Vintage Dirt Bike Q & A

Hi, I have a 79 rocket and among the modifications I have carried out was to fit 43mm forks & an alloy swing arm from an 82, plated to carry twin shocks. Forks were serviced before fitting & shocks are new ohlins. when racing it I feel the frame exhibits too much flex, has anyone experienced this & are there any accepted ways to strengthen the frame ? There doesn't appear to be any cracks or damage which would account for the way it behaves. any ideas would be much appreciated.

Check the small tube which runs under the tank (the one which the HT coil sits on) they break out from the headstock end but you cant see it because that area is covered by the gusset plates. I would advise Mugen copy top engine plates made out of 6mm alloy, also replace front engine mounts with alloy ones.

If you ride one hard on modern tracks they do break, i was going through about 4 frames per season!

I found the 43mm forks too big, the 39mm mugen forks were great but finding them isn't easy so perhaps try some 41mm if you have some available.

It's a known problem with the 1979 frame and resulted in the double cradle design of the 1980. Frame flex will be shown up by fitting a stronger set of forks and swing arm. I would suggest you have the frame stripped and re-weld the seams particularly some of the gaps behind the headstock, perhaps add some cross-bracing under the foot pegs? The thicker Fox-style engine mounts might help and they look trick, too. My pace is too leisurely for this to be a huge issue....

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Thanks guys, I will check it out, that gives me somewhere to start looking.
Things are flexing enough to make the cable ties break that hold my crankcase shield unto the bottom frame tubes when landing off moderate jumps. It worried me enough that I couldn't make full use of the bikes potential speed in case something went horribily wrong. (I have visited the hospital enough times over my racing career, thank you).
I already have alloy front engine plates but would love a mugen copy top engine mount, any ideas on where I could get one of those ?

does the 78 frame have the same problems?

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Both the 78 and 79 were like it.

One thing i forgot about when i posted yesterday is that its a good idea to weld triangular gusset plates in the 3 corners of the engine cradle and like Michael said weld up any gaps.

As for the Mugen top mount, the L/H is a copy of a standard Honda one but the Right was more like a V shape which then went up to the bolt which holds the Exhaust mount. Once you have the modified top mounts it will help stiffen up the top tube (I bent one bad enough that the top triple clamp was hitting the fuel tank on full lock.)

I have seen copies of the Mugen mounts on a website so i will see if i can find a link for you.

One problem i did come across which nobody seems to mention anywhere is that they do crack at the bottom of the headstock so if you do have your frame welded up it would be a good idea to have that area done at the same time.