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FIVE DIRT BIKE BELIEFS: FACT OR FICTION?


MXEditor

Even though our sport isn’t that “old” compared to some others, it has its own share of beliefs, some rooted in truth and some not so much.

In this feature we’ll be looking at different things riders and racers say about off-road motorcycling and try to determine whether they are true or false…or maybe somewhere in-between!

We sat with a few of our staff and asked them to name a few of the pervasive and persistent beliefs that they felt were indicative of the theme stated above, so lets look at each of them and see what we find out.

(Editor’s note: In researching this article, many that were interviewed gave very long technical explanations to our questions. We normally edit these answers for the sake of length and clarity, but in this case we let the responders give longer, technical answers. Part of this is due to reader demand so let us know if this “works” for you, the reader, thanks!)
 

#1 HAULING YOUR BIKE WITH FORKS COMPRESSED BLOWS YOUR SEALS

We’ve all seen different and sometime creative ways of transporting off road motorcycles but the ubiquitous tie-down strap arrangement has to be most common. This involves attaching tie-downs to the handlebars and cinching the forks down until they don’t move…there’s no science to how far to pull the forks down and everyone seems to do it a bit differently so right away you have differences in technique and implementation.

But is this a good way to blow fork seals? Is this phenomenon fact or myth?

Obviously anytime fork seals are under compression they are being stressed, let’s agree on that. But are they stressed enough to help blow the forks seals?

We spoke with James Burry of Risk Racing who had this to say:

“Your forks and fork seals are designed to take big hits, and therefore a lot of pressure when that happens.  Of course that is for short period of time, which they are good at.  The issue occurs over time, and is compounded when people over tighten their tie-downs.  A new fork seal is soft and will “stretch and flex” with the added pressure, as there are designed to do, but as they age they lose their flexibility and their ability to hold the pressure over long periods of time.  Eventually they will leak.  It is best to just leave them at rest or reduced pressure during transit if possible.”

OK, so keeping them stressed all the time can be an issue…how about using a fork brace that sits between the front tire and underside of the fender?

He continued: “The fork brace can help protect your fork seals because it prevents the fork from being over compressed and therefore limits the overall pressure.  When the fork brace is squeezed between the fork and the tire, the tire becomes the “flexible” member of the group rather than the suspension.  The real benefit to the brace is to prevent the bike from compressing during transit.  If you use tie-downs, and are nice to your suspension by not over compressing, then you stand the chance for your suspension to compress further during transit when the vehicle hits a “g-out” style bump.  This can compress the suspension more causing the tie-downs to lose tension and possible become disconnected from the bike or vehicle…end result is a bike flopping down the highway.  So, the fork brace is easier on fork seals because it allows the user to tightly secure their bike without over-compressing the front suspension, and also prevents the suspension from compressing any further during transit.

OK so fork braces are a good accessory to use with tie-down(s) to prevent additional stress on the form seals, except when hitting a large bump which can loosen the whole arrangement. What about the newer stationary systems that attach to the floor of the carrying vehicle and to the footpegs or frame of the bike?

Burry continued: “The Lock-N-Load system responds to all concerns when transporting a bike.  It reduces pressure on the fork seals, limits the travel of the bikes suspension (and) eliminates the potential for a tie down to break. Of course the trade off is the (expense compared to) a cheap pair of straps.”

On this same subject, we had a look at two other factors that may play a role in raising or lowering pressure during transport and we came up with two items to explore:

Atmospheric Pressure - In theory, air pressure in your fork tubes stays static if all environmental factors remain identical, but that doesn't happen in the real world. One factor would be the altitude at which you transport the vehicle, because as you increase your height geographically, atmospheric pressure decreases. For example, atmospheric pressure is approx. 14.7 PSI at sea level, but drops drops to about 10 PSI at 10,000 feet...

So that means atmospheric pressure increases with decreasing height! So the pressure in your fork tube can rise or fall depending upon your location, but not dramatically at no more than a 5 PSI swing for 10,000 feet. So tie down solutions that exhibit static pressure in the fork tubes can have that value actually increase, causing even more stress on the seals.

Air Temperature - It doesn't immediately come to mind when thinking about suspension components except at the pro level, air temperature can also contribute to stress on fork seals when under load as in transporting.

Temperature affects air pressure by causing the air to either become more or less dense, which expands or lowers its pressure. Warm air is less dense than cold air, and as air becomes less dense, its pressure increases.

Standard rule of thumb for evaluating pressure to air temperature ratio is tire pressure will increase by 1 PSI for every 10 degrees of ambient temperature increase, and this is true in reverse as well. So it's not a huge figure but between it is a contributor to elevated (unexpected) fork pressure.

The pressure in your fork tubes can increase as the temperature rises, and this again can cause additional pressure in the fork tubes causing even more stress on the seals.

Conclusion: Pressure on fork seals can be high and for long periods of time when transporting a motorcycle using the tie-down method, potentially leading to premature failure, and using a fork brace or stationary transport mechanism can diminish or eliminate this pressure extending the service life of your fork seals.
 

#2 HANDGUARDS CAN BREAK YOUR ARM

Riders and racers we spoke to had strong opinions about this statement but lack of real world examples hampered their arguments.

First of all let’s define what we mean by handguards…this would be a wrap around metal or plastic “bar” that stretches from the end of the handlebar around the rider’s hands and attaches to the front of the handlebars, creating a loop.

This “loop” of metal is the culprit at hand so to speak, in theory and in practice it can create a situation in which your arm can go through the loop and then be at the mercy of anything else that happens. You may leverage your arm and snap it…maybe get your arm caught in there as the bike drags you into an injurious situation - the possibilities are endless when you think about it.

But does it happen often? Is this phenomenon fact or myth?

Since we didn’t actually know any riders who this has happened to, we searched the Internet for some clues. Many of the responses came from threads just like these:

handguard_poll.JPG

https://www.thumpertalk.com/forums/topic/556585-handguards-causing-broken-arms/
https://www.thumpertalk.com/forums/topic/645172-handguards-and-broken-wristsis-this-an-urban-myth/

The theme of these threads seems to be “it can happen…but usually doesn’t” and most riders/racers have never seen these happen…and if they have, it may have been due to other factors such as mounting the guard too high or so loose it wrapped around and “bit” the rider.

Conclusion: The myth of handguards being the culprit in broken arms and/or wrist injuries just doesn’t hold water. We’ve spoken to countless racers who’ve admitted they’ve never seen this happen. We aren’t saying it doesn’t ever happen but the notion that these components are so dangerous because of it just isn’t true, and most racers agree that the benefit of the guards far outweighs the risk of injury by not running them.
 

#3 GOTTA HEAT-CYCLE THE ENGINE TO SEAT THE PISTON

We just received a big bore 2-stroke engine back from our builder and we asked him…”how do we break in this engine, and is there a certain way you like to do it?” and as with almost everyone we’ve spoken to, he has his own way to “break in” the engine. But with today’s tight tolerances, computer machining techniques and improved quality control is this really necessary? How different are the requirements for a 2-stroke vs. a 4-stroke?

We figured asking some engine builders would be the best way to find out as they deal with this question all the time.

One of the best responses we got was from Tom Zont of TZR Racing and his extensive insight and experience dictated that we publish his comment in entirety.

Tom Zont: “The need to methodically heat cycle a new engine has changed over the years. With better materials being used, higher precision in the manufacturing of the parts themselves, and with most engines being liquid cooled, lengthy and methodical break in procedures are generally not as necessary on today's engines as they once were. This is particularly true with the newest 4 strokes.”

“On any new motor, 2-stroke or 4, parts like pistons, rings, valves and cylinder walls will indeed ‘wear in’ as the engine is run. The piston rings (contact) against the cylinder wall is an area that has a measurable effect on overall output being as good as that engine can be. There are some differences between 2 and 4-strokes in what is critical during the initial ‘break-in’ however. “

“On modern 4 strokes, there is no need to "seat the piston" thru methodical heat cycling. With electro-fusion/Nikasil cylinders, ultra precision cast and forged pistons, and the relatively uniform temperatures achieved with liquid cooling in a cylinder with no ports, damaging a 4 strokes piston is extremely hard to do as long as the engine has oil in it of course. With so much quality oil being splashed and pumped to lubricate the cylinder walls and piston skirt, there is really no need to ‘seat’ or ‘wear-in’ a 4-stroke piston when new. The rings themselves will indeed ‘wear in’ to the cylinder walls over time, creating a better ring seal at the 1 hour mark than when they were brand new. This will happen regardless of how many times you warm up the motor and let it cool (heat-cycle). We have and can put a brand new 4-stroke motor (bike) on a dyno, and as long as we simply warm it up to full operating temperature, we can run it wide open to measure its power output and not damage the piston or rings. The power will go up slightly but measurably, as the parts like the rings wear-in, and the engine becomes a more efficient air pump.“

piston_831.JPG.ca3ca67e532a810c1d7023cafbf5dbdb.JPG

“On a modern 2 stroke however, there is some merit into ‘heat cycling’ a new piston. Because of the elaborate casting of the ports throughout a 2-stroke cylinder, the temperature of the cylinder itself is not as uniform as in a 4-stroke. Temperature variations mean that the cylinder will not expand as uniformly as the engine temperature changes. This can lead to parts of the cylinder that do not expand as much as others. (Aluminum expands dramatically as its heated) You also do not have as much oil available to cushion moving parts as in a 4-stroke. Three to four ounces of oil per gallon of gasoline is not much when you think about how long that one gallon will run your engine for. With less oil to stay between the moving parts, the chances of parts rubbing together without adequate lubrication to prevent seizure or heavy wear are increased.”

“We want the piston to be very close fitting in the cylinder bore. That way it cannot tip or rock back and forth, so the rings will stay tangent to the cylinder walls and create a good seal. That new, exceptionally tight fitting piston is at risk for seizure against the cylinder walls if it expands too much or too quickly, in comparison with the cylinder that it is in. This is where the heat cycling can be a benefit. By methodically warming the piston up to incrementally hotter temperatures, we would gently, GRADUALLY scuff away material where the piston is running out of room to expand. The key here is the very gradual ‘scuffing’ away of material, ONLY in places where it has run out of clearance. Running a new 2-stroke engine for short periods, each time slightly longer, getting it slightly hotter than the last time, can indeed "seat the piston" gradually enough so as to prevent a full on seizure the first time the engine reaches maximum temperature under the most severe operating conditions.”

“The term ‘seating in’ is more appropriate to the pistons rings themselves, and I prefer the term "wearing in" when referring to the piston. ‘Wearing in’ the piston essentially means that you will allow the new piston to very gradually rub away areas on its skirts that become too tight in the cylinder bore because of un-even expansion, both of the piston itself, or the walls of the cylinder. Heat cycling is a cautious way of letting this process happen in a manner that is gradual enough so as not to have what we call a piston seizure. By engine builder’s standards and terminology, a piston seizure is not always a piston that becomes completely stuck, melted or wedged in the cylinder. A heavily ‘scuffed’ piston skirt on an engine that never quit running can still be considered ‘seized’ by many, to varying degrees anyways.”

“Many factors involved can influence how critical it is to ‘heat cycle’ a new 2 stroke engine to ‘wear in’ the new piston, and too many to list here. But as a general rule, it would never hurt anything by heat cycling a 2-stroke a few times before running it at full race pace. Don't confuse ‘heat cycling’ (to break in or wear in new parts) with a standard "warming up". Every modern 2 or 4-stroke should ALWAYS  be warmed up gradually, as close to full operating temperature as possible, before going wide open down a holeshot straightaway. Letting all internal moving parts expand to their normal operating size somewhat gradually, will reduce wear on parts that are expanding at different rates. Not just in new engines, but for their entire lifespan.”

Conclusion: The belief that “heat cycling” your engine before full operation is important, even more so for a 2-stroke versus 4-stroke. It’s not detrimental to your new engine and can result in an engine that will run longer and realize its full performance potential.
 

#4 OFF-ROAD BIKES REQUIRE THE HIGHEST OCTANE FUEL AVAILABLE

Most of us love our motorcycles and want to give them the best fuel available…but what does “best” really mean when it comes to off-road motorcycles. With bikes like Honda’s CRF250R coming stock with over 13:1 compression, this is becoming more important.

Higher octane doesn't give your bike more power, it burns slower to avoid detonation in higher compression engines. Detonation is a very destructive force in an engine and should be avoided at all costs. You can find more in-depth reading on octane HERE.

What does the manufacturer of your bike recommend? This is extremely important because all engines are different. You must base your decision on what grade gasoline to use by knowing the minimum grade recommended by the manufacturer. If they don’t recommend high octane gas for your bike, then you're just throwing away money by using it…there is no real benefit…except if you detect pinging or knocking when using lower octane gas, that would require you raise the octane rating to compensate.

Conclusion:  The answer here is a lot more evident than some of the other items we’ve covered in this article. Always use the fuel with at least the octane rating specified in your owner’s manual. Using a higher grade is of little detriment in most cases except to your wallet…but using a lower grade that could encourage detonation can do a lot of damage and why risk that for the sake of a few pennies per gallon?
 

#5 BANGIN' OFF THE REV LIMITER IS BAD FOR THE ENGINE

If you’ve been to the local MX track in the last few years or watched Arenacross/Supercross on television, you’ll hear riders and racers revving their 4-stroke bikes right up to the limits…until the rev limiter kicks in and interrupts the ignition circuit, lowering the revs and then allowing the circuit to re-energize and do it all over again, causing that familiar 4-stroke “panic rev” sound that used to associated mostly with trying to lift the front of your bike before impending doom.

Justin Barcia comes to mind…

Now many riders just do it for a variety of reasons that we won’t get into here…what we want to know is whether it’s bad for the engine? It sure sounds like it would be…but we see racers run their bikes like this constantly during a racing event without seeming to cause damage…is it because of the rev limiter?

We’d just assume that this is a bad way to run your engine, bouncing off the rev limiter when not needed, but many newer riders use this technique and report that it actually helps them concentrate and stay focused, almost blurring out their opponents and the outside world with this wall of noise.

So we reached out to some a few professionals who have a better insight into the specifics of how and why this technique can affect your engine.

First up was Brent Kirk from Fastheads, who crafts amazing motocross cylinder heads and valve train components for all motorcycle brands. They offer world class precision seat machining, modifications, porting repairs, and general head servicing, so we figured Brent would be one good guy to ask about this.

Brent Kirk: “Rev limiters keep the RPM’s within the limits engineers have designed the engine to operate. On 4-strokes the valve train is the most crucial factor in setting limits for RPM (because) valve springs are limited to how high of RPM they can efficiently be operated. At a certain point they can't keep up with the speed of the valve and cam and this is due partly to harmonics. As a shock wave flows up and down the length of the spring and can actually deaden it ability extend and when the cam can not control the valve due to the spring, all kinds of devastating problems can occur. Valve float is when the valve is moving so fast that it slings itself of the end of the cam lobe and if it doesn't meet up with the back side of the cam before it closes, the valve will slam the seat and bounce. During this uncontrolled time valve shims can fall out, valves can break along with lifters, retainers, keepers and springs. The best engineered coil springs won't perform much over 14,000 RPM.”

Kirk continued: “On a stock 4-stroke race motor engineers limit the RPM’s so the valve spring keeps the valve train under control and within its operating range…this done by retarding the timing when the crank reaches and per determined RPM. We normally only see engine failures when the valve train is tampered with or not maintained.”

Next up we spoke with Derek Harris of Harris Performance Engineering, who specializes in building custom racing 2 and 4-stroke engines in his state of the art performance shop located in Marion, Texas.

Harris: “With the involvement of the factory teams into amateur racing at an aggressive level, (Justin) Barcia was signed to a large salary with endless bikes, equipment with a full-time mechanic.  He would rotate practice motors once every 2 weeks or so, or more frequently if it broke. Matt Biscgelia was on the same program, and while Matt doesn't ride like Justin - it was at least once a month he would have pieces thrown out of his cases....Kids saw Barcia and the video coverage at the same time and the rest is history.”  

“So a production motorcycle IS built to run on the limiter all day, however, not many people follow OEM service manual suggestions. Example; Honda suggests cranks every 15 hours with full engine inspection/tear down on their 250F. Parts are stressed proportionally to RPM.  The more RPM, the more stress. So if you spend time on the limiter - the bike will wear out more quickly. What's most sad is all the engines with exception of the new KTM 250F's do not make good power at the limiter.  It's faster to shift before then.”

“In summary - the more you rev your bike consistently - the shorter it will live. “

Conclusion: The belief that “hitting the rev limiter all the time can ruin your engine” has some basis in truth. Yes, rev limiters are set to kick in before potential damage to the engine occurs, but only in a perfect engine. Any weakness in engine components is magnified and there is a much higher potential for failure of these components at high RPM’s.

####

 

:prof: What do you think of this article? Where did we hit? Miss? Have something to add or correct? Share your thoughts in the comments section below.

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On 5/1/2017 at 8:56 AM, cjjeepercreeper said:

One other comment.  Concerning octane, good advice.  But, no matter what you are using keep your ears open for the sound of pinging.  Not all gasolines are created equal, what is labeled 92 octane at one gas station may equate to what is labeled 89 at another.  You never know.  I try to stick with one brand if at all possible, we have 2 CRF450Rs and a big bore higher compression XR250R, all need premium fuel.  The other 2 bikes will run on about anything, a stock XR400R and a stock TTR230.

Most gasoline brands all pull their fuel from the same refinery(s), is chosen based on the price, and can be sourced from a different refinery every for every delivery. They then mix in their own additives if any. The fuel may be slightly different octanes, but it would be because of the different refineries, not different stations and would vary even at one station (if it varies at all).

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