Author Topic: '78 CB550 fork springs  (Read 323 times)

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Offline FourUsTry

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'78 CB550 fork springs
« on: January 05, 2019, 11:49:58 am »
I'm new here and just getting started on a '78 CB550 restoration. I've got the forks disassembled and was surprised by two things:
  • The springs look progressively wound. I thought stock springs were straight wound but I've seen some references that say that some were progressive. (I don't have the mojo here to post pics yet.)
  • They have a free length of 18.75" and they gave a typical ~3/4" pop up when the cap bolts were removed. I can't find the specs for the length for a '78 but http://manuals.sohc4.net/cb500/service_manual/HSM500550_16.pdf is for K3/'77 and it says 17.46" when new. Assuming that negative sagging is unlikely, anyone have a guess whether these are stock fork springs? Anyone know the correct free length for a '78 CB550?

Based on the collective wisdom here I think I should get new springs (if these are 40 years old) but it would help to know the correct length. I'm also confused how some of the replacement springs say they are for all these different CB models/years when clearly the lengths are varying between years.

Finally, will the emulators make a huge improvement and can I handle that myself without a drill press? Seems like a moderately complex mod.

Thanks!
1978 CB550K

Offline Deltarider

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Re: '78 CB550 fork springs
« Reply #1 on: January 05, 2019, 01:37:41 pm »
Why would you replace them? Mine are older than yours and still function as they should.
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Offline Bankerdanny

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Re: '78 CB550 fork springs
« Reply #2 on: January 05, 2019, 04:34:37 pm »
They are progressive and none of my 40+ year old Hondas have needed new springs
"The problem with quotes on the Internet is that you never know if they're true" - Abraham Lincoln

Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki 80 (K11 probably, it's been 30 years), '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200

Offline FourUsTry

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Re: '78 CB550 fork springs
« Reply #3 on: January 05, 2019, 06:41:45 pm »
Good to know that they are probably OEM despite being progressive -- that eliminates one surprise.

I've read some folks here that say they should be replaced based on age alone, but if this is not necessary and I can confirm that they are not too sagged by time then I'll keep them in the rebuild unless I get the bug to try the emulator upgrade.

Thanks for the info!
1978 CB550K

Offline Bankerdanny

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Re: '78 CB550 fork springs
« Reply #4 on: January 05, 2019, 06:48:11 pm »
The manual has measurements for spring length. You can also measure the distance the tubes move down from on the center stand to off the stand
"The problem with quotes on the Internet is that you never know if they're true" - Abraham Lincoln

Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki 80 (K11 probably, it's been 30 years), '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200

Offline FourUsTry

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Re: '78 CB550 fork springs
« Reply #5 on: January 05, 2019, 08:14:38 pm »
The info I found with spring lengths is 17.78" in the CB500-CB550 manual and 17.46" in the '77 addendum. The '78 Owners Manual doesn't list it. Since my springs are longer I assume that either '78 springs were different or I have non-stock springs. I have not found a reference that shows the '78 spring length, but maybe someone out there knows where to find this?

Given preloading I'm not sure how to compute the spring free length from the fork free travel but I see how they are closely related.
1978 CB550K

Offline Bankerdanny

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Re: '78 CB550 fork springs
« Reply #6 on: January 05, 2019, 09:11:07 pm »
The '78 springs should be the same as '77. There was a slight change to the fork internals in '77.
"The problem with quotes on the Internet is that you never know if they're true" - Abraham Lincoln

Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki 80 (K11 probably, it's been 30 years), '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200

Offline FourUsTry

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Re: '78 CB550 fork springs
« Reply #7 on: January 05, 2019, 10:21:09 pm »
OK, thanks... that makes sense since the '77 and '78 had almost the same specs. But that leaves me with the mystery of what fork springs I've got in there since they are ~1" longer than stock. The bike has only ~5K mi on it and I believe was laid up since 1982 and I don't think the mechanic dude who flipped it on ebay would have put in new springs (and the fork oil was thick and murky).

If I keep them I guess that'll mean the front rides a bit higher than designed, which might work out fine since I am going to a 4.0" rear tire to replace the stock 3.75". Of course that depends on whether these (non-stock?) springs are more progressive so softer at low deflections. I can always add some washers to raise it a bit but I'd rather not grind springs to lower it.

Fun with bike sleuthing!
1978 CB550K

Offline Deltarider

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Re: '78 CB550 fork springs
« Reply #8 on: January 06, 2019, 12:06:47 am »
I think you have given the answer yourself. With that low mileage, not much chance the springs were replaced. Moreover p 56-7 in the Part List do not show any modifications that matter here. You must know that in more than one Honda parts list the springs have not been depicted correctly... Imagine you been the schmuck that had to depict all those windings, what would you have made of it? ;) http://www.honda4fun.com/pdf_parts_list/pdf_spac500-550/CB500-550_K3-K4.pdf
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Offline FourUsTry

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Re: '78 CB550 fork springs
« Reply #9 on: January 06, 2019, 12:45:36 am »
Thanks for the parts list doc -- that is one I hadn't found yet. At least now I have a part number and I found a picture of it that look like mine with progressive winding and narrowing at the top end. I am not surprised that the parts diagrams don't bother showing the progressive winding.


I still can't find any other source that says what its free length is so either the K3 addendum is wrong or they made a design change at some point with the '78 model or (I agree, unlikely) my springs are not stock. I'll probably use them as is and see how they perform with fresh fork oil and seals -- can always do an upgrade next winter!


Thanks for the help (both of you).
1978 CB550K

Offline RustyF

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Re: '78 CB550 fork springs
« Reply #10 on: January 08, 2019, 05:27:16 am »
I recently replaced the fork springs on my 78 550.  At first I just changed the fluid but the forks remained "soft" and would fully travel under braking.  Checked this with a zip tie to measure how much travel I was using.  The old springs were progressively wound, I replaced them with a set of Progressive Springs Part Number: 11-1106 and 10 Wt fork oil. This corrected the sag, excessive dive under braking and gave me a more controlled ride.  I was very please with the results on my bike.  On a side by side comparison the new springs are about 5/8 longer than what came out of the bike.  No problems getting the top nut back on.

Hope this helps.
1978 CB 550 K

Offline FourUsTry

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Re: '78 CB550 fork springs
« Reply #11 on: January 08, 2019, 02:08:00 pm »
Thanks, RustyF. Super helpful. Do you recall the free spring length of the original and/or replacement springs? Mine are 18.75", which is longer than any spec I can find so I'm not sure if they are sagged or not.


I was thinking of adding a few washer spacers to adjust for sag if I can figure out how much sag I've got. I should do the cable tie thing and see how close to bottoming out I get.
« Last Edit: January 11, 2019, 09:20:34 am by FourUsTry »
1978 CB550K

Offline Deltarider

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Re: '78 CB550 fork springs
« Reply #12 on: January 09, 2019, 12:34:31 am »
What sag? I'd install things just the way they are and testride first.
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