Fork Flex Fix... thicker tubes?

[cultofthumpers]

If weight isn't a big concern, would it be possible to have a machine shop turn a pair of 38MM Outside diameter and 35MM inside diameter tubes and use 35MM internals?
I once heard of some place in the UK that used a gun barrel bore to make tubes.
38MM is the maximum diameter for Historic class tubes in AHRMA, but a heavy 4 stroke like a TT500 and a middle-aged rider would cause a lot of flex on those smaller tubes.
What say you? possible?

[Husky500evo]

Instead of going to go to all the trouble & expense to get new tubes machined up , you would probably be better off looking for a set of 38mm Simons forks with 9 inches travel . Otherwise , ( & I stand to be corrected ) the only other 38mm forks from '77 would be off a Maico AW . The Maico forks are pretty good & would be the cheaper way to go .

[cultofthumpers]

It would be cheaper to switch to Maico forks(Simons are out of this world when you can find them), but 38MM tubes with the same wall thickness would be nearly as flimsy as 35MM wouldn't they?

[Doc]

or look fit a fork brace and stick with the originals

[Nathan S]

YZ250/400D have 38mm forks.

Assuming the length and the material stay the same, the formula for stiffness of a tube is:
(D x D x D x D) - (d x d x d x d)
Or D^4 - d^4 if you'd prefer

D is the outer diameter, d is the inner diameter.

So a small change to the outer diameter makes a large difference to the stiffness, while the same change to the ID makes less of a difference.

To answer the original question: It would be possible, and they'd be the stiffest set of 38mm forks around, but you'll probably lose enthusiasm once you hear how much it will cost...

[JC]

Spot on Nathan. Larger OD makes considerable diference.

Maico forks were actually 37.75mm OD apparently. Also, Mk9 250/370 Pursangs & Mk 10 370 Frontera ran 38mm Betors that were plush-city. KTMs Marzocchis may have been 38mm by then also

[Husky500evo]

Well there you go . You can learn something new every day on this forum. I always thought that the D model YZs only had 36mm fork tubes . Never had much to do with Bultacos , so I didn't know about their forks either .

[Maicojames]

Spot on Nathan. Larger OD makes considerable diference.

Maico forks were actually 37.75mm OD apparently. Also, Mk9 250/370 Pursangs & Mk 10 370 Frontera ran 38mm Betors that were plush-city. KTMs Marzocchis may have been 38mm by then also

JC the Katoom Zokes were 35mm til 78. The YZD or Maico Aw forks seem a good bet run their clamps with them, I suspect it easier to machine a stem than bore out clamps...and personally, I would feel better not having smaller clamps bored out .

[cultofthumpers]

Located a set of '77 AW forks with Rusty tubes. for $50 USD I guess I can have a replacement set of tubes made by Forking by Frank or these re chromed.
My concern was a super heavy TT500 engine with a Super cHunky guy (me, not Rick Siemen) with a big hit would bend them under easily. But, I guess Aberg ran Husky forks back in the day and in '77 they were 35MM or 36MM I think and Lord knows he went higher and faster than I ever will.

[cappra]

I would be hesitant about getting tubes from Forking by Frank.
Tubes for choppers might be one thing, but most fork tubes
for mx are hard chromed.  Normal tubing will bend too easily.

[brent j]

Are you limited by year of manufacturer or just size.

If the year is the cut off I would use 38mm YZ250/400D forks. Consider most of the HL replicas running around are using 38mm forks. I run 38's on 2 of my XT500's and have never had any problems.

If it's a size issue then any common 38mm front end should do.

Getting your suspension right will reduce the flex even more.

Brent

[cultofthumpers]

If not from Forking by Frank then where else can you get replacement tubes made?
Does anyone know of a good set of AW tubes down there that needs something from the USA? Maybe we can work a trade?

It isn't a matter of a year cut off it is because AHRMA has rules concerning travel. 9" max front and rear and a maximum diameter of 38MM. They are going on a TT500 in a Husky frame with a 103-113 Kilo rider (16-19 stone) so I want the most rigid forks possible, that is why I thought of the thicker tubes idea. My thinking is if it cost $700 to have them made then it would be a one time investment. Buy a cheaper thinner set and bent them then I lose it anyway considering the down time while obtaining new tubes.

[Maicojames]

.I'm confused-are you in Ausrtalia or the US? If in the US , the AW tubes are easy to find. If you are racing it in AHRMA's Historic classes ( should be elgible for Hist 500, and Hist 4 stroke)-yes 38 mm is max and I check with a caliper myself if in doubt.