CrowSeph

Hello! I'm kinda new to the gravel, in the past i used a custom roadbike (vintage) frame to hold 35mm tires with a full drivetrain and parts from a vintage mtb.
The bike was definitely fun to use but then i decided to sell it for a many reason.

Now since then i have in mind something similar to do, another custom frame modified in the read dropout to hold even bigger tyre.
Let's say, on my mind i want to make something more in the direction of a monstercross than a gravel itself with a steel frame 1x10/11s setup and 650bx2,40.

A bike with those very fat tyre how would act in different terrain? with a smaller wheel and huge tyre would improve the driveability on the offroad section?
I don't care if will be slow on the road but i want something very capable off road, not even like a trail bike but definitely more like an aggressive xc.

is my idea right? or i'm going to regret the build soon or later?

mstateglfr 

Your idea could ride well or it could ride terribly. Who knows- geometry isn't mentioned and that is critical to know.

You can adjust head tube andgle and fork rake to account for the larger tire and make the bike steer quickly like a road bike.
You could also have it ride stable and steer slowly.

Totally depends what you want.

CrowSeph

On my mind the best idea is to find something with the headtube angle of an mtb, or use an adapter to make it more aggressive.
Since the key is offroad capability i'm basing almost everything from mountainbikes theories.

mstateglfr 

Yeah, not trying to be difficult here, but I interpret your comment above to be conflicting. That may be how I use some words to describe the feel of a bike's geometry vs how you use those words.

To me, an MTB head tube angle is slack and when paired with a typical MTB fork(rigid or suspension, doesnt matter), the steering is very slow and stable. That, to me, is the opposite of aggressive. But at the same time, if you take that bike down a steep hill, you want that steering geometry because it will help keep the front wheel out in front of you for stability and the bike will more easily stay pointed straight ahead, which you again want for confidence/stability in that scenario.
When I think of 'aggressive', I think of a steeper head tube angle with a fork that has more offset than a typical MTB fork. This will typically produce a bike that is quicker steering and can turn in a tighter radius.


Hopefully you see how I am using the terms and that may be different from what you are saying?


Anyways, to maybe answer your initial questions...
- A 650b wheel with 2.4" tire is plenty big for gravel roads and single track(offroad) riding. Yeah the rim size is smaller than a 700c rim, but the overall wheel and tire combo you are thinking about is the same diameter as a 700c wheel with a 40mm tire on it. And the circumference of a 700c wheel with a 40mm tire is pretty commonly accepted as more than capable for gravel/unpaved riding from an approach angle(the smaller the overall wheel and tire, the steeper the approach) as it rolls over obstacles and debris.
- A bike with that large of a tire can act however you want it to act since with a custom frame you can adjust the head tube angle and fork rake(offset) to match the feel of riding that you want. When a bike gets a larger tire added, the amount of trail increases. From a very simple view- higher trail means slower steering and more stable steering. Now again, that applies to a bike that already exists and you add a larger tire into the mix. You could easily have your bike designed to take that large tire and have lower trail for quick steering, if that is something you want.

Me personally? My gravel bike has 72.5 head angle, 50mm fork, and 43mm tires which results in 60mm of trail and I really like that from a steering and feel perspective. That trail number is close to my main road bike and thats intentional since I ride my gravel bike on gravel roads...its just a road bike with wider tires and adjusted gearing to increase comfort and range.
Funny enough- if my gravel bike had 27.5(650b) wheels and 60mm tires, the trail would be the same- 60mm.

If you went for something like 71 degree head tube angle, 50mm fork offset, and 60mm tires, then your trail number would be 70mm. This is a fairly middle of the road trail number for gravel riding and underbiking(gravel bike on technical singletrack) because it give you more stability going down hills, but shouldnt feel too slugging when turning.
For context, a Trek Procaliber XC hardtail has 100mm of trail because it uses a 68.8 degree head tube angle and 42mm fork offset along with a 2.2" tire.


Hope some of this helps.