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Discussion Starter #1
Have been struggling to smoothen the ride on my 83’ Honda CM400t for some time now. Started out with me wanting to scramblify it slightly. Knobby tires on larger rims, slightly longer rear shocks, cross bar and a custom made seat.
03CC36D1-0053-44AE-9F70-6F84DD3CA361.jpeg

It turned out quite good, but when I drive it it feels really unstable passing 70-80 km/h. This is on a straight stretch, with good asphalt.
I am currently running Michelin Sirac 90/90-19 on the front and 120/80-18 on the rear. Tried some Heidenau K60 first, but those were the same. This winter I got a new set of identical reverse Comstar rims and changed the bearings to see if that helped. The steering stem bearing is serviced as well as the front forks. The bike is still unstable.
Today I changed over the stock sized wheel set from my 79’Honda CB400Twin and the CM400 runs like a dream. The CB400 Twin has the same instability when driving this with the Sirac tires.

I know that the rear tire is perhaps slightly wider that what the rim can take, but could this be the problem?
Would like to keep these tires, but not sure what I can try next.

Would appreciate any suggestions from the group.
 

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1st problem is you've removed the fork brace, fender, so each fork can move independently instead of traveling the same.
Given the knobby tire I would use 15W fork oil instead of the 10W I usually recommend.
The above should at least improve the stability but I'm doubtful it'll be the cure.
I can't tell if you're running the original rear shocks or not. If so then those need to be replaced. The swing arm should be checked for bushing wear also.
Knobby tires on the front end of a bike are fine for dirt riding or if the suspension was designed for it, rake/trail/srpeing rate/shock valving/fork size/etc. Trying to use a 40 year old street design for trail type tires is going to leave a lot to be desired.
 

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Discussion Starter #3
Thanks!
The fork brace is still on, maybe not in that picture, but that was one of the first thing I put on again.
I have changed up to W15 in the front. I am running new YSS shocks but 3-4 cm longer than original. Have also checked and relubed the swing arm.
I think I have done a pretty solid job with the bike. And as I said, everything feels exactly the same when I run this wheel set on my completely original, but well maintained 79’ CB400Twin also. Feels unstable.
I think I will have to buy a new set of normal street tires.

308425
 

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Discussion Starter #4
This is the 79’ CB400 with the Sirac tires on both front and rear, they are not really that knobbly..
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Ah, that looks much better with the brace.
Pictures on Google look more aggressive in the tread than your pictures. Still a knobby though on old street tech suspension. You could do like the CL350's and run the knobby rear and more street front. I'm using Heidenau K33 front and K37 rear on mine.
 

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Discussion Starter #6
Will try to run it with a normal street tire on the front tomorrow to see if that helps. Will also try with street tire in the rear and Sirac in the front and play around with the combinations.

Do you think back wheel alignment could play a role in this?
308431
 

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I think you've already narrowed it down to the tire treads or damaged wheels.........
 

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Stupid question but did you balance the wheels? At 40mph+ it will feel like you’re running a serpentine pattern. My local shop charged $15 to balance both wheels. You have to take them off the bike yourself though
 

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Discussion Starter #10
Thanks Dr.
Yes, both were balanced when I bought the new tires back in October last year.
I just had the rear re balanced after I changed the rear rim. The guy told me that the vibration most likely comes from the rear tire being too wide for the rim. I am running dual-sport 90/90-19 vs stock 3.60-19 front and 120/80-18 vs stock 4,10-18 rear.
I am not giving up on this yet, and to be honest have plenty of time on my hands at the time...
 

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The 90/90 front is fine. The rear is 2 sizes over which is not good, 100/90 or 80 should be the rear. You've actually decreased the tire contact patch by doing this.
When the tire bead has to move inward from it's normal construction point the sidewalls curve inward which in turn pulls the outer edges of the tread portion up out of contact with the road. The sidewalls are the support of the wheel to the tire so instead of a more or less vertical plane for support it's now at an angle.
 

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Discussion Starter #13
Hi again! Unfortunately the story continues. Just had the dealer fit brand new Dunlop TT100 on the reverse Comstar rims. Stock sizes 4.10 rear and 3.25 front.
310730
The wobbeling is the same on both bikes with this wheel set (Golden Reverse Comstar) and they are both smooth like silk with the other wheel set I have. (Silver Comstar)
310729

What to do next?
How can you tell if a rim is damaged?
Could the sprocket or the rubber inserts cause this wobbeling?

Would love to get a hold of new rims, but they don’t grow on trees..

Gratefull for any assistance
 
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