michaelmoore

Thank you very much for that info! 1.75mm is .068" which sounds like 16g to me (1.5mm as I said above is .059" which would be on the low side of a nominal 1/16" for 16g). It is nice to know that at least in this case my memory wasn't way off. cheers, Michael

I'd appreciate it if someone could put a caliper on the stanchion tubes of their early or late (banana) leading link fork and tell me the diameter and wall thickness. I have a hazy memory that the banana fork is 1.75" /44.5mm OD but I don't have any notes confirming that. I think the wall thickness was .062"/1.5mm. I've never had a chance to measure the early 1960s fork so I don't know if the tubing size was the same. If while you are there you could tell me the size tubing in the link that would be nice. thanks, Michael

Frank held the major Greeves parts stocks in the USA for many years. http://www.greevesguru.com/ That page on Frank's site says that Frank died this morning, but Kenny Sykes will sell the remaining stock at some future point. Michael

A friend's road race bike had an unexpected but thankfully very small and quickly extinguished fire from that. We certainly were surprised at the flames! cheers, Michael

http://www.amalcarb.co.uk/TechnicalDetail.aspx?id=14 That seems pretty official to me. If your pilot screw isn't doing anything then I'd suspect something blocking the pilot circuit. Be sure to blow air backwards to try and dislodge any foreign matter that might have crept in. cheers, Michael

Did you check to see if the float level is too high? cheers, Michael

I've used 2024-T6 25mm tube for a roadracer steering stem and I'd have no issues with that as the diameter is big enough to work OK. But a solid 15mm aluminum round and a 15x8 steel tube both have nearly the same (the solid is slightly better) second moment of area, so the aluminum axle will be about 1/3 as stiff as the steel tubular axle. Most vintage teleforks are pretty bad in twisting and the less stiff 15mm aluminum axle isn't going to improve that and it wouldn't be something I'd care to do, but YMMV. You have to run the numbers and see how they stack up. Some people think "oh, I'll make a sneaky high performance lightweight frame by keeping the standard OD but significantly reducing the wall thickness" and ignore that while they've saved a bit of weight they've also reduced the stiffness of the frame, irrespective of if they are using 531/4130 instead of mild steel or not. cheers, Michael

Some of the high-strength aluminum alloys are as strong as some steels but they are not as stiff. Here's some numbers off of Matweb: 7075-T6 83ksi Ultimate Tensile Strength, 73ksi yield strength, 10400 ksi Modulus of Elasticity, 9% elongation at break 2024-T6 60ksi UTS, 46ksi yield, 10500 ksi MoE, 5% elongation 4130 97ksi UTS, 63ksi yield, 29700 ksi MoE, 25% elongation note that the MoE for the steel is almost 3X that of the aluminums even though 7075-T6 is almost as strong in UTS and has a higher yield point. But ductility and stiffness on those strong aluminum alloys is much lower than the 4130 steel. Aluminum is great stuff when used properly. But you wouldn't normally swap aluminum for steel without changing the part sizes (presuming that the steel part was not massively overdesigned). cheers, Michael

FWIW, the online Scorpa and Gas Gas specs show 2.15" rear rims used by both manufacturers. cheers, Michael

Aluminum is 1/3 as stiff as steel and every time it is cycled it is one step closer to failing from fatigue. Also, the smaller the diameter of the axle the more the stiffness is reduced. You'll probably be better off with a tubular steel axle for both lighter weight/performance as well as longevity. Remember the problems that BSA had when they duplicated the steel scrambles frame in titanium (which is less stiff) and had the thing flexing so badly. You can make aluminum axles, but do as John Britten did and go up a LOT on the diameter and plan on replacing them on a schedule. Let's say you've got a 15mm solid steel axle now. You'd want to go to a 20mm aluminum axle to have roughly the same stiffness. Harry Hindall used to offer tubular 4130 axles that had a mild heat treat to strengthen them some more. You could also use 4140 or 4140HT that comes with a moderate heat treatment, but that only improves strength, not stiffness. As the hardness goes up ductility goes down and you want to take even more care with avoiding sharp inside corners that will act as stress raisers. I've had a front axle break (Suzuki TS185, sheared at the shoulder where the diameter stepped up) and I've been behind a Duc single that had a rear axle break in a road race, and I'd suggest you won't find a broken axle to be a lot of fun. The recommendation of going to a breaker and rummaging through their big box o' axles to find something to shorten/rethread (as needed) sounds like a good plan to me if you are wanting to reduce the amount of aggro. cheers, Michael

Ace's can ride on stuff that mere mortals will struggle on. Generally, something responsive enough to be on the edge of immediate disaster for anyone else is just barely responsive enough for someone like Rossi, Bou, etc. Motorcycles are intended to be single track vehicles. I don't want to have to remember different modes of turning response that vary with the direction of the turn. A gyroscope would be best to help my bikes balance. Or maybe a better rider. cheers, Michael

The length of the spacer I made to replace thr speedo drive that was on my bike is 33mm. I wonder if the speedo drive was not correct, or they normally used a 30mm spacer and just jacked things apart in the unlikely event someone wanted to run a speedo. One person measured their 250 rear wheel and came up with the same offset I found on the countershaft sprocket, so with that and the various reports of "wheel/engine/swing arm are centered" I'm going to presume that the engine is intended to be centered in the frame and the wheel aligned to the centerline. Turning the bike into a two-track vehicle instead of a single track vehicle sounds questionable to me, even if SM did advocate it. Two parallel tracks doesn't sound any better than the wheels never being in line (and if the rear wheel is cocked to the side at some point the front wheel will be turned off center just enough to make it parallel to the rear wheel so it should act just the same as having the entire rear wheel spaced over). I think the next step is probably seeing if I can get the wheel offset without worrying at all about spoke thread engagement. Then I'll mark the spokes and pull a few and see how much longer spokes I need on the right side to get enough thread engagement. Buying half of a spoke set might do the trick. I was looking at the front wheel and noticed a LOT of thread is visible above the nipples on most all of the spokes. I guess I should check those for spoke engagement too. The bike has been through a lot of hands before it got to me so it is difficult at times to tell what is factory specification and what is Dreaded Prior Owner work. cheers, Michael

Did you ever see any clarification from SM on why he thought that? I can't think of any reason why having the wheels out of line could be of a benefit, unless maybe you were doing flattrack/speedway and wanted to bias things in the only direction the bike ever turned. cheers, Michael

I've got wheel alignment problems with my Model 159, even after straightening the bent swing arm and triple clamp. I've measured and drawn up the swing arm and cast aluminum rear engine mount and it sure looks like the engine case center is supposed to be on the centerline of the bike with the swing arm centered on it too. Yet with good sprocket alignment (fresh sprockets too) to get the rear wheel aligned on the front wheel the rim has to be pulled over towards the sprocket to the point where I've got serious concerns about the amount of thread engagement between the spokes and nipples on the right side of the wheel. Perhaps the new Buchanan spoke set is off, with the long spokes on the right not having been made long enough. They seemed pretty similar to the spokes I took out of the wheel. Several people have told me "it is just a trials bike, don't worry about wheel alignment" but while I could do that if it is only a mm or two, 10-15mm is getting a bit out of hand. Other people have mentioned that they suspect Bultaco may not have been too concerned about wheel alignment when they made the bikes, though I'd like to think they'd show a reasonable level of concern. Since the swing arm/pivot/damper mounts seem to be pretty symmetric around the engine case split I'm going to presume they meant to hold the rim on the centerline too. Since the engine is out I figured I'd verify the sprocket offset from the case split. First I measured the (mildly worn) motor mount widths on the engine: front 1.230-1.235" upper rear: 1.230-1.235" lower rear: 1.220" 31mm = 1.2204", 32mm = 1.25984 (1.260)" So I'll use a nominal width of 31-31.25ish mm I set the engine on 2-4-6 blocks (hardened and ground steel blocks 2" x 4" x 6" sold as a matched set) at front and upper rear motormounts with the sprocket side of the engine upwards. I then used a screw jack to bring the bottom motor mount upper face to the same (basic) level as the other two mounts, measuring all this with a vernier height gauge. The 520 sprocket has roughly a 1mm shoulder on each side (which would make it a 525 sprocket at 8mm (5/16") full thickness when adding 1mm to each side of a 6mm sprocket). I wasn't going to disassemble the left cover to measure it exactly and I'm working off the outer face of the sprocket anyway, since that needs to be in plane with the outer face of the 6mm wide rear sprocket. The outside of the 6mm thick sprocket to case centerline is 73.5mm as the best approximation measuring from the outer face of the upper rear motor mount. That would make the sprocket centerline 70.5mm. Would they have set an even number for the centerline (like 70mm)? 70-70.5mm is a .020" difference which is pretty negligable for chain alignment. Since the measurements were coming out closer to 70.5mm than to 70mm I'm willing to accept the former as the dimension for the sprocket centerline that the factory was intending to use. So if you are building a rear wheel and you want to presume that the engine is on the bike's centerline you can subtract 1/2 of the rim's measured width from 73.5mm and have the offset from the outer face of a 520 sprocket. Or subtract it from 67.5mm if you are measuring from the flange on the wheel that mates to the inner face of a 6mm thick sprocket. If someone has a Bultaco service bulletin or similar "official" document that calls out a sprocket offset dimension I'd be interested to hear what they said it should be. I don't know what kind of manufacturing tolerances they held on machined parts like the engine motor mounts vs welded/built up parts like the rear rim or swingarm pivot or rear axle plates. FWIW I've got 190mm for the nominal width at the rear axle for the backing plate/hub/spacer (or speedo drive) assembly. So you should be able to take an assembled wheel (at least for a 159, I don't know if Bultaco changed things on earlier or later models), subtract the rim width from 190mm, divide the result by two and then have the rim be that number inwards from the outside face of the backing plate or the outside face of the spacer on the right side of the wheel if that is easier for you to measure. Presuming of course that the rim is really supposed to be centered. ETA: I forgot to mention that the 159 sprocket is counterbored to slip over the flange on the hub, so depending on your bike's arrangement (counterbored or not) you may need to adjust to get a good offset dimension for the flange on the hub if building a wheel without a sprocket on it. cheers, Michael

Shouldn't all trials bikes come with a shovel like that one in Bozeman? cheers, Michael