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Valve Spring Install Seat Pressure !

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Can any body give me an Idea what is the correct ,, "Installed" seat pressure of a typical single cyl MC/ Dirt Bike ??

 

Try a CRF230 F Sport Bike ,, compared to  CRF 250/450 MX ,, (Yes I'm a Honda guy) I know pressure may vary according to what kind of Parts Yu may using and then Shimmed accordingly !! 

 

I jus don't know a correct starting point ,, STOCK ,, Installed "Spring Seat Pressure"  I have all my part ready for assy ,, first rebuild !! It is a CRF230 Head

 

Thanks Guys :thumbsup:

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In the first place, there is no such thing as a "typical single cylinder dirt bike", unless you believe that seat pressure required for a CRF150L  and that needed for a YZ450f are going to be remotely similar. 

 

I also doubt very much you'll find any OEM specifications for this listed, but you could buy a spring for well under $10, look up the assembled height, then measure the seat pressure on a spring tester. 

 

Failing that, contact a cam grinder and ask for the seat pressure needed with one of his products for a similar engine to the one you are building.

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In the first place, there is no such thing as a "typical single cylinder dirt bike", unless you believe that seat pressure required for a CRF150L  and that needed for a YZ450f are going to be remotely similar. 

 

I also doubt very much you'll find any OEM specifications for this listed, but you could buy a spring for well under $10, look up the assembled height, then measure the seat pressure on a spring tester. 

 

Failing that, contact a cam grinder and ask for the seat pressure needed with one of his products for a similar engine to the one you are building.

 

 

:cool: A few examples would be great !! Like the 2 bikes I mentioned ,, somebody around should know !! 

 

I do have a new cam and the company jus said I should get "these springs" !!

 

I have a new set of springs but no DATA on even a stock setup !! I know this measurement varies from bike 2 bike and the setup weight of the valves ETC ,, usage and broken-in springs that need shimming !!

 

I could use a few examples ,, modified and stock would be great !! TIA

Edited by Scottyyaa

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Cam grinders recommend springs for their aftermarket cams for various reasons. Often it depends on how different their cam is from the original. The ability of the spring to compress far enough to allow full lift without coming too close to cool binding is at least as big a concern with major lift increases as having sufficient pressure to control the valves at full lift and speed. Seat pressure can be a complicated matter, as it needs to be high enough to keep the valves from bouncing when they close without exacerbating face/seat wear.

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Na ,, I am not running any kind of radical lift ,, just an extended duration drop in cam ,, I saw several videos depicting that 'bounce" as yu said and also a few of " Spring and Valve Rotation"

 

I would jus like 2 know a good number for, ohh say a CRF230 compared with a 4 valve CRF250 ,, something like that !! 

 

As for seat and face wear I would like to get my stock seats out and swap with good quality material \seat

Edited by Scottyyaa

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The seat pressure is determined of the weight of the valve, the lift, and the RPM the motor will be turning. The key is to control the valve. You don't want to much pressure though because you will get premature parts failure, and take away power. You cant compare a CRF 230 to a CRF 250 because the 250 uses Titanium valves on the intake side and revs to 12,500 rpm. Where the 230 is a steel valve and if it seen 12,500rpm it would only be for a second and the rider may need bionic legs because the rod and piston removed them.

What size valve are you using? All detentions. What material are the valves made of? What rpm are you trying to achieve? And what lift and duration cam are you running?

As for a seat material depends on the material of the valve you will be using. 

There are people out there that have figured all of this stuff out for you already. And if you don't see it being used today, it is probably because the juice was not worth the squeeze. 

Just my $.2

Edited by Jeff@bigborethumpers
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The seat pressure is determined of the weight of the valve, the lift, and the RPM the motor will be turning. The key is to control the valve. You don't want to much pressure though because you will get premature parts failure, and take away power. You cant compare a CRF 230 to a CRF 250 because the 250 uses Titanium valves on the intake side and revs to 12,500 rpm. Where the 230 is a steel valve and if it seen 12,500rpm it would only be for a second and the rider may need bionic legs because the rod and piston removed them.

What size valve are you using? All detentions. What material are the valves made of? What rpm are you trying to achieve? And what lift and duration cam are you running?

As for a seat material depends on the material of the valve you will be using. 

There are people out there that have figured all of this stuff out for you already. And if you don't see it being used today, it is probably because the juice was not worth the squeeze. 

Just my $.2

Hey Jeff ,, thanks for posting ,, thats why I am asking coz I'm not finding the info through the www !! yur website looks good but not racing right now !! jus building something for the Street !!

 

Essentially ,, I am working on an XR230 ,, I have  SSteel valves ,, My problem is the springs ,, and I want to jus know what is a good "Install Load" !!  :thumbsup:

 

BTW what is an install loads for the CRF for comparisons sake !! and I wont be turnin 12.5k RPM's of course

Edited by Scrammer

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Any time you can make a double spring work it is better. It is sharing the load. You also want to try and get a double spring with an interference fit to reduce spring surge. But if you can only find a single spring that will work you will be fine. just try and get one that is progressively wound. This to will help prevent spring surge. 

As for the comparison on the CRF seat pressure there are to many factors. But its between 30 and 65lbs depending on the valve material, cam profile, and the purpose of the bike. 

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Nothing generic as all valve/cam combinations have unique spring requirements.

 

Double, or triple, springs are used to control spring surge, a single beehive spring can accomplish the same.  The interference fit between springs creates friction that dampens spring surge.

 

Manufacturers spec spring installed length, from which you can calculate seat pressure using the free length and spring rate. 

 

Valve seat pressure is not an issue, cylinder pressures will provide pressure to seal the valve. The main issue is  spring force  to keep the valve train in contact with the cam. Since there are many different lift and lowering rates for cams the cam manufacturer is best able to provide required springs for a particular application.  

 

As previously posted cam grinders know what springs work with their grinds, so just follow their guidance.  

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Nothing generic as all valve/cam combinations have unique spring requirements.

 

Double, or triple, springs are used to control spring surge, a single beehive spring can accomplish the same.  The interference fit between springs creates friction that dampens spring surge.

 

Manufacturers spec spring installed length, from which you can calculate seat pressure using the free length and spring rate. 

 

Valve seat pressure is not an issue, cylinder pressures will provide pressure to seal the valve. The main issue is  spring force  to keep the valve train in contact with the cam. Since there are many different lift and lowering rates for cams the cam manufacturer is best able to provide required springs for a particular application.  

 

As previously posted cam grinders know what springs work with their grinds, so just follow their guidance.  

DAMN HARD to find in our sizes....

wmr made them and still has them for crf250s.

i cant find them for my application.

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A true cam designer must use the spring force/stress envelope as a design constraint. Whoever designed your cam should be able to give you the spring specs as he needed them to design the cam in the first place. Assuming he is a true cam designer......chances are he is just a hack that will tell you he has twenty years experience.

Reality is that as long as you have .040 less lift than coil bind the stock spring will work just fine. I've been doing this a long time and rarely is a spring beyond stock required.

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Reality is that as long as you have .040 less lift than coil bind the stock spring will work just fine. I've been doing this a long time and rarely is a spring beyond stock required.

 

I agree with that, for the most part, particularly when the discussion centers on an engine originally produced for a racing application, like MX or off-road.  But the OP here is building off a lower powered trail bike engine, which might have springs not quite adequate for the RPM range he will end up with if he's successful.  Still, the comments about the cam grinder being the best source of spring advice for the application are well taken.

 

There are a couple of weaknesses to a standard, single rate, cylindrical spring.  One is inertia.  As the valve is opened, the top of the spring is accelerated by force, while all of the coils under it remain momentarily unmoved.  As the lift continues, the force applied to the second coil by the top coil starts to move it, and it in turn begins to move the next coil, and so on down the spring in a kind of a wave manner, much like a pressure wave moving through air.  When the cam goes over the top and lets the valve begin to close, that wave may be still moving down the spring, and depending on the force that it carries, might contribute to a tendency to float the valve.

 

The second weakness is the sensitivity to harmonic resonance.  It is possible for a spring to be exposed to a vibration that will act in opposition to the springs own natural resonance and cause the spring to effectively loose most of its tension.  This was handled in the past by adding a coiled flat band inside the spring to act as a vibration damper.  Dual springs also resist this as each of the two springs will resonate at different frequencies.  Metallurgical advances have also contributed a great deal to resolve the matter.

 

"Beehive" springs accomplish 3 things to some degree.  For one, they allow more compression than a comparable straight coil because the coils can be wound to "stack up" inside one another instead of one atop the next.  They are also a progressive rate spring by the simple function of their variable coil diameter.  Thirdly, because of the progressive rate, they have some resistance to harmonic collapse.

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All this spring pressure what material is the camshaft? What about wear on the lobes?

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I agree with that, for the most part, particularly when the discussion centers on an engine originally produced for a racing application, like MX or off-road.  But the OP here is building off a lower powered trail bike engine, which might have springs not quite adequate for the RPM range he will end up with if he's successful.  Still, the comments about the cam grinder being the best source of spring advice for the application are well taken.

 

There are a couple of weaknesses to a standard, single rate, cylindrical spring.  One is inertia.  As the valve is opened, the top of the spring is accelerated by force, while all of the coils under it remain momentarily unmoved.  As the lift continues, the force applied to the second coil by the top coil starts to move it, and it in turn begins to move the next coil, and so on down the spring in a kind of a wave manner, much like a pressure wave moving through air.  When the cam goes over the top and lets the valve begin to close, that wave may be still moving down the spring, and depending on the force that it carries, might contribute to a tendency to float the valve.

 

The second weakness is the sensitivity to harmonic resonance.  It is possible for a spring to be exposed to a vibration that will act in opposition to the springs own natural resonance and cause the spring to effectively loose most of its tension.  This was handled in the past by adding a coiled flat band inside the spring to act as a vibration damper.  Dual springs also resist this as each of the two springs will resonate at different frequencies.  Metallurgical advances have also contributed a great deal to resolve the matter.

 

"Beehive" springs accomplish 3 things to some degree.  For one, they allow more compression than a comparable straight coil because the coils can be wound to "stack up" inside one another instead of one atop the next.  They are also a progressive rate spring by the simple function of their variable coil diameter.  Thirdly, because of the progressive rate, they have some resistance to harmonic collapse.

Well said. 

Not all cams are created =. One cam may have the same lift as a stock cam but has a different duration and the ramp angles change and the change in this ramp angle causes strange harmonics in the stock valve spring and it will cause it to break.  Sometimes you can throw a small shim under that spring like a .015 shim and this cures the issue, but sometimes you need another spring that is another progressively wound or a duel spring. Always best to use a duel spring that has an interference fit, and they must be wound in opposite directions to portent binding and breakage. 

Most Upgrade cams require more spring pressure to control the valve. You don't want to throw the valve off the end of the cam. If you do you run risk of crashing the valve into the piston, and you are going to ware valves out quickly. 

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Wow ,, Thnks guys ,, I've been off the pace for a while trying to finish my Frame ,, and got the TT notification for my thread ,, 

 

Yes great responses ,,  and yes, I am jus working on an engine similar to the CRF230 F but my plan is to branch out later so I need to know about cyl. head valve train stuff ,, I am actually finding myself with a unique opportunity ,, then I have a webcam for my setup ,, jus a standard grind they offer, for now ,, until then ,, I manged to pick up 2 other used heads for my effort

 

good info on the spring loading dynamics ,, thanks ,, its all cumulative !!

Edited by Scrammer

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I am not trying to obtain MotoX 'RPM's and HP ,, Limited by a rather Long Stroke ,,  but I am trying to get what I can out of my engine !!

 

Learn this as a small Hobby buisness ,, make a reliable setup and offer my service to the locals ,, I am not in the U.S. (retired american here)

 

Thanks Jeff and GrayRacer

 

I know I am limited ,, but with yur infos,some quality parts (other than stock), and good parts selection ,, I think I can get what I want ,, and what might be sellable and reliable !!

Edited by Scrammer

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