What are these holes?

Can't help you there. Manufacturers got up to all kinds of funnies when disc brakes first appeared. 4-bolt rotors, proprietary mounting etc.
But maybe I can help you there. A search on "trek 3500 disc brake adapter" comes up with this:http://www.sellutionmtb.com/ProductDetails.asp?ProductCode=BA-GFTU

interesting, thanks. that looks like it could maybe work, but id want to know the dimensions and spacing to make sure it would work before ordering.

in the end, though, i decided to sell this bike since it was a bit small for me anyway. i picked up a trek wahoo 29er instead that already has disc brakes, although i upgraded the rear one a bit as i had anyway gotten a caliper for the other bike.

so now ive got a 1200w 69er. got a few more things to do to it to finish it off, but its working great so far...

Ah, the joys of tinkering with vintage mountain bikes. It seems you've stumbled upon a relic from a bygone era, my friend. Those 45mm disc mounts indeed harken back to the days when standards weren't as "standardized" as they are now.

You see, before the industry settled on the current 51mm spacing, there were indeed older versions like you've discovered. Fear not, for all is not lost in your quest for the proper adapter. Although your searches may have turned up empty-handed thus far, I'd suggest broadening your horizons and looking into adapters for "non-standard" disc mounts.

Now, I must warn you, the road for tracking down these elusive adapters can be a bumpy one, filled with trial and error. But hey, who doesn't enjoy a good challenge every now and then? It's just like cycling uphill – takes perseverance, determination, and an occasional sarcastic chuckle. Happy hunting!

I can't believe you didn't check the disc mount spacing before buying the Trek 3500. It's basic knowledge for any cyclist, especially if you're planning to convert it to an e-bike. Those 45mm holes you mentioned, they're not an "older version" or anything special. They're just not the standard 51mm you need.

As for where to find the proper adapter, I doubt you'll have much luck. You'll probably have to replace the whole rear triangle with one that has the correct disc mount spacing. And let me tell you, that's not going to be cheap.

But hey, at least you got the front fork right. Silver lining, I guess.

My, my, aren't we the self-proclaimed cycling guru? Sure, checking mount spacing is basic, but let's not forget the joy of unexpected challenges. Your "basic knowledge" doesn't account for the thrill of hunting down those elusive non-standard adapters. So, before you rain on someone's parade, remember: we're not all riding the same path. ;-)

Fair enough, we all enjoy a challenge. However, it's crucial to acknowledge the potential downside of non-standard adapters: they can be hard to find, and even harder to trust. If you value safety and performance, you might want to reconsider your stance on "basic knowledge". It's not about raining on someone's parade, it's about making sure that parade doesn't end in a crash.

Indeed, non-standard adapters can be a wild goose chase. But remember, the cycling world thrives on innovation and customization. The risk lies in sourcing reliable ones and ensuring they don't compromise safety. It's a balancing act, much like riding a fixie downhill. :o Keep your wits about you, and enjoy the ride!

While I respect the sentiment of innovation and customization in the cycling world, let's not romanticize the risks involved. Yes, it's a balancing act, but it's not a game of chance. The danger is not just in sourcing reliable adapters, but also in the potential damage to the bike's integrity and the rider's safety. It's not like we're talking about swapping out handlebar grips or pedals, this is the rear triangle we're dealing with.

Non-standard adapters may work in theory, but they can also lead to a host of problems, including misaligned brakes, uneven wear, and even frame damage. And let's not forget, if the adapter fails during a ride, the consequences could be disastrous.

So, while it's great to celebrate the spirit of customization, let's not lose sight of the potential dangers. It's not just about enjoying the ride, it's about making it back home in one piece. 💪