The 15 inch rule...

[lizrdbrth]

Can someone explain the safety benefit/logic/history behind the 15 inch minimum rim diameter for motorsickles?

[John Noonan]

Can someone explain the safety benefit/logic/history behind the 15 inch minimum rim diameter for motorsickles?

I can think of a few, what is the problem with the rule the way it is?  What do you want to run and have you ever tried rollerskating in the loose dirt or salt? 

[lizrdbrth]

Not challenging the rules, just trying to understand them.

One of our basic platforms in consideration came stock with 32-spoke 4.50x14 rear rims. So if we have to maintain production rim width but increase diameter we're looking at our first unplanned major expenditure and a boatload more tire mass than we bargained for .  If we're allowed to go narrow we can lace a fairly commonly-available 32-spoke 2.50 out back and call it good.

That's my practical question, but I'm still wondering what led to the rule in the first place.

[John Noonan]

Not challenging the rules, just trying to understand them.

One of our basic platforms in consideration came stock with 32-spoke 4.50x14 rear rims. So if we have to maintain production rim width but increase diameter we're looking at our first unplanned major expenditure and a boatload more tire mass than we bargained for .  If we're allowed to go narrow we can lace a fairly commonly-available 32-spoke 2.50 out back and call it good.

That's my practical question, but I'm still wondering what led to the rule in the first place.

Sometimes going slow can cost more than going faster...


Like being the Worlds tallest midget...

[Dean Los Angeles]

It's been in the rule books a long time. It separates motorcycles from everything else.
If it's a true motorcycle then you can contact the motorcycle rules committee and see about a waiver.

[lizrdbrth]

Having two wheels sorta automatically seperates motorcycles from everything else, no?. Just sayin'. lol.

The bike is an oddball, but is a road-legal model that has been in continuous production for 26 years. You can run down to the dealer and buy one today. Maybe the waiver process will get us through our initiation.

Down the road there's sort of a method to the madness in choosing this frame, but we had hoped to use it in Production while we got our licenses and learned the ropes. Rims of stock width and spoke count are a strictly a custom proposition andI'm not popping for a custom rim just for a few runs because we won't run them once we move up, so I think we may need to use another bike for that phase of our education.

Thanks.

Edit: The verbage here states nothing about rim WIDTH. Can we run a narrower than stock rim of at least 15" in Production? If so we're good to go.


7.B.10 Wheels:
Wheels shall have a minimum nominal diameter of 15 in. except in the Sidecar and Streamliner classes. It is highly recommended that strict attention be paid to wheel alignment, wheel balance, spoke tension and tire run-out. Non-cross ventilated front wheels are not allowed except in the sidecar and streamliner classes if the wheel is fully enclosed by the body work. It is REQUIRED that front wheels be cross ventilated by an area equal to at least 25% of nominal rim circle area. Non-cross ventilated wheels rear wheels are allowed. No front wheel discs are permitted. Wheel disc may be installed on the rear wheel only, and must be installed in a secure , workmanship like manner. Installation methods will be closely scrutinized.

[RichFox]

I think the 15 inch rule was to keep out scooters. Why that is I don't know. Maybe some very fast scooters out there.

[lizrdbrth]

Can we run a narrower than stock rim of at least 15" in Production?

[Glen]

contact Tom Evens or Van Butler on the SCTA web site They are the MC people to ask

[lizrdbrth]

I have emailed Mr. Butler and will resume lurker status.

Thank you, sir.

[PatMc]

I don't see the dia as a safety issue, as many street bikes have 14's and go over 110mph, which is faster than many Harleys will go.  And of course side cars can run 10" and 5" rims. 

There really is no safety problem with a 14" tire EXCEPT, the highest DOT speed-rated 14's are like 118mph@600lb but are not recognized by the SCTA.  So you can only go 70mph on them.  They don't recognize the modern designations; L (75mph), P (93), S (112), T (118), (V) (149+), W (168) or (W) (168+).  Which is odd, since the W's, (W's) and (V)'s are the high speed DOT tires, not H/V/ZR.  The ZR (150+) rating is only valid if you aren't running at full load.  It's an obsolete rating that is on the books because of equipment laws in some countries.  Modern tire designations are a number and a letter.  ZR is included also sometimes, but isn't the speed rating, it's a legal insignia.  The tires on my car say ZR (150mph+ at under the rated weight) but they also say (96Y) which is sustained speeds of over 186mph.  Sadly, the dumb thing came with a factory speed limiter of 175mph.  Wifey won't let me reprogram it.

But rules are rules.  If you want to go over 70, you need to first find something that says H, V, ZR on it.  If it just has the modern designator, it might cause you trouble.

[lizrdbrth]

My tires are "H" rated, and in stock form this bike is in no danger of outrunning the rating  If they were 15 or better we'd just get a speed limit imposed. I got it rather pleasantly hashed out at El Mirage.

No matter. We'll just go Modified frame with the stock motor right from the git. Maybe our speeds will be a few mph less embarassing while we're learning.

[1212FBGS]

are you planning on running a production class or a modified/altered class..... you should be perfectly able to run the 14 in production class as long as you are within the speed rating of the tire... when you move into "m" or "a" you will need to conform to size..... also the tire speed rating you will need is not dictated by what you are planning on running but what the speeds are of the current record
kent

[lizrdbrth]

The official word is that I have to lose the 14 for a larger diameter rim, even in Production.

We're now cool with it, just needed to hear it from the horse's mouth, so ta speak..

The decision to run Modified was due to the fact that it's now about the same amount of work to prep the bike for either class since we gotta do the wheels in either case.  We'll just run the stock motor during the learning curve and the rest of you will have plenty of time to go to the snack shack during our runs.

 

 

 

[1212FBGS]

hi can you tell me or PM me what "official" told you that you can "not" run your 14" in production?
thanks
Kent