Motorcycle rear suspension -- or not

[av8ford]

I'm in the planning stage of building a small-displacement motorcycle for LSR racing and have a fundamental question about rear suspension. From reading build threads and looking at a number of special-construction motorcycles over the last several years I see that many are running without rear suspension. My question -- and a concern -- is how are handling and stability of rigid rear "suspension" affected by chatter-marks (washboarding) and overall rough surface conditions?

I'm open to new ways of doing things, so long as they work, and a rigid rear end would certainly simplify the build process.

Mike Bishop

[JonAmo]

How fast do you want to go? I do not ever remember any 220+ bikes not have suspension. Some have tried but later went back to suspension. Cars a different story.

Jon

[Bob Drury]

  Jon, this is probably a stupid question, but do you bike guys add much or any weight to run on the salt and if so on a open bike like you last rode, how much?
 I was just wondering if any of the non suspension bikes tried more weight in a effort to make them work?                      Thanx, Bob      

[av8ford]

How fast do you want to go? I do not ever remember any 220+ bikes not have suspension. Some have tried but later went back to suspension. Cars a different story.

Jon

I have several 160cc and 175cc Honda twins (early 360-degree cranks) and will be satisfied to run better than 100 mph unblown gas in a APS configuration. I think that 130-plus mph wouldn't be unreasonable with one of these motors running in BF.

My concern is primarily with getting what little power we'll have onto the racing surface.

Mike

[55chevr]

I run on pavement and the salt flats with a rigid drag bike frame ... there is a bump, bump, bump on runway pavement when you run over the expansion joints but is not disconcerting. Salt flats I never noticed any bumps. Fastest I have gone on pavement is 153 mph.  Aaron Wilson has gone 192 on the salt flats with a similar rigid frame and he said he had no handling issues.

[av8ford]

I run on pavement and the salt flats with a rigid drag bike frame ... there is a bump, bump, bump on runway pavement when you run over the expansion joints but is not disconcerting. Salt flats I never noticed any bumps. Fastest I have gone on pavement is 153 mph.  Aaron Wilson has gone 192 on the salt flats with a similar rigid frame and he said he had no handling issues.

How much weight are you carrying, how heavy is your motorcycle? Also, what tire/wheel sizes and tire-pressure range are you running?

Mike

[55chevr]

Buell XB9 engine - 72" wheel base drag frame ... 200 lb rider ... Pirelli Super Corsa front 120/70ZR17 and rear 180/55ZR17. 40 psi. in both.

[fredvance]

Forstall Deluca Knecum's Guppy is rigid in the back, 254mph. I tried a strut on my bike, last year, I did not like it at all. Put the shock back on after the first run.

[av8ford]

Thanks for your answers, guys. I'm still not clear about the effect on a small, lightweight motorcycle with less than 50 hp.

Mike

[Trouble]

I only did drag racing.  Struts suck.  Unless you are in a perfect straight line, shifting upsets the bike.  I went quicker with 1" of shock travel.

[dr j]

Mike,
My main LSR bike has been a 94 RS125 chassis with a Honda CRF250R motor.  When I 1st took it to Maxton I used a rigid rear since I had to move the upper rear shock mount back to clear the head and carb.  Maxton was very bumpy and I found myself trying to avoid the main bumps rather than concentrating on the engine, tuck etc.  I hated it so found a pitbike shock that fit and loved it.  Have used it eversince.  At that time I was before nitrous and my more streamlined tail so was running about 107mph.  Bike weighs 225lb.  I would expect the rigid rear to be no significant issue at Ohio where I usually run now.  But having just come back from Bonneville I would not be thrilled with turning off the track into the crunchies with a rigid rear.  

[wobblywalrus]

Mike, the folks I associate with are pretty evenly divided between those with and without suspension and have success with either setup.  My race bike is my street bike, too, so suspension is desired.  I never felt it handicapped the bike in any way.  The thing about boingers is it gives you an opportunity to set things up wrong - like having no damping.  The rigid guys never have to worry about that. 

[salt27]

When I put struts on my H1 it eliminated the hinge in the middle feeling and was well worth the rough ride.
I drag raced it and rode it on the street, never took it to the salt.

  Don

[beerbellykelly]

we ran rigid on the salt this year and found it easy -thanks to all the efforts everyone has put into getting the salt to the best shape for a long while-
heavy bike ran 164.429mph

[SaltPeter]

In 2010 at Lake Gairdner, I ran a stock Suzuki RGV 250cc (2 stroke V twin) at 119mph, it was stock with no fairing.

It is a light bike 306lbs/139kg dry weight and I am glad I had suspension.

I eased it to full RPM in 1st then accelerated as hard as I could till it was topped out.

It had some small amount of wheelspin once up to speed, the rear tyre wore to flattish profile in the middle after several runs, but it tracked really straight.


Pete