I'm building up an old MTB frame into a road bike. The thing had an incredibly-slack headtube (71 degrees) with its original loooonnngggggg rigid fork (39cm from axle to crown). The damn fork weighs a ton.

My plan is to put on an after-market 650c carbon fork that is only 36-ish cm from axle to crown. The effect will be pretty dramatic on the effective head-tube angle....moving it from 71 degrees to over 73 degrees, if I did my arcsin calculation right. The new fork's rake is fairly typical (in the low 40s).

So...my question is....am I missing something about the effects on the bike's handling? Other than the fact that the bottom-bracket will get a couple centimeters closer to the ground (which is fine for this application), I'm just assuming the handling will be fairly normal for a road bike. If that's not the case, please let me know!

Thanks!

That's an inventive project.:idea: You will be the first to know how it will handle - when you get on the bike. Good luck, and don't sink too much money into it.

So...my question is....am I missing something about the effects on the bike's handling?

Handling depends just as much or more on rake/trail than on head angle alone. Trail can be sort of hard to figure out when all the variable aren't known. I predict that this will be the worst handling - perhaps even most dangerous - bike you have ever ridden, but there's also a slim - very slim - chance everything will turn out purple. You're not going to know until you finish it!

That's a first I'm hearing about. Good luck!!

I was thinking of converting my hybrid to a "road" bike with Shimano 105 maybe.. But, i thought, nahhhhhh... Or wait... Lemme know how it turns out, I might try it out... But then I'll have to change a lot of stuff.. And what the heck am I gonna do with my old handlebars etc?

I predict that this will be the worst handling - perhaps even most dangerous - bike you have ever ridden

Oh gee thanks a ton! :)

The longer story is that this is an old Softride frame (with the Classic beam) which I acquired because I injured my back. All reports are that Softride is where back-injured roadies go for prevention of recurrence (before they go recumbent). Also, I had a very nice old set of 650c wheels sitting in a corner looking lonely.

SDizzle, I'm really open to hearing more about your guess on the handling. If it ends up with a head-tube angle of 73.5 on a fairly normal wheelbase, and the new fork is identical in rake to the old, where would the danger come from?

Your seat tube will get really steep , and don't allow you to sit back as you should. Are you sure about those fork length numbers ?

If the trail is at least 40-45mm you should be OK, but I would worry about the seat position.

BTW, 71 degrees is very steep in mountain biking and it generally defines a quick handling XC bike. So in my opinion 71 degrees would have been just fine for the road and unless the trail is huge, it would be OK for climbing also.

Time to report in.

It worked! In fact, that's an understatement. I can ride no-handed, and the handling is reasonably stable. Big success!

Also, since I started this thread, I have spent hundreds of hours and thousands of dollars (mostly belonging to my health plan, but some of my own) trying to move back toward normalcy in my back. Overall, I have to rate that effort as a success. I still have a major issue, but it appears that I'll be able to ride the Softride for long distances without symptom exacerbation.

That is a real relief. In fact, I'm feeling good enough to sign up for RAGBRAI with a riding buddy!

Got any pics?

I'm building up an old MTB frame into a road bike. The thing had an incredibly-slack headtube (71 degrees) with its original loooonnngggggg rigid fork (39cm from axle to crown). The damn fork weighs a ton.

My plan is to put on an after-market 650c carbon fork that is only 36-ish cm from axle to crown. The effect will be pretty dramatic on the effective head-tube angle....moving it from 71 degrees to over 73 degrees, if I did my arcsin calculation right. The new fork's rake is fairly typical (in the low 40s)

I think the change in the angles will be more like 4 degrees. The change in angle will be (approx.):

ArcSine( ( Change in fork length ) / ( Rear axle to crown distance ) )

A typical MTB will have a rear axle to crown length in the range of 40 - 45 cm, so a change in fork length of 3 cm will result in an angle change of about:

ArcSine( 3 cm / 42 cm ) = ArcSine( 0.0714 ) = 4.10 degrees

The seat tube angle will increase by about the same amount.

This sounds like you are trying to turn an MTB into a TT bike. I know someone who has successfully done something similar, but he converted a cyclocross bike (similar fork length as your MTB fork) and used a regular road fork (37 cm long), so his angle changes were less than 3 degrees.

I think the change in the angles will be more like 4 degrees. The change in angle will be (approx.):

ArcSine( ( Change in fork length ) / ( Rear axle to crown distance ) )

A typical MTB will have a rear axle to crown length in the range of 40 - 45 cm, so a change in fork length of 3 cm will result in an angle change of about:

ArcSine( 3 cm / 42 cm ) = ArcSine( 0.0714 ) = 4.10 degrees



Yeah, I basically agree, except you assumed that the bike is for a six-year-old [grin]. My actual rear-axle-to-crown length is about 100 cm, or over double your rule of thumb.

But no matter...it works! And it's fun to mess with bikes.

I'd be weary of breaking a CF fork seeing as you'll be riding the frame in a way it was not intended...

Yeah, I basically agree, except you assumed that the bike is for a six-year-old [grin]. My actual rear-axle-to-crown length is about 100 cm, or over double your rule of thumb.

Oops! I forgot to convert units. For odd historical reasons, MTB frames are frequently measured in inches, whereas road frames are usually measured in centimeters. A typical MTB frame has a wheelbase of about 42 inches, not 42 centimeters (Oops!). so the expected change of frame angles is more in the 2 degree realm, not the 4+ degree realm.