Springs.

[Big Vern]

The read my post!
There are two lengths that could be used for the book. 13" open length and 14" open length
18" open length will not fit the book chassis without major rework. As TT has built his chassis I'm guessing he won't to chop it about
Further 18"open will give something like 6"+ of travel and with only around 4" ground clearance there would be all sorts of issues.

If you have something to offer please make sure it's based either on knowledge or experience rather than a random guess.....
Please remember there are people who could go off and do something based on your random remark wasting their time and money!

BV.