Modified Class Wheelbase

[Old Scrambler]

I see the 2013 rules will limit the wheel base on Modified chasis to +10% over production. What about axle adjustment for switching sprockets?  My current factgory swingarm is extended and is about 1-inch longer than the new rule will allow if based on the published manufacturer data. My Triumph Cub is listed in the books at 49 inches.......but that is with the standard (early) front forks. Thousands of later models where shipped with the slightly longer T100 forks which extend the wheelbase by approximately 1-inch to accomodate a 19-inch front wheel versus the original 17-inch units.  I am searching for authorized data to support the longer wheelbase......but have not yet found it.

I could machine the axle-slot in the swingarm and use a longer adjuster bolt to meet the requirement.....but only when the axle is foward to the end of the slot.  If I change to a smaller sprocket with the same chain will there be an adjustment factor allowed?  If so, how much?

If I shorten the swingarm the shock mounts will scew the chasis from its current GREAT status by changing the angel of the shocks. 

[Old Scrambler]

Partial answer was found.......According to MOTORCYCLIST Magazine, Oct. 1966, the Mountain Cub had a wheelbase of 50-inches as printed in the TRIUMPH sales brochure. So my 55-inch wheelbase should be good to go in the modified class......But would still like to know what adjustment factor will be allowed.

[Old Scrambler]

So I am still looking for an answer...........the new regs say the measurement is from the rearward-most position of the axle.  The stock bike has a 50-inch wheelbase (as published) when the axle is about a 1/4-inch from the front of the adjustment slot in the original swingarm.  It can be moved toward the rear of the slot with the standard adjusters another 3/4 inch.  That means to me that the stock wheelbase is 50.75-inches when measured from the rear-most position.  So 10% equals 5.075 inches and the rear-axle position of my modified would give the bike a maximum total wheelbase of:  50 + .75 + 5.075 = 55.825-inches.

My existing maximum wheelbase is 57.12 inches................my shortest adjusted wheelbase is 56-inches.  I plan to cut 1.5-inches to give me a maximum of 55.62-inches.
   
Am I OK?

[Grandpa Jones]

Not the way I read it. If the factory documentation says 50", (regardless of where the axle is located in
the slot when measured by the factory) you are allowed 55" max measured at the rearmost position. So
the smallest rear sprocket you want to run will have to be adjusted at or below that 55".

Have you set your ride height where you want it? Like you said, those longer forks increased the
wheelbase, so  shorter forks should reduce the wheelbase.

Cheers, Dave

[Old Scrambler]

Yes, I will slide the forks up in the trees about 1.5-inch.  So if that's the rule I have to cut about 1.25 inch from my swingarm, get some shorter shocks, and do a lot of other mods to make everything fit. OK..........its all about fun

[Grandpa Jones]

Yep, change one thing, and you have to change a half-dozen others! 

[Uncle Jimbo]

Yes, I will slide the forks up in the trees about 1.5-inch.  So if that's the rule I have to cut about 1.25 inch from my swingarm, get some shorter shocks, and do a lot of other mods to make everything fit. OK..........its all about fun

I'd be very careful with shortening the front end. It will change your rake & trail numbers most likely reducing trail. I learned my lesson on that in 08 - resulted in a rather unpleasant head shake at 115 mph. A Serious wake up call -  I went back to stock length & added 5 deg. to the steering head to bring up better rake & trail numbers.
Have fun with the build, & get to a drag strip for some needed handling & engine testing. Don't wait till you get to the salt to see if everything is functioning properly.
Like W would say - The harder you work at it, the luckier you get.

Jimbo

[Old Scrambler]

Thanks for the kind advice.............I need kindness after someone in a red Pontiac rammed his car into my trailer hitch conveniently left in my receiver.  A little whip-lash and a 2-hour headache..........but I feel good today. Just red paint on my hitch...........the Pontiac's bumper is shoved under the hood and has a nice-looking "belly-button" in the center

To accommodate the shorter swing-arm the 21-inch rear-wheel needs to be replaced with a 19-inch unit for clearance. The height adjustment to the front-forks should result in the same rake and trail as before.  I use a large parking lot surrounding a warehouse to break-in the motor and fine-tune the chassis..........then to a local road and / or airport for a few speed runs for plug-chops, sprocket-gearing, and carb-tuning to get close to attempting a speed-run on the salt.  In 2011 we removed the baffle from the mega and were ready to go

[nrhs sales]

I wonder just how many folks this is going to affect this year? I can see a lot of folks having to run A class if this is properly enforced.

[salt27]

I wonder just how many folks this is going to affect this year? I can see a lot of folks having to run A class if this is properly enforced.

Correct me if I'm wrong but I don't believe this was a 2013 rule change.

It does make it a little tricky when the manufacturer gives the wheelbase at the minimum adjustment.

  Don

Sorry I just realized this is for Bubs not SCTA. Don't pay any attention to me, I have no clue.

[ol38y]

I wonder just how many folks this is going to affect this year? I can see a lot of folks having to run A class if this is properly enforced.

It will probably be enforced like the seat rule or the front fender rule. hap hazard

[nrhs sales]

It does make it a little tricky when the manufacturer gives the wheelbase at the minimum adjustment.

Salt,
Do you have anything to back up that assertion as I can see that being very important and could lead to a change in how the rule is written.  Of course it can also lead to bad abuse as somebody could have 5 inches of adjustment in their swingarm and bring it to tech with a little sprocket and then throw a big one on for the actual pass.  

Larry,
the front fender rule was radicaly changed a few years ago and has been a non-issue since it was.

[salt27]

It does make it a little tricky when the manufacturer gives the wheelbase at the minimum adjustment.

Salt,
Do you have anything to back up that assertion as I can see that being very important and could lead to a change in how the rule is written.  Of course it can also lead to bad abuse as somebody could have 5 inches of adjustment in their swingarm and bring it to tech with a little sprocket and then throw a big one on for the actual pass.

Nrhs,
The only thing I have to back it up is my tape measure and the factory specs for my bike.

 Don

[Stainless1]

It seems most of the Japanese Crotch Rockets give their wheelbase at minimum... sounds nimble to the carvers but the reality is they usually longer than advertised.  The SCTA ruled the number in the book is the number... AMA may vary

BTW it doesn't matter if a bike is in compliance until record certification time.... them it is up to the inspector to determine compliance of all the rules, not just displacement.  I remember several years ago there was multiple bikes that set records with compliance problems, AMA determined they would let them slip through the crack and keep their records.  There was a thread on this site with all the pictures and problems.  Records only matter if the rules are followed.

[Uncle Jimbo]

  Records only matter if the rules are followed.

Excellent moral to the bottom line of the question Stainless.   

The AMA rule plainly states; "Maximum total wheelbase shall not exceed factory OEM dimension by more then 10%"
Seems rather straight forward to me.  Whatever your Factory Manual says your stock wheelbase is, you can lengthen by 10% & I would read that to be at the furthest adjustment of the rear axle, even if you are not using that furthest to the rear adjustment.  Doesn't matter if you have 6in or 12in of adjustment, so long as the furthest point does not exceed that 10% margin.  Key words - "Maximum total Wheelbase".
 
Just my humble interpretation.  If still in doubt, just check with the AMA Tech steward. I hear he's a pretty good guy.

Jimbo