: School me on Triumph Front Ends please !


TheFrenZ
12-16-2008, 01:39 PM
Just like the title says,I need some input here.What are the exact differences between a pre-unit and unit front end ? What are the differences visually...difference in tube diameter,difference in width,top trees etc.,etc.I just would like to know everything !

Already searched for info on here and found some interesting stuff but I want to know more. Pics would be very cool and any kind of help is appreciated !

bobscogin
12-16-2008, 03:14 PM
Exclude the OIF forks in this discussion. The pre unit and unit both have the same tube diameter, and the same 6.5" tube center to center difference distance except the '69 and '70 models in which that dimension was increased to 6.75". There are different versions of pre unit lower legs, the 2 most basic versions being whether half width brake with "push through" axle, or full width brake with "clamp on" axle via caps. Fender mounts can vary by model and year, as well as brake lug anchor points. There are 2 basic differences in pre-unit trees. Single downtube frames ('pre 1960) had a steering stem that was not parallel to the tubes. It angled in. The Duplex frames ('60-'62) used trees with a different angle, as did the units. Don't mix top and bottom trees between the two. Unit lower legs have variations on fender mounts, but all use the axle caps. Springs are external from '65 on. Way too many variables, such as steering stops being different on various models, to discuss here but this is a few of the basics.

Bob

TheFrenZ
12-17-2008, 09:58 AM
Thanks much Bob ! I'm pretty familiar with Harley stuff and this Triumph stuff is "new" to me.Some suppliers offer MCM-style fork tube covers for pre-units and units seperately.So there must be a difference in mounting bosses,maybe something else,if I'm not mistaken,right ? You most definitely know more.

I saw on some upper triple trees risers and the tree itself are one part and others just have mounting holes.Is this also a pre-unit/unit thing ?

And do you know by any chance the length of those forks measured from lower triple tree to center of axle ? And,I want to shorten the fork...how much can you take out there without getting any issues ?

bobscogin
12-17-2008, 02:40 PM
About the fork covers -- the pre-unit and unit forks up to '64 inclusive have internal springs, and small diameter seal retainers, so the pre-unit fork covers are a smaller diameter. 1965 and newer have external springs and the seal retainers are much larger in diameter, thus the covers are larger to fit over them. On the handlebar mounts, all pre units use 1" bars and there are two types of mounts. The nacelle models used U-bolts to bolt the bars into the top tree, the non-nacelle models had "risers" made as an integral part to the top tree. Unit models had 7/8 bars and used the built in risers or the rubber isolated "P clamp" mounts depending on model and year.

Bob

TheFrenZ
12-18-2008, 11:51 AM
Thanks again Bob ! That all really is of great help and makes finding the right front end for my project easier.What you say about the seal retainers and covers makes sense and that's also what I actually wanted to say but somehow didn't do.

One more thing...that front end will be going in a Harley frame.Found info on conversions on here but however,how exactly is the fork stem mounted to the lower triple tree on pre-units and units ? I just would like to know what trouble I put myself in before purchasing any parts.

Also found this while searching for more info in the net.Lots of VERY useful info and pics...

http://www.raskcycle.com/techtip/webdoc18.html

Billy Whiz
12-18-2008, 12:16 PM
Hey, this might help: I used some Triumph pre-unit forks on a little chopper I built the frame for, it was a low-rider deal and I lowered the forks by cutting the spring length on the internal springs. If you (or provided you) use strong enough springs - sidecar model springs are good for this - you can reduce the standard fork length by 3- 4 inches safely. To do the job properly you should really increase the internal sleeve length (this goes inside the slider between the hydraulic stop nut and the fork seal holder) so you control the decompressed fork length - although I didn't do this if I remember, and everything worked fine. As the springs are all contained internally nothing much can go wrong but be careful you don't bottom out your tire on the lower yoke (or tripletree as you guys say!) Unlike harley teles the tire will hit before the fork seals touch!!

Billy Whiz
12-18-2008, 01:17 PM
To clarify - the sleeve thing I mentioned is the "damper" or oil control sleeve and one thing it does is control the fully decompressed fork length by being trapped between the two fork bushes if you lift the bike off the ground (or jump bridges!?!) So if you don't get air that scenario is not going to happen..

VA Rebel
12-18-2008, 03:46 PM
Not to be argumentative, but I think all the unit construction 650's from '63-'70 had external fork springs when they left the factory.

bobscogin
12-18-2008, 05:20 PM
Not to be argumentative, but I think all the unit construction 650's from '63-'70 had external fork springs when they left the factory.

Your comment got me to thinking about that, so I did some research to refresh my memory. I previously stated that '63 and '64 unit models used the pre unit internal spring fork, but it was the '63 unit only. '64 had the external springs.

Bob

TheFrenZ
12-19-2008, 12:47 PM
Thanks again for everybody's input !

Billy...3-4 inches sounds good.Do you maybe still have some pics of the bike ?

And one more question...can I mount pre-unit lower leg assemblies to unit triple trees/yokes ?