Monster-cross build on the road, will it be fun? Don't ask why, don't ask how much but this is what I'm aiming at:


29er entry level MTB with light Al frame stripped down
Carbon rigid MTB fork (for clearance)
Carbon drop-bar handle bars
35c tires
Road levers
Convert the 3x8 to 1x8 (46T x 11-42T)
Disc brakes


Since I've sold my road bike a year ago (after triathlons) I kinda missed the pure fast aerobic workout of a road bike but still have some gravel roads around me and wanted the versatility of CX/gravel to do some mellow XC around (for real off-road I'll use my friend's FS :) )

Gravel / CX market here is sooo pricy and I cannot get a good deal even on a used bike. So this beast would be way cheaper than that.

Me: 1.83m (~6'), weighs 95kg, Since I really liked the road bike aero position I thought about getting size M (17.5") as it will allow me to get low and forward on the handlebar.

My question is, would it be fun? would I go reasonably fast on paved roads? What do you think?

Thanks!
It will be a fun bike to ride, but it won't feel as fast as a road bike.

I've been building up variations of this bike for the last 15 years and there is simply no replacing the feel and acceleration of skinny tires and light wheels. Geometry also matters. Head angle, trail and wheelbase can be very far off the dimensions of a "normal" road bike. 

Much of the "feel" of a road bike is not it's absolute top speed, but it's ability to accelerate quickly and be nimble in it's handling. Any changes you make in tire width and geometry will be compromises. The question that only you can answer is "Is it good enough?".

Tire choice will make a big difference in how fast the bike feels. Tires in the 35c range are all over the place in terms of weight, tread and rolling resistance. A light skinwall 35c tire with smooth tread can be reasonably fast and certainly makes bad roads and the occasional trip off the road's shoulder much easier. I really like having a bike that I can easily pull off into a dirt should and not worry about loosing control.