FT: Frame (Twin) Rear shock absorber alignment

[timetraveller]

Here is the picture that Highbury tried to send. It was too large so I have done a screen copy, pasted it into WORD, copied the WORD image and hopefully it will be below. The grey plastic bushed can be clearly seen.

I don't have one here to compare but it turns out that the difficulty fitting was due to the diameter of the steel tube fouling the top of the RFM between the lugs.

 

[medat727]

I found out the hard way that you shouldn’t let it float at the RFM, my coil over Works rear shock snapped in the middle of a race during a high speed sweeping right, the whole rear end collapsed almost throwing me to the floor. The fix was spacers to keep the rear of the damper located and making the piston rod out of high strength steel using a 1/2”x 20 tpi thread instead of the stock stepped 3/8”.

 

[bmetcalf]

With my Thornton rear shock, that is too scary, please delete your post!

Spacers may be in my future.

 

[greg brillus]

I don't believe that set up is good enough for something being pushed hard.........I made up a steel bracket that wrapped around the center of the casting to anchor it securely, and also add a couple of small gussets from the forward tubes to the oil tank on the upper frame as well........These parts will readily bend if you don't..........You get away with it on a road bike, but on a racer these parts need to be strong.

 

[medat727]

Greg, what damper are you running?

 

[greg brillus]

On the twin racer they were a Fastace pit bike coil over shock absorber with adjustable spring pre load and adjustable rebound damping.........very handy........These had a 270 mm center to center length and came with a 250 Lb spring........I had some other springs made in 300 and 180 Lb rates to suit the front and rear of the bike with a fully sprung seat. This jacks up the rear of the bike quite a bit, so the rear chain looks quite loose until you sit on the bike........It also made the cornering very quick, and I could out turn most other riders. I used the coilover on the front as well so no spring boxes up front........The front suspension was very supple and no friction anywhere.......... That bike was probably the best handling Vincent I have ever ridden.

 

[Martyn Goodwin]

On the twin racer they were a Fastace pit bike coil over shock absorber with adjustable spring pre load and adjustable rebound damping.........very handy........These had a 270 mm center to center length and came with a 250 Lb spring........I had some other springs made in 300 and 180 Lb rates to suit the front and rear of the bike with a fully sprung seat. This jacks up the rear of the bike quite a bit, so the rear chain looks quite loose until you sit on the bike........It also made the cornering very quick, and I could out turn most other riders. I used the coilover on the front as well so no spring boxes up front........The front suspension was very supple and no friction anywhere.......... That bike was probably the best handling Vincent I have ever ridden.

Just confirming - a 300 Lb spring at the front and 2 x 180 Lb at the rear?

Is that correct?

 

[greg brillus]

Single coil over front and rear, 300 rear, 180 front similar to the spring rate on Brampton forks.

 

[Bill Thomas]

I rode my Special "D" 130 miles home, After snapping a rear "D" unit,
Like a Pogo stick !, Started very steady, Then got quicker, Maybe too quick !,
So back to steady again.

 

[Oldhaven]

Medat,
It looks like you made a new lower spring seat in addition to the new piston rod. I am doing the very same piston rod fix on my Brampton / Works conversion, the result of the same mode of failure. My spring seat was shot into infinity during the incident so I am making another. Is the spring seat continuous or does it have the slot milled into it as in the original? I don’t feel the slot is necessary since the lower mount is removable.

Thanks very much for the pictures and details