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hardtail is set up with 100mm forks would putting 140mm forks on make that much difference......
Yes most likely it would make a big difference. To the betterment or detriment would depend on the frame. What’s driving the possible change? What are you trying to achieve?
The effect of going up 40mm at the front will be around 2 degrees off the head angle, maybe three quarters of an inch higher in the bottom bracket, a shorter feeling front centre (can’t say by how much) and a slacker seat angle (again not sure by how much).
It will be quite a big effect. If the frame is designed to feel right with 100mm forks and 140mm forks then you may like the effect. Most likely though you would want an on the fly travel adjuster if you’re used the bike with 100mm forks.
Lots of people, myself included, run their hard tails with 140mm forks, but those frames are probably designed around that.
You should also check the warranty of the frame.