Hello, i am designing a bicycle frame for first time. I am making a hardtail trail MTB with 27.5 wheels. What should be the minimum Bottom bracket height?

I made a 29er hardtail with a 70mm BB drop which is considered low for an MTB. But because it's a 29er (big wheels) that works out to about 312mm from the ground to the centre of the BB. The person I was making it for likes a low BB and isn't usually riding on big rocks. He hasn't had any pedal strike issues.

So it depends on the use case but I think quite low makes a nice riding bike.

Agreed that riding style, suspension presence, crank arm length and other factors come into play. This is one design spec that duplicating what has worked before on a production bike (independent on wheel size) is a good starting dimension. Andy

Yes although many of us in the real world don't "send it" as much as the designers of production MTBs anticipate. So it's often OK to go a bit lighter gauge on the tubes and a bit lower BB.

Not sure production designers anticipate how far their customers will "send" their bikes but I di suspect they heed how well those riders can land their bikes Andy.

I like around 300 to 305mm sagged with 170mm cranks and 130/140mm travel forks. Remember that hardtails change their geometry quite a bit as they go through their travel, unlike full suspension bikes. If you plan to run a long travel fork, the changes are more dramatic than a little XC fork. Keep that in mind as you design the bike.

Recalling Cannondale, still on US soil back then, had a 'Beast Of the East' MTB with a higher than the regular BB.