Posts
46
Joined
5/12/2020
Location
Redlands, CA
US
Edited Date/Time
2/26/2021 2:49pm
So i just picked up this Triumph tr5mx, I have always wanted a vintage 4stroke mx bike so this is the perfect base for that. I will be changing the geometry a bit to take some of the slackness out of the rake and increase rear wheel travel. I'll be making my own tank, seat and exhaust. The engine will just get reliability upgrades most notably a Carrillo con rod and improved lubrication.
My dad passed away one year ago and he was a bike nut, restored many older British bikes so I'm building this in his memory.
So here is what I am starting out with.
My dad passed away one year ago and he was a bike nut, restored many older British bikes so I'm building this in his memory.
So here is what I am starting out with.
sorry for your loss.
The Shop
But I missed your earlier post on geometry. I have to say that 30 degrees of rake was quite typical of that era, 30-31 actually; the only brand running 27-28 degrees successfully then was Maico, whose leading axle forks restored the amount of trail needed for straight-line stability, especially in the rough. I forget how much trail, maybe 4.5", which most other brands were getting with more rake and enormous offset by modern standards, like 55mm.
Maico was the indisputable leader in handling and towards 1980 other brands just gave up and copied what Maico was doing, including swingarm length/angle, countershaft location, etc.; the 1982 Honda CR480R is reputedly a single-shock copy of a 1981 Maico 490 chassis. In fact, contemporary bikes simply derive from the Maico formula.
So if you crank a 1970 bike's rake down and use straight-leg forks, you may run into serious headshake and other instability. A huge difference in modern chassis dynamics is the use of low-speed compression damping to reduce fore/aft pitching. With the old bikes, we tried to enter the corner with enough braking to compress the front end & reduce its rake, diving on purpose, as well as to load the front tire better for traction. It worked if we did it all correctly, and the long rake protected us with stability down straights--we needed that protection with short travel.
Gary Jones told my brother that as he got older (his 60s I think he meant), he had to give up riding vintage bikes & modern ones on the same day because the chassis dynamics are so different that good technique required two wholly different sets of habits to go fast and he was mixing them up and scaring himself.
That said, many, many people are putting Race Tech cartridge emulators in their forks to get separate & adjustable low speed compression damping there along with shocks that have speed-sensitive damping like your NJBs, and running longer shocks to bring the rake in for better steering without needing the front end to dive on entry. More modern chassis dynamics so far as suspension travel allows, in other words, and often less of a cut & thrust "two-stroke" line choice, more like rolling corners fast in modern four-stroke style. Not uncommon to see aftermarket triple clamps to make rake/trail more Maico-like on CZs and Husqvarnas too.
It's obvious you will work out what suits you, I just thought I would essay a long-winded explanation of what was going on with the stock geometry. With old-school four-stroke engine braking on that Triumph, you will have another option for loading the front on corner entries, and of course it's inherently closer to modern chassis dynamics overall as a four-stroke.
Regarding swingarm angle, it was interesting to learn earlier this year that when Roger DeCoster had to do an ad hoc long-travel rear suspension at his race shop in Belgium to salvage his chance at the 1973 500 championship, he grafted in 20mm vertically to the main frame cradle, one effect of which woukd have been to restore more of the original swingarm angle with the shocks moved up. Most of us modifying our own frames in 1974 did not know to do that; even manufacturers like Montesa didn't address it for several years. Sag was a different matter back then too, and we were more or less blindly working with rear preload, spring rates, air pressure added up front, whatever we could to try to make the bike steer into and drive out of corners and go down straights.
Again, beautiful bike, and keep doing it your way, it is truly becoming your bike as you work through the issues--
Will be following this build with great interest.
@CDswineheart has a really good point there about footpeg position as another thing to either get used to or change. Using lower and narrower handlebars than fifty years ago is another variable to play with. I can hardly look at old race pictures anymore without thinking the rider would have been happier in that turn if their bars had been a couple inches lower and narrower, and some guys were figuring that out ca. 1973.
I moved my footpegs back a little but left the height stock. I will be trying bars from both my KTM's 350 & 500 to see which is better the 350's are very low rise.
It was almost like they were riding choppers back in the day! I started racing mx in 1980 on a 78 rm250.
I am building a Rupp 125, which was a US-made brand 1971-75 with the ubiquitous Sachs engine. I had a Monark with motor work by a famous tuner and managed to keep the engine & some bits all this time. The Rupp frame looked like it might be the best one I might find here for that engine--need more wheelbase & swingarm length with a built Sachs with a light ignition rotor, plus I hoped to get overall weight down significantly, from 91 kilos to under 85 if I could, and it looked a good basis for that.
Rupp claimed in ads to have bought & analyzed Euro bikes to arrive at their frame geometry; might be some CZ and Maico in there, possibly Cheney Sachs, I just know where it sits on paper compared to Monark, Penton/KTM, and DKWs I rode a lot, and it's worth trying. An unusual detail is that the footpegs are below the swingarm pivot; the motor gave them leeway to put them farther forward per usual practice, so I am sure they did it intentionally.
I wish I had your fabrication powers--I am only using the Rupp frame and swingarm, everything else is from something else, so I have been sussing out how it should or could all fit and doing the small bits I can. Fortunately there were shared measurements for many major pieces then, fork lengths, hub widths & such, they built their prototype that way, judging by the photo I ran across, so it's working out.
I am going to have to send the engines (wound up with two, long story for later) to a specialist for bearing renewal/lower end rebuild due to not having the special tools needed for the multiple idiosyncrasies embodied in those engines, so if I am waiting on them and the chassis is coming together I may start the thread then just to spread out the posting to a manageable level. I like a build thread more when the pacing is good; I can't really get into the "here's a bunch of stuff in its packaging" photos, would rather see more concrete progress, exposition of problems and solutions, etc. I mean, I have quite a pile of interesting old bits now, but the real story is in making them work--
That's too bad you have to send the bottom end out to get the bearings changed, usually a simple job on two strokes.
When I get to working on the engine the posts will no doubt slow down. I have worked on all kinds of two stroke engines but four strokes are alien to me. All I have ever done on four strokes is change the oil, rebuild carbs and set the valve clearance on my KTM's. Valves and cam timing might be the biggest obstacle of the whole build!
Pit Row
First, just a suggestion based on practicality--while I have never built a heavily modified vintage mxer, I have built a number of vintage road racers, and the problem I always had with getting a modern swingarm angle in a vintage bike was that the countershaft sprocket was usually too far away from the swingarm pivot to allow that much angle, because the chain would rub solidly on the pivot at full extension at lesser angles than your 11 degrees. Although guys road race and dirt track these too, I have never worked with a B50 BSA frame like yours so I don't know whether that is the case for this frame too, but it may be something you will want to pay attention to. The swingarm angle on one of my vintage roadracers, a 750 Triumph twin built in a C&J Champion dirt tracker copy frame, was extended right to the max, and it was maybe 7 degrees, as an example, and the chain was just kissing the pivot at full extension. I notice there is no chain on the bike in the pictures, and if you haven't already done so, you might put one on it, to see where you are. If it is a problem, then you might try to put bigger front and rear sprockets on it, to see if that helps, although there is a practical limit to that.
Another problem I encountered on road racers and dirt trackers in getting modern numbers into the older frames is that usually the countershaft was too high relative to the swingarm pivot, which is hard to fix without major surgery except by stink bugging the chassis attitude a bit, lowering the front and raising the rear, to get the swingarm pivot up some in relation to the c/s. And that "solution" is doubly hard on vintage bikes, because of the distance of the c/s from the swingarm pivot I mentioned above limits how far you can go. Don't know if that is an issue for your frame too. I have also heard about the Don Jones method of sectioning and lowering the cradle to do basically the opposite, to take some swingarm angle away with longer travel, but never tried it. They used to do that to DT1's, I think that frame mod was published in Dirt Bike Magazine around 1974 or so, when people were torching their frames to get rear travel. But if you go too far with this, it can itself cause problems by putting too much percentage of the travel above the point where the axle, the swingarm pivot and the c/s are in a line, and that can take away too much antisquat and introduce problems all of its own.
On general numbers, natch the Maico numbers are a great place to start for a vintage bike, and I am assuming you would copy say, a 74 1/2 or 75 frame, which had travel numbers more or less like yours. Another chassis tuner I knew, Gil Vaillancourt who started Works Performance Shocks, who rode a lot of desert and mx, always told me that a vintage dirt bike should have 60% of its rear wheel travel below a line where the axle, swingarm pivot and countershaft are in a precisely straight line, and 40 % above, to preserve a proper balance of antisquat and compliance, and he really knew his stuff. Ray Hensley, the guy who did Trackmaster frames, told me for max rear wheel traction and satisfactorily finishing turns, he liked to put the swingarm pivot center 5/8 of an inch above the countershaft center on dirt trackers. These two ideas lead basically to the same place, finding a balance of aggressive antisquat and suspension compliance while on the gas, and these approaches work great on road racers too. I have never built a vintage motocrosser so I don't know exactly what altered numbers might work well (I started mx racing in the 60s when vintage was modern on through the early 2000s but only torched one or two frames in the mid-70s, and in the 90s I vintage raced a 500 Triumph twin Metisse for quite a while, but I left its geometry stock and raced it in stock geometry as a short travel bike). But mxers have the same problem of needing antisquat in the rear to keep it from bottoming and keep it steering with the power on, so you might think about that kind of approach of a balance of antisquat and compliance to evaluate where you are. Basically, less antisquat lets the rear sag while you are on the gas, and gives more rear bite, but takes away from front bite and makes it push, and too much may actually slow the bike down on bumps. I always thought my Maicos (I had a 79, 80, 81, 82 and 83 when they were current) sat down too much in the rear in corners for my style, and I sort of solved that by using a 4.00 Metzeler on the rear instead of a 4.50 and clutching it a lot to get it spinning and rotated from the rear in corners, but that is a question of riding style--I never had much luck with the larger tires on the Maicos, though, which was in part related to swingarm pivot height. For me, the bigger tires made the bike grabbed too much forward traction and made the bike squat too much in the rear, and lose front traction off corners, Of course, antisquat also makes the bike harder to bottom while on the gas, by making the rear suspension harder--that is a blessing and a curse because while it helps with bottoming, if it goes too far, it may slow the bike down across bad chop--whatever little bit the Maicos might have lost coming off the corners they certainly made up by being able to leave the gas on hard across rough ground, and they definitely had the overall balance nailed. As someone else said above, many bikes copied their numbers an even then they didn't all work as well at first.
You seem to be on the right path on swingarm angle, and as you proceed, you will become increasingly aware that swingarm angle, pivot point height, antisquat, and the way the bike behaves are all closely interrelated.
Also, a rough principle of geometry is, in about the length of a motorcycle wheelbase, raising the rear one inch takes away about one degree of fork rake--for example, along with increasing the angle of the swingarm and slightly increasing pivot height, which are also good, simply raising the bike 3 inches in the rear would tuck the fork rake in roughly 3 degrees, the amount you are trying to change it. That is a big change, more than may work well in practice, but maybe half that much might give you a very noticeable improvement. Another bike with a relatively low swingarm pivot was the CZ, also a bike that was copied a lot in the short travel days by the Japanese manufacturers (I remember one of the early 70s short travel Kawasakis measured about identical to a CZ). I remember what an amazing difference it made simply to put a pair of 6 inch travel 15 1/2 Works Shocks on my CZ in place of the 13 1/2 inch Konis I had been using. It tuned better, finished corners better, and of course went over bumps better too, with just that one change, which moved this same bunch of related geometry numbers on the chassis--swingarm angle, pivot height, fork rake, antisquat effect.
And a suggestion--if you don't already have one, you might get a good angle finder with a magnetic base to stick on the forks and swingarm, to add to your tool collection. It is pretty helpful, to try out changes on the chassis table and accurately measure the differences with it immediately. Anyway, sorry to go on so long, good luck with your fun project.
Right now I am pretty much set with my swing arm angle to first ride the bike at, the good thing is my NJB shocks are adjustable length so if I find my s/a angle is too great then I can shorten the shocks down to compensate although as you mention this will throw the rake back out as well.
The s/a pivot is more or less at the same height as the centerline of the c/s sprocket which is a big help, it would be nice if they were closer together though. I have had the chain on the bike and there is a little rub as the bike sits with the suspension fully extended, nothing close to how modern bikes run though.
I will definately look into those Maico geometry numbers for sure.
For sure I have one of those angle finders, I used it extensively on the last bike I built (a 1975 Suzuki GT500).
Again thank you for your impressive input!
You will need to thin out that flange so it isn't such a big heat sink or you will burn through the tank. Try making a 3/16 " wide step so the 2 pieces are closer to the same thickness:<
What are your plans for the forks, springs, emulators?
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