Topic: Odd wheel shudder

[Mr.Mazza]

Hey all,

Got a K4 with 4" over forks, got a weird issue appearing as I get the bike reready for rego (Still got a tank to seal )
When I pull up hard, the last 1-2m the front wheel shudders, not side to side but rather toward the engine and out, like the forks bend toward me, then get spat back into position.
Would this is not enough preload on the springs? (There is some 'float' at full extension, hinting I may need longer spacers)
The head bearings are new, not right tightness? Or are my forks cooked?

Any help, open to pissing off the long forks (Cool is cool but confident riding is much better!)

Cheers

[ekpent]

Maybe more preload needed or a fork tube is bent. I had to replace a fork tube once as it was sticking and binding a little bit.

[PeWe]

Or too much fork preload.
If progressive springs from Progressive or Wirth, no spacers.
I have tried 3-5 mm, too hard fork.
It is not a problem if fork will sag down some when straighten it up from side stand.
Opposite with progressive springs.
Front will have lower setting which is more stable than higher.

Check front tire for strange wear,  "wavy" wear.
This is my learnings from the last 2 years.

[HondaMan]

A way to check the forks for 'bent' is:
block or jack under the front crossmember  of the engine to barely lift the front off the ground, then loosen the lower axle clamp and one side's fork clamps at the top and bottom just enough that you can rotate the upper shaft by the top (filler) nut. Rotate it a full turn, watching for sideways movement of the fork's lower to tell you if that fork tube is bent. If not, retighten that one and try the other one.

It is very common for extended fork tubes to get bent on these bikes because they are not well supported: running into a curb at 5 MPH is usually enough to do it.

Normally, extended fork tubes must be made of stronger/thicker metal, or else use of a different triple tree on top is needed so larger-diameter forks (complete) can be installed with longer, thicker tubes. That is expensive, so [too] many just cheaped-out in the day and bought longer tubes. A bent tube is very common: I have 2 of those in my garage now from a 750 that came here a while ago with this very issue: they are just 4" oversize. They would bind about 1" into the travel: rotating the fork tube 90 degrees would completely bind them, while 180 degrees of rotation (of both) would let the lowers move again. But, they shuddered and 'stuck' at varying points of travel, no matter where they were turned, so we replaced them - with OEM length.

Do NOT install "progressive" springs if you can help it. The 750, in particular, hates them, as the forks lose almost 50% of travel for you just sitting on the bike, leaving 2" of fork travel while riding - this will soon bend the new fork tubes from frequent bottoming-out on rough roads (here in Colorado that takes about 1 month of typical commuting). My two cents: use straight-wound springs.

[RAFster122s]

Who makes straight wind springs for 550 and 750?

[newday777]

Who makes straight wind springs for 550 and 750?

Racetech

[PeWe]

Only using the Racetech springs will give performance increase compared to progressive?
My progressive has not bottomed out. (Yet, maybe not riding on that bumpy roads)

Found this for CB750 K6.
(K3-K6,  341 lowers) forks)https://www.ebay.com/itm/204191585741

Their emulators are needed too, right?
https://www.ebay.com/itm/155405938822

Added some fork spring photos.
- YSS progressive in plastic. 467mm long
- Used old stock look tight. 474mm long.


#
#
-Wirth at top  (in plastic too)
-Progressive under.
Same lenght 482mm

[HondaMan]

Do those progressives hold the forks up high when you sit on the bike?
All of the progressives I have seen will sink more than 2" as soon as I sit on the bike, so they end up with not enough travel over bumps (2" isn't enough!).

[PeWe]

Adjustments today.
I added 3.6mm washers under each fork cap for pretension.
Total sag 35-37mm (upper blue arrow vs scraper) when I sit on bike without leathers, boots and helmet. (Ca 88 kg).


I sat on bike, made a line on tube with marker pen. Bike back to side stand. Measured line to scraper where I wrote the line.
Green lines without washers, blue with 3.6mm pretension washers.
Highest markings when sitting on bike, lowest when bike is raised up from side stand.


I measured 87mm travel, (parked on side stand) after last 350km ride, no washers used. Rubber scrapers removed the dust on the chromed tubes.

My K6 has the Wirth springs.
With 3.6 mm thick washers the initial sag down is 13mm less than no washers.
When sitting on bike 8mm less.
Differences green vs blue arrows above. (Higher lines of rach color when sitting on bike.)


Wirth did not ship with spacers.

Adjustable fork cap nuts would be fine. If they will not leak oil.
But could work to find out the total depth of fork cap nut. Then use stock with washers to get same height.

This is another reason for a ride!

This bike, my blue K6 has Ikon shocks with progressive springs.
Good and comfortable with springs at highest (less pain in hip joints) and dampening at softest (1) (less PITA). Now changed to 2.
4 numbers in total.

My brown K2 has also Ikon shocks but not progressive springs. (FORK has progressive springs made by Progressive). 
The result of Ikon shock settings will either be too soft or too hard.
Not much better than the cheap Honda stock knock-offs bike got first.

I'm sure this will end up in test rides testing the settings.
I hope it is OK now after changing fork and shocks today thanks to this thread

[HondaMan]

When I 'toured heavy' I used the air forks up to 15 PSI and full preload on the rear (Betor or Mulholland - now ORCA) shocks, with 115 lb. rear springs. The payload ranged from 525 to over 600 lbs (not for cafe' riding!) and speeds ran 55-100 MPH on freeways/interstates. The daylight hour rides ran up to 15 hours in the saddle, 2-up riding. The tires were Continentals or (Avon rears + Dunlop TT100 front) at different times. The Conti Twins of old were pretty taxed with the heavier loads while the Avons were stellar and the TT100 with 40 PSI was to die for (they are not made since the 1990s). The Bridgestones could barely handle the payloads at all, and the first year I had my K2 I ran 10k miles and totalled out 2 sets of the 'Stones that year alone, prior to the improved rear shocks (Betor) the next year. The air forks went onto this bike almost as soon as I got it home from Honda, due to the Vetter being aboard.

By Year 2 (1973 season) I had added 0.75" of preload to the fork springs, which is still there today. If I remove the Vetter (for some test ride or something) I can feel the forks top out with this much preload over, say, railroad tracks or a steep driveway entrance step, so a little less preload would help with that. Any 750 or 500/550 I build always gets some preload in the (a little more for the 500/550).

[Tracksnblades1]

Hey all,

Got a K4 with 4" over forks, got a weird issue appearing as I get the bike reready for rego (Still got a tank to seal )
When I pull up hard, the last 1-2m the front wheel shudders, not side to side but rather toward the engine and out, like the forks bend toward me, then get spat back into position.
Would this is not enough preload on the springs? (There is some 'float' at full extension, hinting I may need longer spacers)
The head bearings are new, not right tightness? Or are my forks cooked?

Any help, open to pissing off the long forks (Cool is cool but confident riding is much better!)

Cheers

Pretty normal back in the +4 and especially the +6 days with rear lowering shock brackets.
I’ve seen +6 spacers in +4 tubes. And +6 tubes that wouldn’t compress without using the front brakes but they flexed when you hit bumps…it’ll,ride right when you put the stock length on it and get the rake angle back where they’ll compress instead of bend..$0.02…

[PeWe]

I missed the small detail about +4" longer tubes.
I had +6" early 80's with not stock fender to make it even worse.
Not good handling. Uneven wear on front wheel happened and made it even worse.
A stabilizer is a must to help some.

[PeWe]

A fork brace like this was very common many years ago when longer forks were popular.
Must be lots of old used ones waiting to be used. $25-50US

https://carpyscaferacers.com/shop/shop-our-store/motorcycle-parts-accessories/honda-cb500-cb550-cb750-parts-upgrades/alloy-fork-brace/

[PeWe]

Adjustments today.
I added 3.6mm washers under each fork cap for pretension.
Total sag 35-37mm (upper blue arrow vs scraper) when I sit on bike without leathers, boots and helmet. (Ca 88 kg).


I sat on bike, made a line on tube with marker pen. Bike back to side stand. Measured line to scraper where I wrote the line.
Green lines without washers, blue with 3.6mm pretension washers.
Highest markings when sitting on bike, lowest when bike is raised up from side stand.


I measured 87mm travel, (parked on side stand) after last 350km ride, no washers used. Rubber scrapers removed the dust on the chromed tubes.

My K6 has the Wirth springs.
With 3.6 mm thick washers the initial sag down is 13mm less than no washers.
When sitting on bike 8mm less.
Differences green vs blue arrows above. (Higher lines of rach color when sitting on bike.)


Wirth did not ship with spacers.

Adjustable fork cap nuts would be fine. If they will not leak oil.
But could work to find out the total depth of fork cap nut. Then use stock with washers to get same height.

This is another reason for a ride!

This bike, my blue K6 has Ikon shocks with progressive springs.
Good and comfortable with springs at highest (less pain in hip joints) and dampening at softest (1) (less PITA). Now changed to 2.
4 numbers in total.

My brown K2 has also Ikon shocks but not progressive springs. (FORK has progressive springs made by Progressive). 
The result of Ikon shock settings will either be too soft or too hard.
Not much better than the cheap Honda stock knock-offs bike got first.

I'm sure this will end up in test rides testing the settings.
I hope it is OK now after changing fork and shocks today thanks to this thread

UPDATE:
I have removed the washers on both my bikes that had a hard feeling at front.

Handling improved significantly with more sag. Feels that center of gravity is now lower, better stance.
More fun to ride.
Both bikes are stable and take corners much better now.
I ride a little bit less aggressive on my K2

Both bikes have 10W fork oil, level 160mm from top when completely collapsed without springs.
Measured as 175ml in each leg when refilled.

No hard bottom out. No fairing. My own load is around 105kg with leathers, boots and helmet. 12 kg clothes

The fork preload should be adjustable with a simple grip like rear shocks.
Something that does not start to leak after a year or 2.

[HondaMan]

The fork preload should be adjustable with a simple grip like rear shocks.
Something that does not start to leak after a year or 2.

NOW you're talkin'! That's what these (and most of the Japanese bikes) need. Hard to do: I once saw an arrangement where the top fork caps were welded thicker, then drilled and tapped (1/4-20 in that case) for a long screw (hex socket head) that then pushed down as it was tightened for more preload. While the welding could have used some nicer finishing work to look better, it was a thoughtful arrangement on a heavily-loaded CB750. The only "catch" to this was that it had those setbacks that move the handlebars toward the rider about 1.5" for a more upright sitting position, which he desired for the long tours (can't blame him for that!) Those were popular items back in the day. I don't like them myself, or I would have improved on his welding finish and made some for my own bike. I still might, someday? The screws were about 2" long above the fork caps, they were 3-1/2" inch long screws overall.

If this was done with the handlebars in the stock position then the preload adjustment range can only be about 1/2" before the adjustable screw would be up against the handlebars. However, if a thin-headed bolt were used, in might make for a somewhat-adjustable front spring preload, still not a bad idea?

[PeWe]

The fork preload should be adjustable with a simple grip like rear shocks.
Something that does not start to leak after a year or 2.

NOW you're talkin'! That's what these (and most of the Japanese bikes) need. Hard to do: I once saw an arrangement where the top fork caps were welded thicker, then drilled and tapped (1/4-20 in that case) for a long screw (hex socket head) that then pushed down as it was tightened for more preload. While the welding could have used some nicer finishing work to look better, it was a thoughtful arrangement on a heavily-loaded CB750. The only "catch" to this was that it had those setbacks that move the handlebars toward the rider about 1.5" for a more upright sitting position, which he desired for the long tours (can't blame him for that!) Those were popular items back in the day. I don't like them myself, or I would have improved on his welding finish and made some for my own bike. I still might, someday? The screws were about 2" long above the fork caps, they were 3-1/2" inch long screws overall.

If this was done with the handlebars in the stock position then the preload adjustment range can only be about 1/2" before the adjustable screw would be up against the handlebars. However, if a thin-headed bolt were used, in might make for a somewhat-adjustable front spring preload, still not a bad idea?

Stock fork caps are rather thick.
The washers I used were only 3.6mm thick and did a big difference.

It might be possible to drill a hole, fine thread M8. Allen bolt, like a long stop screw

A washer the bolt presses down. The fork cap must be lower than stock so it will have same thickness (height) with washer as stock.

The problem is a seal that can withstand the pressure and not leak. Plus a lock function of the adjuster.

Should be enough to adjust max 5mm down. For those in need of more, add another washer on top of the spring for the initial preload. Fine adjust during rides.

[HondaMan]

Should be enough to adjust max 5mm down. For those in need of more, add another washer on top of the spring for the initial preload. Fine adjust during rides.

Good idea, Per: add some washer(s) to get close to the desired lift, then 'fine tune' it with a shorter screw under the standard bars.
Hmm...could be good for when occasionally carrying heavy loads, but not always.