vmax front end prob.- add valve springs? & how much oil & air pressure please?
February 3rd, 2010 | Articles | 2 Comments »
philo asked:
i need the correct oil measurement for the forks and the correct air pressure for the forks…my friend has an 86-7? model vmax here in the philippines. the front end is a real dog… very ‘spongy’… i own a xs650 yamaha and he had a real hard time riding with me last weekend. i hated having to slow down for him to catch up. i realise the springs are probably shot too… could adding a couple of automotive valve springs to his forks help? thanks fellow riders….. go the xs650!!!!
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i need the correct oil measurement for the forks and the correct air pressure for the forks…my friend has an 86-7? model vmax here in the philippines. the front end is a real dog… very ‘spongy’… i own a xs650 yamaha and he had a real hard time riding with me last weekend. i hated having to slow down for him to catch up. i realise the springs are probably shot too… could adding a couple of automotive valve springs to his forks help? thanks fellow riders….. go the xs650!!!!
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You can use valve springs, but even better is 3/4″ pvc as you can cut it to a desired length for pre-load.If you add pre-load cut them to a length that is such that you have to push down oh the spring or spacer 1″ or so to put the caps back on.
The ideal spring load leaves you with 25-33% of the travel as sag or top out.
As far as air pressure ( assuming the seals are good and air tight) forks run on very little pressure , like 5-7 lbs.
The fork oil capacity will be in the manual or you may have to contact a dealer for the specs. I don’t work on metrics too much any more .
One additional tip, when replacing the fork oil ,try a medium or heavy weight, this will slow the speed at which the forks compress and return.
Hi Philo,
Vmax has handling problems even when they were brand new.
I’ll answer your questions,then tell you some other things about it.
””””””””””””””””””””””””””’
Front suspension:
Front fork travel ……………….140 mm (5.51 in)
Fork spring free length……..386.5 mm (15.2 in)
(minimum)………………….381.5 mm (15.0 in)
Collar length………245 mm (9.65 in)
(this is the long spring spacer in the fork)
Spring rate:,,,3.75 N/mm (0.375 kg/mm, 21.O Ib/in)
Oil capacity:
619cc (21.8 Imp oz, 20.9 US oz)
Oil level:
123 mm (4.8 in)
Oil grade:
Yamaha 10w Fork Oil fork oil or equivalent:
Enclosed air pressure:
standard: ( 39.2 Kpa ),,,(0.4kg/cm2)….(5.7psi)
Minimum to Maximum:
39.2 ~98.1 Kpa ,,, (0.4 ~1.0 kg/cm2) ..(5.7 ~ 14.2 psi)
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About Adding Valve Springs:
Vmax Fork Springs are VERY Soft
That has 2 Bad effects upon Handling.
#1 the Front end SAGS,,,this changes the Steering Geometry.
This is ALL the Time,,,while sitting still,,riding normal,riding in a straight line.
#2 the Front end DIVES…this is SIMILAR to SAGGING,,but Worse.
It’s Dynamic,,,it CHANGES while You ride.
Every slight Increase in Load from Braking,Cornering,or even from Bumps in the road triggers an EXTREME SAG.
And it happens suddenly without much resistance,,,,so it moves Very Far when it Dives.
Adding “Booster Springs” Will help the SAG.
It will raise the Bike’s ride height and correct the steering geometry.
But it will NOT help the DIVE very much at all.
And unfortunately, this is where VMAX’s need the most help.
There’s 2 Basic Properties to Springs.
PRELOAD,
and
RATE
Booster Springs Increase the PRELOAD
but they do NOT increase the Rate.
So they’ll Raise the Bike to correct the SAG,,
but do almost NOTHING to help the DIVE.
The Resulting Effect is that under Braking or Cornering Forces,,,the Dive will be about as bad as ever.
Think about RIDING that .
Now–It’s Sagged and rides a certain Way.
When it DIVES,,,it only goes So far before it Settles.
That represents a certain Percentage of CHANGE.
If You Raise it with Booster Springs,,that Corrects the SAG.
But when it DIVES,,,it dives at exact same rate.
Except it Dives Relatively FARTHER.
So it is a Larger Percentage of Change.
Kind of like falling off a Higher Cliff.
The “TRICK” is to use More Booster Springs to Replace the Factory Spacer.
This cause the Main Spring and the Boosters to SHARE the Load,,,which Increases Overall RATE,,,,and Reduces DIVE.
Imagine a Very Soft Spring and STIFF spring,,,pushing against Each Other,,end to end.
The SOFT one Collapses first,,,before the STIFF one moves at all.
At That point,,,the SOFT spring becomes Nothing but a SPACER because it’s Coil Bound.
And the Only “SPRING” you have left is the Stiff One.
As you narrow the difference between the 2 Springs,,,they begin to SHARE the Load.
Soft spring compresses a Lot and Stiff Spring compresses a Little in response to the Load of the Soft one’s Compression.
If they were MATCHED Equal,,,they would Compress equally under a given Load.
If their Rate was 2:1 for example,,,Soft one would Compress 1″,,Stiff one would compress 1/2″
OR,,,Using a 10 pound Load Example ,,and a 10lb and 20lb Rate Springs.
10Lbs would compress 10lb Rate Spring 1″
Or,,would compress 20lb spring 1/2″
Using BOTH Together would Split the Difference,,
10lbs would compress them 3/4″
20 lbs would be 1.5″ vs either 2″/10rate or 1″@20Rate
50lbs would squash a 10Rate Spring,5″
or a 20Rate spring,,2.5″
Together,,,it would be about 3.5″
Valve Springs,Booster Springs,,are SHORT.
As soon as they are Loaded Enough to become Fully Compressed—-then they become “Just Spacers”
And the RATE quits RISING.
Think about it a minute and it makes sense.
Using MORE Boosters>Stiff Springs,,,keeps them acting As Springs Further thru the range of Travel,,,before becoming “Just Spacers”.
………………Anyway,,,,
Remove the Factory Spacers,,,and Cut them ,,,OR Better yet,,,just make some Shorter Ones from some cheap pipe.
And use 2 Valve Springs,,,or even 3.
You gotta Experiment and see which way works best
If You’ve got a “Bathroom Scale” You can Measuer the Springs FORCE and Preload and Rate.
*Put a Rod Thru the Springs to keep them together
*Push straight down on the SPRING,,,and note the Weight on the Scale
*I THINK the TRAVEL is about 5 Inches on those forks???
–Check it on the bike to make sure.
You want to Check STOCK spring & spacer Setup.
Measuer the “weight” at the existing ,current Ride Height
And the amount of Preload.
And the Fully Compressed “weight” of the spring.
When you know all 3 of specs about the Stock Spring and Spacer…..
You can then Easily figure out How Many Valve Spring Boosters to Use,,,,and How Long the Spacer needs to Be
Recall the STOCK Spring is 21 lbs per Inch.
Each Inch of Preload adds 21lbs of weight to scale.
Full Compression @5″ Travel =105Lbs>>>5 x 21,,,,
PLUS however much Preload.
Ya Want the Equivalent of about a 35~40Lb Spring under under Full Compression.
A)Find out the Forks Full Travel
B)Figure out how much Preload the Stock set-up has.
Both in Inches and in LOAD.
Example,,,1.5″ Preload and pushing the Spring Down 1.5 ” on the Scale =30Pounds
c)You want to get about 1.5 to 2 times the Total Load at full fork compression.
And about That much Preload,,1.5~2X the Stock amount
You Cant get it Ideal or Perfect,,,not even with a Lot of Luck.
But You can get it CLOSER than it is now,,,and make a BIG Improvement.
It’s TEDIOUS,but not real difficult or complicated.
Just simply adding a Spring,,,MIGHT make it WORSE.
Because it WILL fix the Ride Height and Fork angle and make it SEEM Better,,,LOTS BETTER.
But if you Do No Increase the RATE also,,,
Soon as You LOAD The Front under Braking or Cornering,,,
The DIVE will be More Extreme.
And your “improved handling” will fall off a cliff.
It will dissapear Quick and upset the bike badly.
You can BUY good Replace Springs for about $100 USD.
Or spend some time experimenting,testing with a scale,making spacers,,trying 2 or 3 valve springs—and get pretty close to the $100 spring set.
Use as Little Air Pressure as possible,,,use it to “Fine Tune” your stiffer springs or to adjust Ride height
………………………………………………….
OIL:
Use Name Brand “high performace” Fork Oil.
Not just plain motor oil.
BelRay is best,,,or my 1st choice anyway.
Yamaha says to use 10Weight Fork Oil.
Use 20 Weight instead.
That helps a LOT to Slow Down any diving.
30 Weight is far too heavy.
It’s BAD to try to Trick suspension into Acting “Stiff” with Too Heavy of Fork Oil.
Heavy Oil does NOT stiffen it,,,it only SLOWS the action.
When it gets TOO Slow you lose Compliance with road-surface following.
Which means you lose traction,,,not a Good Thing to do..
You CAN add a Little Extra Oil to raise the level.
That Reduces Air Volume in the fork.
Which makes the Air-Assist “Spring Rate” Rise Faster.
It’s another Trick to make the Forks act Stiffer than they actually are.
Some people Like doing that,,and on some bikes it works well.
I prefer to do that as little as possible,,,and rather Try to get actuall Spring Rate as close as I can.
You CAN use more oil (a LITTLE MORE) to help Fine Tune the fork—but keep it to a minimum.
The Space is already SMALL,,,and as an Air Chamber it gets TOO SMALL very quick from Too Much Oil.
………………………………………………………
SHORTER SHOCKS
–lower the Rear and Help add Rake to Front end.
It helps stabilty and takes away some of the Wobble Vmax’s do sometimes.
Shocks ,,are Expensive,,,$300 range and Up.
You can look around the internet and see “lowering bracket kits” for various bikes.
They are easy to make.
You can make simple ones and drop rear end 1/2″ to 1″.
It’s a Small amount,,,but You CAN feel the differnce .
It just simply moves the Shocks farther back.
The “Bad News” of a Lowering kitt which simply moves the shocks rearward is;
It Shortens the rear suspension Travel ,,because of the Geometry change,,altered leverage,etc.
If ya dont change it MUCH,,,it’s OK and works well without too much compromise.
It also “stiffens” the rear end a bit.
Easy to make the brackets for folks who are into doing that kind of stuff.
It’s Nothing Fancy at all.
You can get a Large Percentage of the Benefits of Shorter Shocks,,,,,without spending $300
……………………………………………………
FORK BRACE:
These things do a WORLD of Good.
They’re not Too Expensive to Buy.
But “One Ride” and you would not care How Much they cost.
They make a BIG improvement.
Good News they are SIMPLE to make—-for anyone who is into doing that kind of stuff.
A “Homemade one” can be HEAVY,,,and UGLY,,,and very Crude Looking.
It can be SCARY LOOKING thing to have bolted onto a motorcycle fork,,,,Garbage-Like Appearance.
A Little extra effort can make even the worst homemade one look good enough.
One Ride,,and it’ll be “most beautiful thing in the world”.
One Any bike they work good,
On a Vmax,,it’s almost a Necessity.
You can Buy them for $75~100 on up,,
or make one for almost nothing that works just as good.
…………………………………………..
Frame Brace
This is the BIG THING for making Vmax’s work good.
Kits are usually $200 and up.
But these too can be rather easily “homemade” very inexpensively.
It doesnt have to be fancy to work well.
This is a Must Have for any Vmax.
……………………………………………………
I Know you Didn’t Ask about All those things.
You are RIGHT about the Forks needing,,,something.
And your idea about Booster Springs will help a lot.
A little extra Thought and Effort,,,You can Do that and get it to work Real Good.
And save almost $100
The OTHER stuff,,
Shorter Shocks,$300
Fork Brace,$100
Frame Brace,$200
Fork Springs,,$100
That stuff is ALL “necessary” to get that bike to even BEGIN “Working Right”.
But it’s Also probly about $800~1,000 USD Minimum if you Bought it all.
All those things have “Homemade Alternatives” that will work just as good.
And can be done VERY cheaply,,,,,almost “free” depending on who’s doing it & their capabilities.
Or at least a LOT cheaper.
And it turns a Vmax into an Entirely DIFFERENT bike,,,handling-wise.
It dont make it just BETTER,,,,it makes actually GOOD.
,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Here’s some Links to those Items.
You can take a look and get some ideas.
Maybe Shop Around and find some Cheap Ones,,
or look into Doing It Yourselves,,or finding somebody to help make them.
Your Booster Spring Idea and Good Fork Oil is BEST Place to start.
Fork Brace is Next.
Lowering Rear a Little can be done pretty easy.
FRAME Brace seems Exotic,it’s really not,,,and it alone will make as much improvement as everything else all total.
………………………………………..
Here’s FORK SPRINGS:
# 11-1129
general info
Maybe the Specs & Info can help you “calculate” some effective booster springs,,or give ya some ideas??
Retail Source
Look at “VMAX” then “Suspension Upgrades”
They have ALL that stuff,,Springs,,the Braces,Shocks,etc.
***LOOK at the SWING ARM UPGRADE also—-thats a VERY SIMPLE Mod to Do “homemade”
…………………………………
Here’s Cheaper Fork Springs and Fork Braces
……………………..
Here’s a buncha Vmax’s you can see the Frame Brace.
Different Styles,,but all same design,,,mounted in same place,same way.
It’s very simple……even though all these are “fancy ,custom accessorty items”.
Nothing but a Pipe or Bar bolted across the frame.
Some Clamp on frame tube,,,some have a Mounting tab welded on to frame at upper end.
……………………………
Look around the internet at Vmax’;s,,,
you’ll see a BUNCH with the Fork and Frame Braces.
And braced swing arms.
Most are very nice parts,,,,but they ALLWORK the Same.
And are pretty simple to make for anyone who can do simple fabrication
Just something to think about.
Sorry it’s such a long answer,but maybe you can get some ideas that will be easy and help