Hi all: here's my $0.02 (of course, its 2 cents Canadian so its really only 1.5 cents US)....
I started on a '75 Honda CB360T (didn't EVERYONE have one of these old puppies?) in 1975 and it was fine, although it didn't have enough power to get me into too much trouble. I did however, put more than 60,000 miles on it without any issues. I traded in the Honda on a '75 XS650B in 1978 and wow - THAT was a pretty big bike for its day. I found the handling to be just fine - although it DID have enough power to get me into trouble (just paperwork...). As I recall it (and others have said this) the tires would usually slide well before any metal was scraping - and so I did wind up in a ditch the odd time, but the beautiful "B" never hurt me. When the Suzuki GS850G shaft drive four came out in 1979, I was in love (again) and so I traded the Yamaha in on a leftover Suzuki 850 (which from my perspective now was clearly a huge error) in late 1981. I recall that I got precisely what I had paid for the Yamaha from the same dealer I had bought it from - more than 60,000 miles before. I loved the Suzuki - but I wish I had kept the Yamaha. Nonetheless, the XS650B served me well and had a very low cost of ownership. The Suzuki had a reputation as a really good handling large motorcycle - and it was, although it was pretty heavy at more than 600 lbs wet. Once again, those late '70's tires weren't as good as the engine, brakes and frame and so....it would slide around, which was
really fun with all that road-hugging weight. After the 850, I got out of bikes for 25 years until last year when I bought a 2007 Honda ST1300 (talk about power and weight...) and my 1976 XS650C, both of which I just love to ride.
The Honda has amazing linked ABS brakes and really good tires - and a frame that you could park a train on. The Yamaha...not so much...
The point is, as others have clearly said, early tires weren't really very good and
neither were the drum and rather primitive disk brakes of the day. That meant that the forces those tires and brakes could generate from friction with the road were simply not very high (especially as sliding friction is much less than static friction). For that reason, 1950's, 60's and early '70's bikes would often slide
before they would wobble. This tended to mask quite a few weak, or at least wonky, frames in many early bikes (hello...Kawasaki triples.....!!!.....holy crapola Batman!!!). Those darned things, especially the earliest ones, were 60-70 HP engines in 30 HP frames with 12 HP brakes, for the most part.
For us on this forum, the fact is that the early Yamaha 650's, and some other Japanese bikes,
would wobble fairly easily -
even on early '70's tires and crummy brakes and so those of you riding pre-'73 bikes, pretty as they are - please watch it - because modern tires will NOT slide as easily and that will put forces into your bike's frame that are much higher than it was designed for. These higher loads from modern tires will be transmitted to those early frames (that are now 40+ years old) and thus wobbling could be induced even more easily than before. I ride every week with a bunch of local guys, some of whom are mounted on beautifully restored vintage British bikes. One chap has a magnificent late 1950's BSA Gold Star 500 that he says can be rather evil on modern tires - whereas back in the 1950-60's when it was new, the Gold Star was an absolutely solid handling bike with no vices. The key point is - your only contact with the road is through the tires - and so THAT is where all the forces that can cause a wobble will come from. Basically folks, here is the physics of the situation....
- Old tires + crummy brakes -> poor traction = low frame forces (little chance of a wobble in most cases).
- Modern tires + crummy brakes -> good cornering & acceleration traction = high frame forces (good chance of a wobble if the frame is not strong and stiff).
- High frame forces on a weak frame = wobbles.
Pete
