Im looking into extending my stem length but as my top tube is fixed I was wondering what the upper limit is (in your opinion) on stem length. I understand that if the stem is too long the handling will become kooky.

I have my fork cut 40mm longer than the head tube, then I added a 40mm extension. I wanted an almost upright position. I hadn't noticed anything wonky with my steering. Of course, my ride is heavy enough so that it is very stable.

I believe that the stem length issue,. is more a safety issue as opposed to a handling issue. At least that is my understanding, I am willing to take the risk of having the longer stem set up.

As it is a lever , fatiguing the steerer tube to breaking is a concern, with leverage of long stem extension ..

It is possible to add an internal sleeve to the steerer tube to double its thickness.

For a different purpose I gort a intenal Quill as a stem riser , made for BBB it tightens inside the stem,

and replaces the star-fangled nut in the process , it could be completely in the steerer tube ,

or you add a few of the included shims, to raise the stem clamping height ..
Its part # BHP-21, in 9/8" type.

Since you talk top tube length I assume you mean forward extension.

If so, the longest standard extension is 140mm, and I prefer staying below 120, but fit trumps all. Don't worry about handling, that's more a question of weight distribution then stem length per se. In any case, you already have the frame so focus on getting the best possible fit using that. If a decent fit isn't possible, then you can shop for a more suitable frame.

yes i meant forward extension, sorry

Your post was clear, it's the previous responders who have misread. As already noted, they sell 140mm stems and they are intended to be purchased and used. A lot of pros use 130-140mm stems.

A longer stem may create a slightly slower feel to the steering but you would be used to it within a ride or two.

Another misconception is the idea that changing the stem length affects the weight balance. It makes very little difference. The biggest contributor to weight balance is saddle fore/aft position, follwed by torso angle. The lower the torso, the more weight on the front.

I rode a Cannondale M700 for years with the stock 150mm nearly-straight stem.
(WTF were they thinking?)
No handling issues at all..but..I finally learned about the concept of "fit" and realized why I was so miserable on 2+ hour rides.
A 125mm stem with a 35-degree upward angle put the bars in a reasonable location.

Guilty as charged, after being up for 16+ hours,it is amazing what a few hours sleep will do for one's reading comprehension.

I'd like to know more about how this works. Please explain!

Cheers

Very simple. With a given torso position, everyone has a certain center of gravity. If you have 45% of the weight on the front and 55% on the rear you know that the center of gravity is located a distance of 45% of the wheelbase, from the rear of the bike.

If you move the saddle back 1cm it will roughly reduce the weight on the front by about 1% and increase the weight on the back by 1%, since the wheelbase of the bike is not greatly less than 100cm. This is easily proven by placing a scale under the front wheel, with the rear wheel blocked up so the bike is horizontal. You can also prove that lowering the torso adds a lot of weight to the front wheel, compared to riding in an upright position.

If you move your nads forward, as you would with a longer stem, there is little change to the weight balance.

EDIT: NVM explained

Thank you for explaining, that's very helpful. Just one further question; What is the effect on bike handling of shifting the center of gravity forwards or backwards?

Some experts think that the weight on the front should be in the 42-45% range. Too much weight on the front can make the steering feel heavy - one reason I don't recommend nonsetback seatposts. Unless you're racing or descending mountains, the average person would never notice the difference. If you ride hairpin corners in the mountains, you will certainly notice a poor cornering bike.

any idea where I can find a long threaded (thats the old kind right?) stem? I have looked on a few places and the straight stems only go up to 120mm.
I basically want the longest stem i can find. (after doing some measurements apparently my bike is too short).

I suppose i could also switch to flipped trekking bars.

I have some Thomsons in 140mm for 1-1/8 fork and 26mm H-bar, and some Oval Concepts similar up to 130mm. If either interest you send me an email at info at the site below my signature, include your phone number & I'll call.

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This is a common issue. The fix I prefer is to use a threaded to threadless quill stem adapter, about $10-15 at a bike shop. Then attach the modern stem that suits you. I would recommend, based on your description, a long stem (130 mm or so) with about 17 degrees of rise. It's easy to think you need more rise than you actually do. 17 degrees flipped up satisfies most folks.

The idea that stem length compromises handling just isn't true. The handling will feel different, perhaps more aggressive or twitchy when going to a long stem, but I wouldn't say it makes handling poorer. There are many pro riders that use very long stems because they prefer the handling and fit that way.

you got any thomsons with a 26.0 clamp taht are close to 100mm?

Yes, I have a bunch of 4 bolt 100's in black or silver, in both angles 10° and -5°. I also have lengths on either side.

I also have some 2-bolt, 31.8mm in various lengths. All are new including the bag they come in.

Email me at my initials @ the site below the them if you're interested, include a phone number for faster response.