Trying to understand the hub/chain logic

[YZ426_Kicks]

Hey Guys!

Thanks for reading...

I'm confused about the hub/chain situation.

It seems the majority are emphasizing the chain tension is too tight. On full extension the tension creates overwhelming stress on the hub/sprocket bolts, and causes the sheering to occur.

Quite a few have stressed that Yamaha's recommendation of chain slack is too tight. Some have even said the "three finger" rule is too close for comfort. The latest said between 45-50 mm, yet Numpsey said he was running at 50mm and still had his hub go. I just don't understand.

Having 4x4'd in lifted trucks for quite some time, I am aware that they too have hub/bearing stress issues. Their hubs/bearings come under stress due to the WEIGHT OF THE OVERSIZED TIRES. The added weight, the forces needed to overcome the inertia to spin the tires in mud, and the weight of the mud, creates the conditions for hub/bearing failure. I've been in plenty of mudholes, and have seen all makes of trucks blow their hubs and bearings because of working the truck to prevent getting stuck.

On the Yammie, the chain is what a Super Swamper is to the truck. Even with Yammies recommendation of clain slack, or the "three finger" rule, I can't see how the chain could put that much tension on the hub to cause it to fail. Or am I missing something?

I've heard the stock chain ( which Im still running ) will stretch. Wouldn't it be reasonable to think that the chain would give before the hub or sprocket? I figured the chain would be the weakest link.

I have gone over what happens mechanically to the bike when landing a jump. ???

Any assistance would be appreciated.

Please be nice, okay. Remember, I just started...I want to learn.

Thanks!

God Bless!

Randy

YZ426 Kicks

[James_Dean]

Randy,

The chain is not always the weakest link. It is steel and other parts are aluminum and may not be as strong.

The chain tension varies as the swingarm goes thru it's arc. The tightest chain tension occurs when the axle, swingarm pivot, and countershaft sprocket are all 3 in alignment. The only concern is that there is still some slack when this tightest point in the arc occurs. When the bike is on the stand the slack may be about "3-fingers" worth.

The hub breakage may or may not have anything to do with chain tension adjustment. Other factors like higher engine inertia than 2-strokes have may be over stressing the hub along with increased bike weight.

James Dean

[This message has been edited by James Dean (edited 10-14-2000).]

[DaveJ]

YZ426K,

Sorry, but there's no comparison between a tight chain and an over size truck tire.

Truck hub bearings fail because of the extended torque applied by the larger tires against the bearing. Much like using a longer bar to assist in removing a tight bolt or nut.

The torque of a 4-stroke that pulls on the drive train of a motorcycle is not an applied force as in your truck tire example. It's a reference to the nature of the engine power output

However, a larger sprocket would apply more "torque" against a hub than a smaller one. In this case, you would introduce forces similar to your truck tire example.

Make sense?

Now, I think there are some hub failures out there that may be caused from poor castings of the hub.

But whenever a collection of mechanical devices breaks, they always leave a trail of evidence as to how and why.

In poor castings, air pockets and shiny surfaces within the metal can be seen along the fractures. That's a given.

If a hub was overstressed, such as under-designed for this type of engine, then the break(s)in the hub would be even or consistent. In other words, no bolt holes would remain on the hub in their original shape - there would be distributed destruction.

The drive train of these bikes (hub, chain, sprockets) are designed for maximum strength when forces are applied within the line in which they travel, (push/pull). If forces are applied in any other direction, the weakest component will give, (in this case, it's the sprocket).

When a chain is too tight, (via the movement of the rear suspension) the forces applied against the rear sprocket and hub are perpendicular to the direction that the chain travels, (against its design).

Since the sprocket can't go inward, it bends outward, and in the process snaps the bolts, and further overstresses the remaining hub attachments.

This is evident by a hub that has two or three attached bolt holes after the failure, (one half is ripped off, the other half is untouched). Other signs include sprockets that are folded, bent main shafts, and broken and missing bolts.

If these were caused by bolt failures, the nature of the breaks, and conditions leading to the hub failure would be very different. That's a story in itself.

You can experiment with this by setting the bike on a stand. Tighten the chain to a minimum spec of 35 to 40mm. Then raise the rear swing arm up in an attempt so that the rear axle is the same height from the floor that the swing arm pivot bolt is, (you may want to disconnect the rear linkage) to make this easier.

When you extend the swing arm up, you'll feel the chain get tight, then you can see the sprocket begin to fold (flex) outwards. If you where to fully apply upward force, with the added force of the engine at the same time, and/or repeat this motion (like over a series of bumps) then the effect would be more dramatic, and of course you would set off a chain of failures, (no pun intended).

So I'm not saying that every hub failure is the result of a tight chain. But in some of these cases, the evidence is obvious.

And I don't think I would ever recommend setting anything to a spec measured by body parts.

Hope this helps.

DaveJ.

[yo_its_matt]

just a thought, when i check my chain tension i usually check it with weight on the bike ive noticed that if i set it to specs and weight the bike they are too tight sometimes! if you check your chain like you would race sag, say a 1/4-1/2 inch if its muddy a little less in dry conditions, you wont ever have it too tight, it may look too loose on the stand but who rides it on the stand anyway!

[MXOldtimer]

I dont even know what my manuel spec's are for chain tension and I have never used spec's to set my chain tension on any bike I have owned. When ever I buy a bike and tear it down to lube everything (before riding) 1. I disconect the shock

2. I straight edge the counter shaft , swingarm pivot & axel

3. spin the wheel to find the "tight spot" in the chain/sprockets

4. set my tension on the chain using some comman sense sop for, mud build up.

5. now I put the bike on a stand pick a fixed point on the swingarm, place the master link over my fixed point press down on the link and measure

6. when setting my chain tension never ever can my measurement be "MORE" than than my first measurment , "lets just say" my first measurement was 1 1/2 inches it cant even be 1 9/16 inches. less of a measurement is ok but "never more"

7. If you ever change a chain or a sprocket(should be changed as a set) you cant use the old measurement go back to steps 1-5

If you have more than one sprocket, what I use to do with smaller 2 smokes is peen my hub & sprockets make all my measurements at one time and write them down, then when I changed or removed, I knew where to place the sprocket & what my measuements was

Doug

[DaveJ]

Doug,

Very smart...and good advice for all!!

DaveJ

[holeshot]

Great explanations in this thread. I plan to use Mxoldtimers method when I service the linkage (soon).

I'd also note that a loose axle nut can also cause a hub failure, because the axle oscillates back and forth, creating forces on the hub that were not anticipated or designed for. When we returned from a ride a while back, we noticed that the axle nut on a CR250 was very loose, so we tighened it, loaded up the bikes and left. When he got the bike home, he noticed a couple of small bolts and nuts lying under the bike, but thought they were just "extras". ?

On the next ride, he went about 50 ft before the hub exploded $$$.

A chain that's too tight will also cause the rear wheel to hop up and down and kick sideways over acceleration bumps/whoops, turning the excellent rear suspension on the 426 into a pogo stick.

[Hick]

Using the method in the manual (bike on stand, wheel off ground, distance between top of rear arm guard bolt to bottom of chain) I compared Yamaha’s spec to what would be MXOldtimer’s method.

They were very close.

Because of some of the hub failures I had read and corresponding opinions that stock spec was too tight I was worried.

Maybe I am doing something differently but at 40 mm I still had at least one inch of play at the chain’s tightest point in the swingarm’s travel. That seemed like enough to qualify for Doug’s “common sense slop [sic].”

Coincidentally I did this before I read his post. ? I should mention that I have a 47 tooth rear on my 426 right now, that will make some difference (as Doug noted in his post).

The Yamaha spec also corresponded pretty closely with the “Three Finger Method” (bike on kickstand, wheel on ground, 3 fingers between rear of arm guard and top of chain.” I agree with DaveJ, it makes no sense to use fingers when a caliper (or tape measure) is handy but I’m not going to lie, I have done this with all my bikes. But now that I know how tight is too tight on my 426 I will use my calipers. I suppose I will have to redo the measurement if/when I regear.

Maybe some track debris (rocks, mud) getting wedged between sprocket and chain is what caused some of these guys hubs to grenade (or a bad casting) but I didn’t find anything wrong (re: too tight) with the method and spec laid out in the manual.

[Taffy]

just a question, right?

4 times as many wr400's out there yes?

no collapsed sprockets & hubs

but plenty of loose bolts that have locked the wheels up etc. and before you say anything we ride em hard too!!!!!!

everything yamaha do is only just good enough for the purpose for which it was designed yes?

400>426cc, improved ignition, carb, more, more, more!!!

the difference between the bikes is the clutch, gearbox AND the rear wheel/tyre.

the rear tyre on the enduro has a lot more give in it. it's the only kindness the geartrain knows. the walls are a lot taller for the rocks etc. how many of the YZ's are dualsporting or not sticking strictly to a diet of glen helen for breakfast, dinner & tea?

if you run a ducati without a rear cush drive you smash the gearbox & clutch to pieces in no time. one trip even. seriously!!

if you beef up a clutch, if you beef up the rear wheel (19" over my 18") if you have a gearbox that makes the engine ALWAYS rev as it bounces into & out of contact surely something somewhere has to give?

when a wr hits the next gear the gap is big enough that the revs die. power is down & therefore softer & kinder. we're only talking small percentages here-increments

is there any give in the geartrain anywhere? is it true that some riders are experimenting with 18"er's?

could they be YZ riders at rocky type tracks?

i don't read the comics enough to know.

lot's of questions & thought provoking idea's gentlemen.

Taffy

[holeshot]

There are "local pro" level riders at our OTHG races in SoCal, who can jump 100 foot doubles with one hand tied behind their back, and they use stock hubs, spokes and rims on their 426's.

The only real difference between them and "regular" people, is in the wiring between the brain and right wrist.

[DaveJ]

Hick -

I would think the 47 tooth sprocket would make a difference, but I'm not sure by how much. It would also effect your wheelbase (unless you cut the chain).

And an inch of deflection could be objective. Even if the chain doesn't make contact with the top of the swingarm, (which is tight enough to cause problems) you could still say you have an inch of slack. Let me know the details of how you went about this.

This would also apply to how tight you pull the chain when you make the 40mm setting as well.

In either case, I'm not trying to start a chain tension riot here. I'm just trying to get everyone to understand why a select group of us would be experiencing a problem that the masses are not.

I also think that Yamaha did a great job on specing the hub design. If not, I and many others would have to replace these things once a week.

DaveJ

[Numpsy]

It seems that we here in Sweden is not accurate enough to tension our chains.

This weekend I rode a scramble type of race (on my fathers brand new CR 250 ´01) and ther I talked to a couple of guys with both YZ´s and WR´s and found out that 50% of them have had a hub faliure.

To add to the list was one guy who DNF:ed with a broken hub and one guy whos hub allmost broke.

By allmost I mean that all the sprocket holes now are elliptic instead of round and all the bolts had tilted.......

The rear sprocket range of us all is from 47-50 and all has the chain a bit "slack" compared to the book.

Numpsy

[Hick]

Originally posted by DaveJ:

…an inch of deflection could be objective. Even if the chain doesn't make contact with the top of the swingarm, (which is tight enough to cause problems) you could still say you have an inch of slack. Let me know the details of how you went about this.

DaveJ

I installed the swingarm (but not the linkage), wheel, and chain and snugged the axle nut. I then lifted the swingarm up with a jack until the countershaft, swingarm pivot and rear axle were lined up. Then I set the chain tension to between one and two inches of play, halfway between front and rear, depending upon how hard you pulled on it. I didn’t measure anything here but I’m convinced it was plenty loose, and you’re right this is subjective. (?) But you could easily move my chain an inch or so with one finger.

My Regina o-ring chain has one two hour plus race on it but it is still in good shape (i.e. stiffer than a non-o-ring).

Then I installed the linkage, let the arm hang down and measured the clearance with a pair of calipers, pulling up firmly with one hand and inserting calipers with the other. It was actually right at minimum spec so I increased the play a bit. I did not remeasure the play at its tightest point because I didn’t see any need to; I knew I had more than enough slack at this point.

The obvious source of discrepancy would be my TM chain slider being taller than stock. I tried to account for this when I made the second measurement but I’m certainly not infallible. Maybe I made a mistake somewhere. This may also have affected my initial setting relative to a stocker. But my adjusters ended up back where they started so I’m satisfied that my chain tension is sufficiently loose (it survived the race with minimal stretch, there was lots of gravel, water and some mud on the course, some of which ended up on my chain/sprocket by the end) but still within spec.

I also use calipers to set the right axle block. I’m sure eyeballing the marks works fine but calipers are actually easier and provide some piece of mind that my axle is straight.

I’m not sure what difference my smaller rear sprocket would make either. I suppose it would depend upon the angle of the arm at full extension, but it seems a smaller sprocket may result in less (not more) slack at this point. But I think a standard chain (especially if broken in) would make as much of a difference, especially given the subjectivity of BOTH measurements being compared here, calipers or not.

I still think that one little pebble of destiny is all it would take to break many things, even if your chain is too loose. Once the dust cleared you may never find any evidence implicating the pebble. It seems to me that a chain set tight enough to do any damage would be obvious at a glance. Of course if this had happened to me I wouldn’t be so sure…

[DaveJ]

Hick,

I'm a little confused. When I read your first paragraph, it sounds like you raised the swing arm, then set the tension. Then lowered it and made a measurement.

Did I read that right?

Have you tried setting the 40mm as the manual says, then testing max tension?

DaveJ

[MXOldtimer]

Numpsy

If the guy finished the race and had elliptic holes in his hub with tilted bolts that would show the hub still had strength since it didnt come apart, besides the bolt holes wouldnt be a bad hub, thats a loose sprocket again someone didnt tighten properly or didnt locktite the bolts so how can that be listed as a bad hub. Im not messing with ya cuz I want to know some fact so I know what to look for before my hub explodes. If you find more people with bad hubs "INVITE" them in here so we can hear there story rather than second hand info.

Doug

[Numpsy]

Hey, I can not speak for the guy with the elliptic holes and pointed this out to him, but he told me he had torqued them acc. to the book.

I am not sure if I belive that, but I like to take a fellow dirtbiker for his word.....

All I spoke to during this raceday got a card from me with this site adress on it, just hope they will show up !!!

I have now ordered a Talon hub and a Hinson basket and will soon be able to hear the lovely sound of my darling out on the trax again.....

Despite of all the problems I really have to say that this is the most awsome bike ever !!!

Numpsy

[holeshot]

Hick:

I finally had a chance to service the linkage, and while I was at it, decided to "straight line" the swingarm to see what the minimum slack would be with stock sprockets. The 40mm listed in the manual seems to be correct ? , because when I had it at this setting, the chain was pulled tight at maximum tension, but not enough to break a hub or cause any suspension problems.

It looks like I'll need a new sprocket soon. What did you do to keep your sprocket bolts from loosening (blue or red locktite?)

[Hick]

Holeshot,

Your results with the chain tension seem similar to mine, thanks for that info.

I haven’t had any problems with sprocket fasteners coming loose but I don’t think the riding I do (even the occasional 2 or 3 hour off-road event) is as hard on the driveline as a moto or two on a hard track. I’ve been using blue loctite anyway though. Ever since I owned a KX500 I put loctite on almost everything with threads (I’m a bit paranoid that way after 2 pinch bolts somehow came out on the KX).

After about my fourth or fifth sprocket change the lock nuts didn’t seem to be working as well, you could almost spin them on/off past the locking tang by hand. So I replaced them with similar grade nylocks.

I think new lock nuts (any kind) are the best insurance (even better than loctite). A new set of stock nuts for the “stock is best” folks will hurt you at $2.20 per (HLSM price) but any decent fastener or automotive store will have hard grade lock nuts of some type. IMO you can only expect a lock nut to survive so many removals before its effectiveness is diminished. If you are really anal you would replace the nuts every time you removed them (esp. nylocks, they tend to come apart eventually).

If the lock nuts on your bike are still tight I would just use blue for piece of mind, torque them and check them after one ride. Personally I wouldn’t use red, you may have hell getting them back off, but if you’re worried enough perhaps it is worth it (I’m sure piece of mind is worth a lot when you pin it up the face of that triple jump, or whatever you wacky moto guys do every weekend ? )

[flyinguitars]

Hey guys,

I have a question...Has anybody ever tried using safety wire on the hub bolts or other important bolts? I worked for a few years as an airframe and powerplant mech. and we safety wired every single bolt that was torqued. on aircraft. Just a question not advice.

Mike

[Hick]

Mike

I love safety wire. Footpegs, rear brake lever bolt & master cyl. pin, grips, drain bolt(s) etc.

I don’t think there is enough threads exposed on the rear sprocket for safety wire to work there. I suppose you could drill the nut AND bolt…

It just never occurred to me since I haven’t had problems here but I don’t see why it wouldn’t help.

How do aircraft mech.s do it (washers, pre-drilled bolts, you drill the bolts)?