pressed001

Hello all,
I have what I believe to be a '95 GT Tempest made from 7005 Aluminum. Wondering two things: Serial number on the NDS dropout says H4L18880. What does all this mean?

Also, a much more technical question: how did GT heat-treat their 7005? I ask because I want to have disc tabs welded on. Just want to know what heat treatment will be required afterwards.

Thank you!

cpach

Honestly if you have to ask, you probably shouldn't do it. There's no way getting good professionals to weld, heat treat, and finish the frame will cost less than a reasonable modern frame with disc mounts. Also discs place different stresses on the frame that the tubing was not intended to accommodate. Discs are great, but cantis/linear pulls can work more than reasonably well. Leave well enough alone--that bike is built up super nice, you'll only ruin its value by altering it.

But hey, I googled it for you! Here's the first result: Bicycle Frame Making with Aluminum Alloy ? Key Processes: T4 vs T6 Heat Treatment, AL 6061 vs AL 7005 ? Bicycles & Bicycling

alcjphil

Adding a rear disc brake is, at best, a very tiny marginal gain. Your bike was never intended for disc rear brakes. A front disc brake is a much better modification. All you need is a disc brake compatible fork

pressed001

I really want the discs so that I can use some high-speed, low-drag carbon wheels. The pickings are thin for 26" rim-brake rims.

alcjphil

What do you mean by "high speed"? This is, after all, a rigid frame mountain bike. "low drag" wheels only become important for sustained speeds over 50 kph

Matt Gaunt

I love that build! Honestly, even if it were possible to add disc brake mounts without compromising safety (which realistically would be difficult at best), I still wouldn't do it.

Keep as is and enjoy. ��

cpach

Those are really nice wheels on there already. If you're on road, the primary impediment to aerodynamics is going to be being on a friggin mountain bike with flat bars with a more upright position with your chest more open to the wind. If you're off road, I doubt aerodynamics is going to be your primary problem.

Anyhow, regardless, it's just a very bad idea from a mechanical standpoint to try to convert that frame to discs.

pressed001

Been there, done that. I've already got 2 companies that can weld the frame and another ghat can T6 heat treat it.

Just wondering what process GT used so I can keep it the same.

If you Google "GT disc brake mount" you can find some pretty ligit adapters for these non-disc frames with their beefy drop outs. They are for the LTS and Zaskars. No tempest specific though.

Thanks for the input though and yeah I might just as well leave it as it is. I could just pick up another frame. But then again, it wouldn't be my 1995 GT.

pressed001

Sorry, dropping some military lingo. I just meant "nice" wheels. Nice as in 320g, 35mm wide, tubeless, hookless nice. 😁👍

dsbrantjr

If you really want some ballsy brakes without the disc hassles,you might consider Magura hydraulic RIM brakes: https://www.magura.com/en/components/bike/rim-brakes/
I have HS-33s on my rigid mountain bike and am totally satisfied. They were dirt-simple to install onto the canti posts and you can shorten the hydraulic line without needing to bleed them if you are careful, just use a guillotine-type cutter. Easy reach and wear adjustments. Highly recommended.

pressed001

First off I want to thank everyone for the replies. Also I want to apologize for any unclarity on my part.

I realize that there is a lot of love for keeping these older (a perspective thing, no doubt) bikes stock. I understand that mentality and sympathize because these bikes really are great just the way they are. That said, I could also pick up another '95ish GT Tempest for less than 100 bucks on the German market with relative ease.

Thanks for the complements on the wheels. I know they are great! I built them myself. Regardless, I want to go disc brakes!

What I am thinking at this point is to either get the mounts welded on and then perform a T6 treatment, or leave it as it is. It could go either way at this point but I am thinking that it will get shelved for a while. I am in the process of purchasing an older steel On-One 456 with original fork. That project will take priority as this baby already rolls.

Andrew R Stewart 

My understanding of 7005 Al is that it uses aging as the method of returning the after welded condition to full strength. Because heat treating costs real money on various levels but the time a bike frame spends (from Asia as most 7005 bikes come from there) in process, shipping and warehousing cost the same regardless of frame material Asian Al production quickly adapted this alloy for cost concerns.

Whether 7005 responds to elevated heat treating (compared to "room temps") I don't know. But as someone already mentioned if you have to ask then you are not informed enough to do the work and have it not be wrong. Andy (who has a GT Colorado, but it's steel and USA sourced)

2seven0

To answer your original question welding aluminum anneals the weld bead and HAZ (Heat Affected Zone). The area will naturally age to @ T4 condition (in 6061 alloy). For heat treat info see link below.
Hope this helps.
West

https://www.fairing.com/7005-aluminum

pressed001

Great link. Thanks for the info.

After hearing back from the frame builders I have decided against welding the frame. According to them the frame would slightly deform from the process and would need to be set, which they are not equipped to do.

I believe that the previous comments about the room-temperature-hardening are true and that these are most likely T4 level hardened from that process.

Again thanks again for the information. One of my questions still remains: what my serial number means exactly. Anyone have input on that?