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Brampton coil-over damper.

Oldhaven

Well Known and Active Website User
VOC Member
By the way, in the photo of the rear Thornton above, and also on the front one,it shows the rubber bumper in an orientation the way I got it. I kept it that way, but it seems it would work better the other way around on the shaft, since now the small end of the bumper hits the seal when the travel bottoms things out. Am I wrong on this?
 

davidd

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VOC Member
By the way, in the photo of the rear Thornton above, and also on the front one,it shows the rubber bumper in an orientation the way I got it. I kept it that way, but it seems it would work better the other way around on the shaft, since now the small end of the bumper hits the seal when the travel bottoms things out. Am I wrong on this?

Ron,

I think the seal is is supported on the inner side, but I will have to look the next time I change the oil. The rubber bumper is said to give .25" of travel before it prevents further movement. I think if you hit the bumper it will slide over the lock nut on the rod which is holding it proud of the screw on end. Carleton hit a hay bale dead center once and the damper appeared untouched. It was not Carleton's crazy riding that was at fault. Another rider snagged some material that the bale was cloaked with and spun it onto the track.

David
 

hadronuk

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VOC Member
Ron,

I think the seal is is supported on the inner side, but I will have to look the next time I change the oil. The rubber bumper is said to give .25" of travel before it prevents further movement. I think if you hit the bumper it will slide over the lock nut on the rod which is holding it proud of the screw on end. Carleton hit a hay bale dead center once and the damper appeared untouched. It was not Carleton's crazy riding that was at fault. Another rider snagged some material that the bale was cloaked with and spun it onto the track.

David
I thought AVO had the bump stop the wrong way round as it would contact the seal more. They said it was fine, the scraper outboard of the seal is easily tough enough.
 

Oldhaven

Well Known and Active Website User
VOC Member
Thanks, George. It is a 21. I used it because they are easier to find tubes for, and I think it compliments the vintage look of Brampton forked Vincents. The front brake is quite nice and I don't find it too "fierce", which might have been a problem. It stops me as fast as I feel comfortable with. I have only had one panic stop and didn't break the tire loose, which pleased me.

Ron
 

Oldhaven

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VOC Member
Hi Alyn,

Not to worry. Here is a picture of the bike with half a tank of fuel, and me not on it. As you can see, everything is good and agrees with recent findings about Girdraulic lowers. I am not sure how Bramptons are affected by slight down inclination, but mine is fine and slightly up with me on it. I could probably preload the spring a bit more, but I mostly ride solo and no luggage. As you can see in the center stand photo and this one the long travel allowed by the forks now renders my front stands useless except for tire changing unless I carry a 2 inch tall stone slab along with me. I miss that and will probably fit a V3 type stand. I already fell over once trying to start it in loose gravel after a stall with only my left foot to hold things up.

I also included one showing remaining Thornton travel with the bike in this semi loaded condition. It's not a lot, but handles things fine. I did not measure with an original spring, but it is probably about the same.

IMG_0480.JPG IMG_0481.JPG
 

timetraveller

Well Known and Active Website User
VOC Member
Just to note that the recent comments on front forks and modified steering heads concern only guirdraulics. The geometry of the Bramptons is different and it would be interesting for any bike owner with Bramptons to remove the front spring and to trace out the front wheel spindle movement as I did. Publishing the track either here or in MPH would educate a lot of us.
 

Oldhaven

Well Known and Active Website User
VOC Member
As an interesting note, and I hope I am not getting too far off track, When I ordered my Dave Hills center stand, I measured the rear pivot bolt distance as he specified, and got 12-3/4 inches. Dave commented that this was about an inch taller than typical, and was the second longest stand he had made, let alone for a B. I think my stand is a bit long now that things have settled in a bit, but the combination of the front and rear Thornton's and the Justin Mackay-Smith rear springs and spring boxes put the thing up in the air a bit. That is probably why my front stands are vestigial appendages now. B's should be lower than C's and a comfort to shorter inseams. Anyway, that is something to consider if you want to change to a Brampton coil over.
 

hadronuk

Well Known and Active Website User
VOC Member
Just to note that the recent comments on front forks and modified steering heads concern only guirdraulics. The geometry of the Bramptons is different and it would be interesting for any bike owner with Bramptons to remove the front spring and to trace out the front wheel spindle movement as I did. Publishing the track either here or in MPH would educate a lot of us.
Are Brampton upper and lower links parallel and of equal length? I photoed some at Founders Day, but I was mainly interested in the spring mounting arrangements. Anyway, if they are parallel, the axle spindle must follow a circular path of radius equal to the link lengths.
If not parallel, the axle path could be calculated by the same method I used for the Girdraulics. The method is fairly simple, the only slightly complicated bit is working out the intersection point of two circles.
 

davidd

Well Known and Active Website User
VOC Member
I did the axle path of a Series A Brampton. It was an arc almost identical to the Girdraulic axle path. I believe that means they are not parallel and are of different lengths, but I do not have those dimensions in my file. There are two major differences with the Brampton. First, there is no preload on the spring and second, the center of the arc describing the axle path is below the axle when viewed from the side (the Girdraulic's is above.) This means the wheelbase does not shorten like the wheelbase does with the Girdraulic. This translates to a fork that can go up and down during braking and is not predisposed to full extension by a high preload. This is why the Brampton's have the reputation for good handling. The Brampton provided a good model for modifying the springs and altering the geometry of the Girdraulic to make it work well.

David
 

davidd

Well Known and Active Website User
VOC Member
As an interesting note, and I hope I am not getting too far off track, When I ordered my Dave Hills center stand, I measured the rear pivot bolt distance as he specified, and got 12-3/4 inches. Dave commented that this was about an inch taller than typical, and was the second longest stand he had made, let alone for a B. I think my stand is a bit long now that things have settled in a bit, but the combination of the front and rear Thornton's and the Justin Mackay-Smith rear springs and spring boxes put the thing up in the air a bit. That is probably why my front stands are vestigial appendages now. B's should be lower than C's and a comfort to shorter inseams. Anyway, that is something to consider if you want to change to a Brampton coil over.

Ron,

I have forgotten how long I made those rear springs, but 9" stands out in my mind. When I had the springs made I specified closed, ground and squared for the ends. I had problems with them cocking on the racer so I went to the coil-over and I did not bother finishing the stock rear spring project. My next step would have been to order the springs with open ends. I was going to thread a small plug of aluminum to the spring pitch, which would thread inside the spring. This would have a threaded hole for the spring eye. I thought that would be a good way to replace the claw on the stock Vincent rear springs.

I also made a contra wound spring that fit inside one or both rear springs in case a higher spring rate was needed.

Carleton wanted to run a set on his racer and I told him he needed spring boxes with a plastic insert to reduce the friction. I urged him to get the ones that Barry in Australia was making. Justin became interested in what I was doing and undertook to make some for non racers to use.

Flash Suspension CHP.JPG

The long rear springs aid the handling somewhat by steepening the steering head. Carleton has added another 2" plus with the Reg Bolton mod also.

To be honest, I had not thought much about the B. You are correct that it would raise the ride height!

David
 

greg brillus

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VOC Member
The links on the Brampton's are of different length, with the top one at 3.375" centers and the lower 3.700" centers, they are also not parallel with the front spindles at 237 mm apart and the rear pair at 219 mm. The Girdraulics have equal length links at 4.00" centers and they too are not parallel with spindle at 230 mm apart at the front and approx. 210mm apart at the rear. Using one of Norman's modified stems the rear lower spindle is 18.50 mm lower than stock and this brings the links much closer to being parallel to one another with front to rear distances of 230.50 mm at the front and 229 mm at the rear, so a difference of only 1.5 mm. Hope this is of help, cheers....................Greg.
 

Oldhaven

Well Known and Active Website User
VOC Member
There is a bit of previous discussion about Brampton's geometry on this penultimate page of the very interesting thread "comet suspension" .

http://www.vincentownersclub.co.uk/index.php?threads/comet-suspension.5504/page-30

My comments there resulted from the beginning of my conversion to Patzke taper roller bearings, and I did a measurement of the rear spindle distance with the stock roller bearings before the change. I was going to follow up after the conversion and do not remember doing that, so I went out this morning and did it. The stock distance was about 8.680, and it is now about 8.760 after the conversion. Tthis is not quite the 3mm difference I was expecting, but it would change the stock geometry. (This is rough measuring using the points of a dial caliper on the center ends of the spindles.) The front distance appears to be around 9.375, which would not have changed. It could be that the stock Brampton distances were based on even fractional numbers of 8-11/16 and 9-3/8, if the engineer was left brained, but that is a guess. As timetraveller says, getting a good plot of Brampton travel or the details of the geometry like the figures Hadronuk had for the Girdraulics would be very interesting. I'm too busy/lazy to disassemble mine for that, but perhaps someone in the midst of a rebuild could do it.

As an aside from the front discussion, the installed extended length of the davidd/Justin rear springs/boxes/Thornton combination mentioned above, spindle to spindle, is ~10-3/4" with the bike on the center stand and the rear wheel off the ground. I had to preload them a lot to get the boxes in with the Thornton fully extended, (scary enough for me), though I added HDPE cups at the ends of the springs so they are just a few thousandths longer than the designer intended. They do not settle much off the stand.

Ron
 

Oldhaven

Well Known and Active Website User
VOC Member
I see my rough measurements for both stock spindle locations are about 1-1/2 mm more than Greg's. Probably due to my method, since that is probably beyond manufacturing tolerances.
 

Oldhaven

Well Known and Active Website User
VOC Member
Just writing to describe a situation that occurred with my Brampton damper conversion. I was attending Coburn Benson's memorial service last month and had a wonderful 125 plus mile ride over to Kingfield, Maine and back......until the last 7 miles just outside Monson. After hitting several large bumps my front Thornton shock conversion failed in rather dramatic fashion, spitting the bottom seat and spring out the front of the fork into the fender, denting the bottom of the headlight bucket, popping out the headlight and rim, and slightly crimping the front brake cables when the upper links came to rest against the handlebar. It looks like the rod broke and the lower clevis flipped forward, bending the spring until it came out of the lower spring seat . I found the bent spring alongside the road but not the seat. I made it the rest of the way home in that genuflected position at less than 15 mph on the back road beyond Monson. The bike handled surprisingly well with no front suspension.
I'm not sure why this happened, but the shaft failed at the threads, and possibly stress or embrittlement and heat treating may have been involved or perhaps I did not preload the spring enough, since I did bottom out occasionally. My clevis mounts were free to pivot so nothing was binding. Anyway, I will now go back to the original Brampton spring, at least for a while. It is on the bike now and I have to repair the fender before I take it out again.
I have looked at the failure point and notice that the hole in the clevis is drilled and tapped just a bit off center, (My fault), Less than 1/32 laterally. This slight non concentricity to the spring center might have stressed the threaded joint with repeated flexing. Another cause might be the difference in angle between the fork motion and the damper motion. See post 28 picture. It could be that having two easily pivoted clevises does not support the assembly, and it might be better to support the rod end more. This angle difference may have bowed the spring and stressed the shaft at its weak point at the thread diameter reduction. Other Brampton conversions have hooked the lower damper mount over the original bottom spring hook behind the lower mounting hole and this would have prevented the clevis from flipping or slightly rotating forward and shifted all the necessary joint rotation to the top. That might have worked and if I do decide to repair mine, that will be the way I do it.

I have found a replacement spring on eBay and Carleton Palmer put me in touch with Pierre Vaillancourt, who was associated with his family's original Works Performance company. He was very helpful. Works performance has been out of business for two years but Worx Shocks is still in business and services works performance shocks and makes new parts. The people that work there used to work for Works performance. Same phone number 818-701-1010 Monday through Friday before noon Pacific time. I called and they have the shaft and will make a new spring seat for me. Pierre also suggested that I could machine a new shaft from 1/2" 17-4 ph H900 that would be twice as strong as the original shaft. This material is available from McMaster Carr. Since I would be remaking my own clevis I could machine the rod end to 12mm instead of the original 3/8-16", making it stronger yet.

I have not decided whether to use the Works coil over again, and will ride the original spring for a while to evaluate if the coil over is that much better. Carleton feels that a Thornton on the rear of a B like I have improves the performance of the Bramptons by damping out the bouncing so it is not transmitted to the front. I have never ridden my bike with the original front spring, since I put the front Works coil over on when I finished the restoration in 2015. I will repair the coil over in any case, and if I want to go back to it it will have the stronger shaft and probably a hook over the original spring hook.

Sorry to put a "damper"on enthusiasm for this modification, but I thought I should at least warn others to keep an eye on theirs. I was super pleased with the ride while it lasted, and despite the concerns I have heard about using the headlight mount for the shock mount that held up fine and it was the shock that failed.

IMG_1131.JPGIMG_1134.JPGIMG_1136.JPGIMG_1154.JPG
 
Last edited:

vibrac

Well Known and Active Website User
VOC Member
I dont think this unit can be confused with my"California" damper I got on a NA rally some years ago
1568298127688.png
That has a neat linkage at the bottom to use the original spring anchor
1568298247321.png
Anyway its fitted for Cadwell
1568298401841.png
 

Robert Watson

Well Known and Active Website User
VOC Member
When I first started playing with the Thornton's for the Brampton's one of the things they stressed to me was that the mounts had to be free to rotate, which only makes sense and all the kits I did had bushings clamped so that the mounts could rotate. One person, said he was quite capable of making his own and just bought the shock only. A month or two later he phoned up raging that I had sold him a piece of *%& and blah blah blah. Result was exactly as you describe. I went to visit him and pointed out the the lower mount was, against my specific instructions, clamped solid. Although the up and down travel appears to be almost in a line parallel to the shaft it is not. I chastised him severely for a self inflicted problem. We repaired the problem by drilling and tapping the rod and loctiting a stud in the end. My idiot brother sold that bike to a friend of mine some years later and it is still working just fine. I have run one on my black B for around 20K miles plus, most with 2 up and luggage and have had no issues.

As a note on the lower mount as well, when they refused to make me any more 15 or 20 years ago "on the advice of the lawyers" they said it was the lower mount that was in question. I measured several sets of Bramptons at the lower lug and calculated the shear strength based on the lowest grade of casting material and it turned out to be slightly stronger than the 5/16" bolt in double shear that was holding it in there!
 

Oldhaven

Well Known and Active Website User
VOC Member
My clevises rotate easily at both ends, the lower one on oilite bushings, and the double nuts assure it is not locked solid. Vibrac's spring is captured in a sleeve so it cannot bow as easily, and the hook is what I have in mind if I do this again. When I bought it it was modified by adding a 1/2" rod spacer internally to shorten travel so it would not cause the lower links to hit the girder. I note that both examples above seem to have less available travel, and I wonder if mine should have been constrained a bit more on the extension to limit the angular difference and bowing forces at the extended extreme.

For reference, if anyone wants to know, the spring is 120 lbs, 6.812 long, and uses .262" wire. The shaft is 1/2" x 5-7/8" long including the piston and threaded ends. If I make another shaft I may eliminate the spacer and make it shorter. I will for sure make it a 12mm thread to get a bit more than the .297 minor diameter I had with 3/8.
 

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