Bicycle Mechanics - 700x23 to 650bx32, will this work or will the stability and handling suffer?

I have a 1978 Trek TX900. It's ideal except for two issues. The bike will take a 700x28 tire, but a 700x32 will not fit. The stand-over height is tight (I can stand-over the bike, but have almost zero safety factor).

I'll need to install a wider tire on this bike for some gravel road events in 2013. I have a modern Cyclocross bike with 700x32, but one of the events I'm considering requires a vintage bike. I like the 700x32 size for gravel roads and for my everyday cycling.

A 650b conversion will allow me to fit a 32mm wide tire like the Grand Bois Cypress 650B, see: http://www.compasscycle.com/tires_gb_650_32.html .

However, I'm concerned that the smaller diameter tire will change the trail of the fork and create a less stable ride? The fork rake is 4.5cm according to the Trek Catalog. What rake would be ideal to preserve the Trail?

Measure radius , of the 2 wheels, with tires mounted, ...

All I can offer , is the concept of pneumatic trail, by having a fatter footprint on the road surface
the steering should not feel as Light.

recycling a link someone else just posted, elsewhere, here.
http://www.bikeman.com/bikeman-blogs/650blog/1771-650b-conversion-guidlines

The geometry change is the least of your problems. Getting the current already long reach brakes to align with the smaller rims will probably be impossible and you will have to replace them with even longer reach brakes.

I figure a trail of 58cm with the 700x28s, and 53cm with the 650x32Bs. You probably won't even notice. :thumb:

I figure a trail of 58cm with the 700x28s, and 53cm with the 650x32Bs. You probably won't even notice. :thumb:

Thanks'. As a follow-up, will the reduced trail improve or reduce stability? Does the handling change? That rake would be best to restore the original geometry?

Thanks'. As a follow-up, will the reduced trail improve or reduce stability? Does the handling change? That rake would be best to restore the original geometry?

The steering would be ever-so-slightly "lighter" -- I don't think it would be perceptibly less stable. You could restore the original trail by going to a fork with 40mm of rake, but it would definitely not be worth it for so little a change. 650x38B tires (like the Soma B-line) would make up half of the difference and be better on gravel if you haven't already purchased the tires. :)

Here's my go-to trail calculator: http://www.yojimg.net/bike/web_tools/trailcalc.php

The steering would be ever-so-slightly "lighter" -- I don't think it would be perceptibly less stable. You could restore the original trail by going to a fork with 40mm of rake, but it would definitely not be worth it for so little a change. 650x38B tires (like the Soma B-line) would make up half of the difference and be better on gravel if you haven't already purchased the tires. :)

Here's my go-to trail calculator: http://www.yojimg.net/bike/web_tools/trailcalc.php

Good to know. If I needed, the Surly Steamroller and Soma Classic Curve fork have a 38mm rake. Do you know of a chart or calculator that provides tire diameter data?

People seem to want low trail with 650B Bikes with Porteur racks on front carrying stuff..

I just work out the diameters by hand, since the tire's height above the bead is roughly the same as its width. A couple mm more if there's a thick tread.

So 32x650B is 32+32+584=648mm, 38x650B is 38+38+384=660mm, 700x23C is 23+23+622=668... well isn't that convenient. :)

And yeah, I like low-trail, based on my limited experience with it. You might have to pay a little more attention, but it's less "work" to steer. I would be very happy with 53mm of trail, especially for long, tiring rides. :thumb:

People seem to want low trail with 650B Bikes with Porteur racks on front carrying stuff..
Did you really mean "low trail" or low rake? The two run counter to each other. Again to the OP, have you addressed the brake reach problem?

...have you addressed the brake reach problem?

This is not the issue, but to reply to your question;

73mm reach Tektro R559 Brakes

http://www.rivbike.com/product-p/brsr559allen.htm

The steering would be ever-so-slightly "lighter" -- I don't think it would be perceptibly less stable. You could restore the original trail by going to a fork with 40mm of rake, but it would definitely not be worth it for so little a change. 650x38B tires (like the Soma B-line) would make up half of the difference and be better on gravel if you haven't already purchased the tires. :)

Here's my go-to trail calculator: http://www.yojimg.net/bike/web_tools/trailcalc.php

Nice post. I might add that less trail is quite desirable for bad roads. The front wheel is diverted less by loose, soft, or fluid surfaces.

+1 As far as handling, measure the overall height of the tires mounted on rims. The rim diameter alone is meaningless, it's the overall height of the tire that matters.


This is not the issue, but to reply to your question;

73mm reach Tektro R559 Brakes

http://www.rivbike.com/product-p/brsr559allen.htm

Yikes... I haven't used these but I would not expect much in the way of braking. I would get some canti studs brazed on if you intend to keep the bike this way for a while. Or just run the caliper in the back and get a fork with canti studs for the front.

+1 As far as handling, measure the overall height of the tires mounted on rims. The rim diameter alone is meaningless, it's the overall height of the tire that matters.



Yikes... I haven't used these but I would not expect much in the way of braking. I would get some canti studs brazed on if you intend to keep the bike this way for a while. Or just run the caliper in the back and get a fork with canti studs for the front.

I hope we can keep this on topic. Discussing any changes in stability after the conversion based on changes in geometry is the topic.

I hope we can keep this on topic. Discussing any changes in stability after the conversion based on changes in geometry is the topic.
True, but in so many of these wheel size conversion threads the problem of brake reach is overlooked until it bites the owner. The OP has taken this into account so we can go back to the original question.


I might add that less trail is quite desirable for bad roads. The front wheel is diverted less by loose, soft, or fluid surfaces.
Are you sure you have this right? Low trail gives highly responsive, more "nervous" handling so the bike turns eagerly but it's straight line stability is poor. Greater trail is slower to react and gives greater straight line stability and it's what you want on rough, uneven roads. I believe you are confusing rake and trail.

Are you sure you have this right? Low trail gives highly responsive, more "nervous" handling so the bike turns eagerly but it's straight line stability is poor. Greater trail is slower to react and gives greater straight line stability and it's what you want on rough, uneven roads. I believe you are confusing rake and trail.
I am absolutely positive of this. You don't want stability on these surfaces, you want the ability to lightly steer through or over them with minimal force. Short trail feels like floating over this stuff. Long trail digs in.

This is the principle behind Gary Fisher's Genesis geometry mountain bikes.

As someone who has done several conversions, one thing I can add to the conversation is that any differences in trail resulting from a switch from 700c to 650b wheels are going to be minimal, and even if the change were to be quite large, the difference in ride dynamics is often unpredictable. For example, much of the way a wide, 650B tire affects steering response is dictated by other aspects of the frame geometry and fit. One of these parameters is reach, which affects how much rider weight is placed over the front wheel. Reach also affects steering because of the lever effect of the stem. A longer stem will have more of a lever effect.

To illustrate-- I did one 650B conversion of a road bike made for 27" wheels. My geometric trail dropped from 52mm to 44mm, and I expected the steering to lighten up and for the bike to handle a front load favorably. I found that this was not the case; on the contrary, steering became more sluggish (which I attribute to the 650B tires being fatter and my having a lot of weight over the front wheel-- my back angle is very low on this bike, around 35 degrees when my hands are on the hoods).

In contrast, I converted a road bike from 700c to 650b for my wife, and in the process built it up as an upright townie. As a result, her back angle is much higher and thus places less rider weight over the front wheel than the frame was designed for. I don't know what the final trail figure is, but I know it's higher than my own 650B conversion mentioned above, probably in the 50s. She complained that the steering was too light. This, despite having higher trail than my bike! To remedy the issue, I installed a handlebar basket and encouraged her to keep some weight in it. So she always keeps her purse and accessories in the basket, and that balances out the weight distribution.

Both her bike and mine have the same tires (650x42B Grand Bois Hetres).

I will soon be taking delivery of a purpose-built 650B road bike frame which will replace my 650B conversion, so I will experiment with reconfiguring that conversion into more of an upright city bike to see if that improves the steering response.

So the take-home message is that you can make all the geometric predictions you want, but geometry parameters other than trail are going to factor in to the ride and handling dynamics of switching from narrow 700c wheels to wide 650b wheels. The best thing to do is to try to get ahold of a set of 650B wheels to test-fit before executing a full-blown conversion. A short ride around the block should reveal a lot about how the handling will be affected.

I'm interested in this discussions as well. I have rims/hubs/spokes in 650b size waiting on my bench to be built so I can experiment on my two favorite road bikes. Brake reach is less of an issue with my target conversion bikes because 25c is a tight fit on both. I'm finding tire width to be one of my limitations --with 650bx32 looking like a tight fit. I'm also considering bending a higher rake fork to go even lower on trail --just to experiment. And feel in loose gravel is one of my target experiments. I have felt like the steering on my 57mm-trail (other) roadie (even with 32c tires) wants to plow a trough while I'm trying to wrestle it back into balance. I had thought big tires would solve my gravel issues but it didn't seem to make much difference. I want to see what low trail feels like in gravel. (I'm also building a loaded tourer from a 1988 Trek 850 with 62mm trail and am anxious about its ability to handle gravel)

I don't have answers yet but I'm pursuing similar experiments and will post what I discover.

I'm coming to the conclusion that most bikes are not going to make good 650b conversions. The best conversions are 56cm sized and smaller, already have clearance for 700x28, or larger tires. Older frames built around 27inch wheel might have brake reach problems, the reach might be greater than 70mm once a 650b wheel is installed. 700c bikes with cantilever brakes won't work without repostioning the studs. Larger frames just look odd with the smaller wheels.

This blog covers all the issues very well: http://www.bikeman.com/bikeman-blogs/650blog/1774-650b-what-and-why

The Soma Stanyan conversion looks good to me: http://somafab.blogspot.com/2012/04/650b-soma.html

and see here: http://www.bikeman.com/bikeman-blogs/650blog/1770-bikemans-650b-soma-speedster

The 650b Velo Orange Polyvalent is looking like a worthwhile investment, it solves all the issues.

http://whbikes.wordpress.com/2010/02/22/jonathans-velo-orange-polyvalent-650b-part-i/

http://whbikes.wordpress.com/2010/03/12/jonathans-velo-orange-polyvalent-650b-part-ii/

If brakes are the last issue you are dealing with, once you're happy with geometry and tire clearance, consider Sturmey Archer drum brake hubs. They have different rear drum hubs for either fw or cass.

So the other option for trying out low trail would be to turn the fork around backwards.
Wait. That would be super high trail. Never mind.

If brakes are the last issue you are dealing with, once you're happy with geometry and tire clearance, consider Sturmey Archer drum brake hubs. They have different rear drum hubs for either fw or cass.

have you used these? I had the rear S-A freewheel drum on a cruiser for a long long time. they are meant to be aux brakes for tandems, and don't develop enough braking force to really stop a bike fast. even when used with long pull ('v-brake') levers, it was barely adequate. And this was a real S-A hub, made in England.

The solid axle bent on this after many years of use, and was an unobtanium part due to the implosion of S-A, so I brazed on V-brake bosses to this old cruiser frame, and WOW, having real brakes made it a whole different bike.

Hmm, don't give up yet. What interests me now is that you say the frame will only take 700x28 tires at present, but that it was a 27"-wheeled bike originally. The tire clearance must have been really tight with those!

I would disagree that larger frames look odd with 650B wheels. Generally, the tires and fender together make up all the space that the old wheels did, and the bike looks no sillier than it did with 27"/700C wheels. :thumb:

Did you really mean "low trail" or low rake? The two run counter to each other.

this is true the heavier front load favors low trail achieved with more fork offset.

I have stuff in my Front Panniers on my Bike Friday All the time,
it has a pretty small trail ..
(the difference between the line through the steerer tube axis,
and the perpendicular line thru the front hub center, as laid out on the ground-plane.)

Older MTB Build,
I've been running SA Drum brake hubs for 25 years,
they now are part of my black ice, studded tire shod bike.
because on Ice, the last thing I want is brakes so instantly strong, I break traction.
they modulate smoothly to bring me, reliably, to a stop.
[it has more trail, for straight line stability, I just have a big Brit saddle bag on it..]

FWIW, rest of the set-up.
I have Mustache bars and old Campag road , pre-aero levers, and pretty Beefy cables .

this is true the heavier front load favors low trail achieved with more fork offset.

But of course. That's one of the reasons touring bikes have more fork rake. And for the slacker head angle, too, of course.

have you used these? I had the rear S-A freewheel drum on a cruiser for a long long time. they are meant to be aux brakes for tandems, and don't develop enough braking force to really stop a bike fast. even when used with long pull ('v-brake') levers, it was barely adequate. And this was a real S-A hub, made in England.

The solid axle bent on this after many years of use, and was an unobtanium part due to the implosion of S-A, so I brazed on V-brake bosses to this old cruiser frame, and WOW, having real brakes made it a whole different bike.

I had a bike set up with Sachs drum brakes and it was fine. More recent direct experience with S/A drum front brakes in both 70mm and 90mm versions.

I'd say drums are better than single pivot road calipers, on par with some canti brakes and cheaper dual-pivot road brakes, but not as good as linear pull or certainly most disc brakes. I'd put the performance of the modern S/A hubs ahead of most vintage brakes they might be replacing and probably better than some of the longer reach road calipers necessary for a 650b conversion. (I have zero experience with 70+mm calipers...)