm, shorten the chain to suit and trial it.

[Wil_K]

Wil

I have two spare swing arms if you want to have a go at chopping one up.
On the plus side if you modify one and hate it swapping it back is a fifteen minute job as long as you have a spare chain the right length on hand.

Thanks Henk, as always you are one helpful and generous bastard. I will probably grab that one off Chris. Unlikely he will ever get on one of those Malaysian commuter bikes again now that he has moved on to his GP framed machines.

[Grumph]

Grumph:
Thanks for your input. That is just the sort of information I was hoping to extract from the experienced folk. So if the swingarm is shortened, then would a solution to the rear wheel weight transfer be to increase the rear ride height to shift more weight to the front wheel and at the same time making the steering geometry more aggressive?

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.

[Wil_K]

I would simply slot out the axle adjusment slot further first... get it as far forward as you can without the middle of the tyre hitting the swingarm, shorten the chain to suit and trial it.

I like your thinking. That is a pretty good idea. Could go 15-20mm further forward by the looks. Guess I would have to modify the brake carrier too.

[MrMarko]

I like your thinking. That is a pretty good idea. Could go 15-20mm further forward by the looks. Guess I would have to modify the brake carrier too.

The brake carrier should slide forward with the axle? can't recall its about 6 or 7 years since i had my fxr bucket. i sold it to the club and its used as the bike for people to try out buckets on at mt wellington. still around apparently.

[Wil_K]

The brake carrier should slide forward with the axle? can't recall its about 6 or 7 years since i had my fxr bucket. i sold it to the club and its used as the bike for people to try out buckets on at mt wellington. still around apparently.

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.

[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 :D

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 :niceone:

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 :D

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' :yes:

[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. :crazy:

[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 :shifty:

[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 .