XRTT Low Boy Frame Dimensions?

[KRTTXRTT]

Thanks XLH for posting the pics! What a swap meet find.. with that kind of luck you should buy lotto tickets. Like BF said, those shocks and swing arm must have been added much later. The shocks appear to be longer than what was used on these? I am guessing the originals were close to 13 inches or less.
You have the start of a GREAT project. What are you doing for a motor and some time I hope to be able to see that blue print in enough detail to read the dimensions! It appears the steering head angle is 27 degrees? Can't really read it though.

[KRTTXRTT]

Originally Posted by barefoot

Did you buy some of the heads that were on fleaBay ? They looked expensive but cheaper than building your own ...


I know. That's why the ones that hadn't been butchered were worth their weight in gold Some people begged and pleaded and got unfinished heads.

Ahh, I see now.

If you are really brave or slightly insane, you could go to Kenny Augustine.

If you survived, your heads would definitely work but I'm not sure it's worth the stress on your sanity. You'll have just as much fun with ten less horsepower ...


Everything sounds good except the steel pushrods. Heavy wall aluminum works better imo ... but if we didn't each have our own ideas it wouldn't be any fun

BF I went with steel to see if it would help the cam accuracy - like in NASCAR. I know they will be noisy, but it is not a street bike. The stock pushrods were thick wall aluminum. If this is a bad idea, I can always switch it back. I assume you prefer aluminum because of the lower weight?

[barefoot]

Originally Posted by KRTTXRTT

BF I went with steel to see if it would help the cam accuracy - like in NASCAR. I know they will be noisy, but it is not a street bike. The stock pushrods were thick wall aluminum. If this is a bad idea, I can always switch it back. I assume you prefer aluminum because of the lower weight?

This is just personal opinion so take it for what it's worth but ...

NASCAR engines have shorter pushrods heavier springs and much more lift than a Sportster and they run cooler. So I don't think what works for them is necessarily straight-across the best idea.

Dragster engines (when they were competitive) use(d) steel pushrods, but they also had much more lift, heavier springs, different cams, and they never got really hot.

Ages ago this was a question - "how about running chrome-moly pushrods like Joe Smith ?" I'm sure people tried, but ... XR's use the heavy-wall aluminum to this day. (That was actually one of Axtell's discoveries.)

Possible reasons would be : People say the steel ones resonate and don't follow the cam as accurately, which is possible. They also don't expand as much as aluminum, which can be worth many thousandths of lift in a pushrod that's as long as a Sporty. Not only that, today's cams have pretty advanced acceleration and deceleration ramps so it's possible that an extra ten or fifteen thou of slack could take the valve spring right out of the cam's control. Definitely harder on the valve seats, slamming down rather than being controlled by the cam.

So for me, I'd stick with aluminum. But nothing ventured, nothing gained so give the steel ones a shot if you like them and see how it works out. Maybe you will discover a new hi-po trick for XR motors

[xlh59]

I will re-assemble the XRTT chassis the next weeks .... my kids are at my parents place for holiday, so have time for another kid!

JUst postimg some pics here, if you want some specific pics, let me know!

The frame has a -67 rear tomahawk, so definately a late version (the sqare tube swingarm tells the same story) -- It seems to be used, at least some kind of engine was taken in and out .... would love to know the story of this chassis. It came with Morris cast wheels, a 18" front (with dunlop street tire) and (yes - makes no sense at all) a 19" rear (with knobby flattrack tires) -- so I assume some XR750 flattrack racer bought it as a spare street chassis ... the 19" rear was what was left ....

[KRTTXRTT]

WOW
Looks like the real deal. And it does appear to be a low boy. Interesting to note this one did not get the added reinforcement near the steering head. Appears to have had litte use? Usually the top tube is a mess from old foam rubber and tape used to keep the tank alive.
Let me know if you ever decide to measure it. If so, I can run the dimensions by Jim Belland and he may be able to tell you who it was mde for. No two are alike supposedly.
On the other hand, send it to me and I will make a print of it!@@!
Very cool find!

[xlh59]

would love to know more about the history of the frame ... which dimesnions would be relevant for the identification? I can measure whatever is needed!

[KRTTXRTT]

I have used the motor mounts as a reference point and managed to make up some crude dimensions before for a regular XR frame. Be interesting to know what the steering head angle is and the height of the neck assembly measured from the bottom cradle. One weird detail is this frame is using a tomahawk from an electric start Sportster and most used these with the 52 series part number?
Years ago making a dirt frame, I used the location of the steering head and the pivot point for the swing arm to build a jig. Then I would set the motor in there and work to get it located correctly so the wheels are centered and going straight. I used a very high tech measuring device to keep the wheel planes true: a long fluorescent light bulb! They are dead straight. If you generate any dimensions, just pull them from a common surface and that might work?

[barefoot]

Originally Posted by KRTTXRTT

One weird detail is this frame is using a tomahawk from an electric start Sportster and most used these with the 52 series part number?

I would not be at all surprised if this was not a "factory" frame at all. These aren't that magic ... Belland built the first of what became the standard XR dirt frame in his garage. Nichols Engineering built all the roadrace frames in one batch, as far as I know. Mert built a bunch of weird frames (actually had Terry Knight make them but same difference.) Ronnie Nunes built really nice frames. Schwerma built tons of good frames with nothing more than a flat plate and an adjustable arm to set the steering head tube on. The rectangular tube swingarm says "non-stock" to me.... none of the pavement XR's they raced in Class C (and they quit racing them after 1973) had rectangular tubing swing arms. None. All those "Lucifer's Hammer, was Mark Brelsford's #1firestorm bike"" stories stink of Marketing .... look at photos, the top tube (on Lucifer) is totally wrong, the swingarm is totally wrong, smells like Madison Avenue from fifty miles away.

It would be easy to make nice XR roadrace frames. There are no stickers or stamped traceable numbers on them at all. I would not be half surprised if people hadn't done this, given the money they bring. I know of some motorcycles in museums that were made much later than the dates on the sign .... there are people in the "vintage" game with plenty of money and not so many scruples.

(Not criticizing your frame, XLH. Looks pretty damn nice to me, I'd take one in a heartbeat. Copy or factory no difference, as long as it's made well. For a user, it's riding it that counts, not whether Roger Reiman sat his sanctified ass on it in 1970.)

[xlh59]

I know that the -67 rear casting and the rectangular swingarm indicate that it is not an early frame or an original 72 XRTT frame. The frame itself is incredibly light (will find out these days) and the welds are perfect. The swingarm has different thicknesses on the rectangular tubes, the inner box section is thicker than the outer one (cool) .... I have seen a picture of Jay Springsteen riding a XRTT with rectangular swingarm in 1978 (not the "Hammer") ...
I could get the round swingarm from John Steel and pretend to be "original" -- which is not my type of thing ... racing has been an aftermarket and custom solution business eversince, so as long a part has any history its fine for me.
"Period correct" is what counts.

I will make some measurements on the frame these days. The steering angle seems quite flat to me, will check the angle as well.

Anyway thanks for your input!

[xlh59]

the weight of the frame is 8,8kg or 19,5 lbs -- the swingarm is 3,4kg or 7,5 lbs -- thats lightweight! All steel!