you can lower it down till the frame sits on the ground - thats as low as you can go...
nder 3 inches you cant ride very fast because you cant lean it over to turn.

a stock rigid frame has about 5" of clearance
a custom rigid frame usually has 3 to 4" of clearance. look careful at, say, a 48 frame - the line from the neck to the seat post to the wheel is NOT straight - guys used a saw and welder to lower the frame to make it straight - then made the front end longer to get the clearance back so it would turn - i saw lots of guys doing that in the 70's install a weld on hard tail or struts and then keep adding to the front end till it quits scraping.. they rode terrible and cornered worse - my stock 79 sporty rode circles around the big twins... but i was careful to back off as to not offend them by making them look bad.
there are struts available for soft tails that just replace the shocks with a bar with a adjuster - that will let you set it at 3 or 4 inches and it wont get low enough that you cant turn. - the stock bike will still run circles around the lowered bike. there are companies now making rigid frames for twin cam softails - those will be very low but still corner ok since the rear wheel wont move.
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