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reason I am replacing my tonneau is that the "old" one has shrunk over time, it seems to me that you wouldn't want to stretch it really, really tight, but just firmly taunt. Thoughts on that view?
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Stever,
A tough call. A lot of top materials shrink with cold & expand plus stretch with heat. Premium materials are much less subject to this problem. Firmly taunt when hot can be impossibly tight when cold. You have to warm it up to get the fasteners engaged. I think it depends upon the temperature range that the car is subjected to. Cheap top material will tend to shrink & distort with age, but genuine Everflex is not so much of a problem. It is the original equipment top material on Healeys.
The down side of a loose fit is that it flaps more in the wind. I drive in temperatures ranging from the low twenties to 100 plus. The expansion/contraction of the material is considerable. I hate the loose flapping when the material is hot enough to be loose, & so fight & tug like crazy to get it fastened when it is cold. Sometimes I have to move into a heated garage to get the snaps fastened. At least it doesn't drive me crazy with it's flapping in hot weather.
I had a tonneau made of inferior material on my truck, & after a couple of years, it shrunk enough that two people couldn't stretch it enough to get it fastened unless the temperature was in the high 90's & it had been sitting in the sun. If I ever unfastened it in the cold, there was no hope of getting it snapped again. Ultra violet from the sun really degrades the cheap vinyl materials. Now running open bed again. I think that you will have the best results with a premium material. Read, not cheap, not low grade.
Some of the premium materials such as Sunfast, tend to shrink & stretch even less with age & temperature changes. If you live in San Diego or similar, much of the problem goes away except for the susceptibility to ultraviolet damage. I have both an Everflex tonneau & a Stayfast tonneau. One for "correct" & the other for everyday use. the Stayfast requires considerably less effort to get snapped when it is cold, & doesn't loosen up so much when it is warm. Not concours original though, & the price will give you "sticker shock". A flapping tonneau drives me nuts, so I fit it pretty tight.
I suggest going with a top quality material such as Heritage Upholstery's Everflex material for the traditional top & tonneau or Sun Fast-Stayfast if you like the canvas look. No gloss. Sunfast is more temperature stable than Everflex & costs a lot more.
Obviously, the larger the sections of flat material are, such as a tonneau, the more it shrinks & stretches with temp changes. On the other hand, on a top you have mechanical leverage to help pull things into place. Much easier to handle a tight top than a tight tonneau, so you can fit a top very tightly. Flapping tops are not attractive either.
As you have observed from the other posts, fitting it when it is hot & at maximum stretch & expansion Helps get the tight fit.
I am speaking from experience on the relatively small tops of a BN2. The larger cockpits of a four seater require extra braces/stiffners to control the flapping, but the same principles apply.
Maybe you can get some ideas here
https://www.heritagetrim.com/
D