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Strength Properties of Ornamental Chrome Plating Under Bending

2003

How far can the base steel be bent before the chrome plating cracks?
I have an expensive classic bicycle that is chrome plated throughout with a high quality ornamental chrome plating job. I want to adjust the chromoly steel tubing of the frame so that it can accommodate a wider rear wheel hub.

At present the wheel stays (frame members that hold the axle in place) are set apart 120 mm. I want to cold-set the stays to a new permanent spacing of 130 to 135 mm apart.
The bike store mechanics I have spoken with see no problem with cracking the chrome during the cold-setting process, but I am not convinced.

The stays need to be bent far apart enough to yield the steel before they spring back to the new position. In addition, an S-shape bend is required to ensure that the frame connections at the axle (dropouts) remain parallel and aligned at the new spacing. How can I determine how far the tubes can be bent before the chrome cracks?

Or assuming I could determine the amount of elongation in the outer fibers of the steel during the bending process what elongation of the chrome plating can be tolerated?

Conversely, what compression on the inner fibers?
Also how does the bending process affect the bond between the chrome finish and the chromoly base metal. If the chrome were to crack during the process, how complicated would it be to repair the chrome plating locally?

What budgetary costs could be expected for stripping, prepping and re-chroming the damage, assuming a high quality job?

JOHN FRANCESCON
hobbyist - NEW YORK, New York

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Hi, John. I don't think anybody can give you the numerical data you would like because you have layers of nickel and of chrome (and possibly copper) that must stretch and not lose their bond to the substrate, and that bond strength is highly variable.

Still, if the nickel was done well, it usually has a good deal of ductility (the shop may periodically bend parts or test coupons around mandrels to demonstrate to themselves the ductility and bond. The chrome itself may crack, but this is only millionths of an inch thick and may not be a problem. People can only guess, but I guess the same as the bike shop, that it will not be a problem.

I can't quote someone else's charges, but plating shops tend to charge about the same for their time as a mechanic or plumber. How much personal attention you think this will take should provide a good estimate of the cost. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
October 2009