Author Topic: fork brace snafu 500 vs. 750 (Read 2203 times)

jvandyke · « **on:** May 07, 2011, 10:13:07 PM »

'72 CB500K
While doing fork seals and boots I discovered the upper fork brace to be cracked at pinch area. Sourced a good 500 brace, refurbed. Noticed it had lower risers for the handle bar, was told I probably had a 750 brace on the bike. Proceeded. On reassembly it appears the difference between a 750 and a 500 upper brace is more than just the riser height. The spread between the tubes is also different. Yes? Well, I know the brace I had on the 500 is certainly different than the one I bought that came off a 500 as the tubes don't line up, not even close. I need either a 500 lower brace or a good 750 top brace then, yes? Is the lower brace as easy to swap out as the top or will the whole head need to come apart? Darn I really thought I was close to getting back on the road, shoulda let the dang seal leak. Of course, finding the cracked upper brace before it killed me was probably a good thing. (no, I didn't over torque it, PO did I assume)

Scott S · « **Reply #1 on:** May 08, 2011, 05:15:16 AM »

Are you calling the triple tree a 'brace'?
The fork brace is usually what goes between the forks at/on/above the fender. In some models, the fender itself serves as a brace.

It sounds from your post that you're describing the triple tree.

Kevin400F · « **Reply #2 on:** May 08, 2011, 06:38:53 AM »

I measured a '78 CB750K triple clamp against a CB500 triple, and they appear to be the same fork separation at approximately 7-1/8 inch.

Perhaps the one you bought came off a CB500T rather than a CB500Four?

Some measurements of each would be useful, to help us determine what you are dealing with.

Kevin in TX

jvandyke · « **Reply #3 on:** May 08, 2011, 08:01:02 AM »

Yes, top brace I meant "bridge" I guess. My bike is a 500k1 (four) I thought it was all stock but the new top bridge I got came off a 500k and didn't match the spread of the tubes so whatever is on my bike isn't a 500K it seems. I think I sourced a 500K lower bridge/stem so hopefully I can swap that in and be back to good. I will post some pics and measurements later today.

jvandyke · « **Reply #4 on:** May 08, 2011, 10:24:31 AM »

Here's pics. Shiny one is reported off a 500 four. Broken one is?

??
Spread center to center tube holes is really close to 7.25" on both.

Kevin400F · « **Reply #5 on:** May 08, 2011, 12:16:57 PM »

Ahh......now I understand what you meant by the tubes not lining up. Unless the angles of the bridges in the pictures are deceiving me, it appears difference is not in the width between fork tubes, but in the offset from the steering stem. You really don't want to mess around with that dimension if you can help it, since the offset affects front end trail and chassis stability!

I STRONGLY RECOMMEND you replace the broken top bridge with another of comparable geometry, rather than using a lower tree to match the "large offset" upper bridge. With the large forward offset, you would dramatically reduce the front end trail and reduce chassis stability.

mjstone · « **Reply #6 on:** May 08, 2011, 12:37:55 PM »

I'll go out and check my '72 500 and see which one it is. You definitely want to use one from a 500k/550k.

MJ

Kevin400F · « **Reply #7 on:** May 08, 2011, 12:52:25 PM »

jvandyke......I pulled this top bridge off a spare 1972 CB500Four chassis I have laying around.

If you want it, send a note to my regular e-mail address and we'll work out the details.

Kevin Hunter
Pearland, TX

kthunter01@sbcglobal.net

mjstone · « **Reply #8 on:** May 08, 2011, 01:33:47 PM »

Yep, the one that Kevin400F has is the right one!

MJ

jvandyke · « **Reply #9 on:** May 08, 2011, 02:30:28 PM »

Kevin, what's the offset on that one?
Here's my measurements.
Done fast and not very accurate but close.

Kevin400F · « **Reply #10 on:** May 08, 2011, 03:13:52 PM »

Measuring the same way you did, I get closer to 1-7/8". Check yours again and see what you think. Mine is a factory original top fork bridge from a 1972 CB500Four.

Kevin

jvandyke · « **Reply #11 on:** May 08, 2011, 03:20:11 PM »

I'm not home but will later.
Here's what I did on my computer.
I took your picture, made is a bit transparent and superimposed it over my picture. I added the blue circles where your 500 four bridge tube holes are, red on the one I got that is supposed to be off a 500 four. Drastic offset difference. When yours is placed over my broken one, they line up pretty good. I think my replacement is NOT for a 500 four.

I'll email you and maybe snag yours?

jvandyke · « **Reply #12 on:** May 08, 2011, 07:27:19 PM »

Quote from: Kevin400F on May 08, 2011, 03:13:52 PM

Measuring the same way you did, I get closer to 1-7/8". Check yours again and see what you think. Mine is a factory original top fork bridge from a 1972 CB500Four.

Kevin

My original broken one is darn close to that 1 7/8" hard to measure to the center of holes. The one I bought and tried to use is starting to seem like it wasn't off a 500 after all eh? It isn't even close at 2 1/4" Maybe my broken was was original to the bike after all.

jvandyke · « **Reply #13 on:** May 09, 2011, 08:48:06 AM »

Could it be the one with the 2 1/4" offset is off a 500F? I'd like to ID it so I can flip it to someone who could use it. (and get some of my $$ back out if it, no fun getting the wrong parts).
Thanks for your help.
Kevin will hopefully get me set up with the right one and I'll be closer to DONE. Weather is great and bike is inop and that is not good.

Flying J · « **Reply #14 on:** May 09, 2011, 08:54:18 AM »

DO you know why it broke? You dont want it to happen again when you get a new one. I think you are missing the washer on the clamp bolts. Might want to check that.

jvandyke · « **Reply #15 on:** May 09, 2011, 09:22:07 AM »

Yes, I've investigated the spacer (washer) in the pinch bolt clamp. I found ONE but both sides were cracked clean through and as soon as I extracted the pinch bolt, the pieces fell out of both sides. I can only assume someone over torqued them and despite at least one side having the spacer, they both cracked. Likely it has been like this a long long time as I know the bike's history since '73. Uncle bought it one year after new, quite likely after someone wrecked it, judging by the dent in the back of fender and dents in each of the chrome top brace/head light bucket holders, they are dented on the back side like the whole front got smushed straight back. I found both aft front axle bolts were barely finger tight too, so the lesson I learned is; go over a "new" bike and check stuff before trusting your life to it. My Uncle passed away about 10 years ago and the bike went to my brother in law who basically let it rot. I got it about 16 months ago and have been resurrecting it as I go. I'm hoping Kevin has another spacer washer he can pass along otherwise I've got washers the right thickness that I will make a spacer out of, and I'll torque carefully when the time comes.

jvandyke · « **Reply #16 on:** May 13, 2011, 03:48:53 AM »

Got new bridge from Kevin. Fits perfectly. Thanks Kev. Seems like it was all OEM parts up front after all. Made clamp spacer from a washer but haven't torqued stuff down yet. Got too late but even so wheel is back on, new fork seals, new fork boots, polished lowers, cleaned everything nice 'en purdy, fresh paint on bridge (Thanks to Kevin!!) and idiot light/handle bar clamp piece (got two lights swapped so now bright is neutral and neutral is bright, but that's no big deal to fix. Freshly painted tank and side covers ready to go on, have to refurb emblems and then I'm just going to ride the thing......a lot.

mjstone · « **Reply #17 on:** May 13, 2011, 02:47:18 PM »

Great to hear it! Maybe you could join us on the Rally when it hits Michigan.

MJ

jvandyke · « **Reply #18 on:** June 01, 2011, 01:18:16 PM »

Can anyone ID the brace pictured above? A guy sold it to me as for a 500k but it obviously wasn't. He's melted back into the dark abyss of well meaning but ignorant "helpers". I'd just like to pass it on to someone who can use it and recoup my cost. I paid $40 for it and it will never do me any good.

Bankerdanny · « **Reply #19 on:** June 01, 2011, 01:23:39 PM »

Does it have a part number on it anywhere?

jvandyke · « **Reply #20 on:** June 01, 2011, 03:17:03 PM »

I don't think they ever put numbers on these. Would be nice though.

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Author Topic: fork brace snafu 500 vs. 750 (Read 2203 times)