dirty tiger

I need you guys to tell me how stupid this idea is:
At the moment I am riding an XL Surly Ogre set up as a rigid 29er. I really love riding the bike, but sometimes, it feels like it's a bit too big for me and I've been contemplating switching to 27.5"/650b wheels....27.5 front wheels seem fairly common, singlespeed 27.5 rears seem to be a niche item that needs to be custom built. So, I was thinking of trying my hand at building up a strong,durable rear disc 27.5" single speed wheel...and I find myself wondering if I could/should just leave the front wheel a 29er. I remember a few years back when the 69er concept was a big deal, so I know there is precedent for running two different sized wheels...but that fad seems to have vanished, so I am wondering if it wasn't such a good idea.


All options welcomed.

steve-in-kville 

That's been around long before bikes were invented.

(sorry... couldn't resist!)

dirty tiger

Phrasing! ...I walked right in to that one...

SquidPuppet

If the bike is too big, a smaller wheel won't make the frame magically fit you.

Dropping the rear that much is going to make it steer wonky. I suspect the change might be big enough to introduce wheel flop, which sucks.

bwilli88 

Dirt motorcycles have been doing this for years. I think a 79r with a 27.5+ on the rear and a 29 up front would be sweet.

SquidPuppet

But everything about them is designed for that from the start, the frame, forks, and swing arm, so it's all balanced.

bwilli88 

But look at the differences between a 29r(622mm) rims and 27.5(590mm) 32mm, a whole inch and a 1/3. Split that and the 27.5 is axle is only 3/4" lower, can't have that much difference particularly if you use a 27.5+.

SquidPuppet

A 27.5" is a 650b which is what OP is contemplating, which is 584mm. That would be a 38mm decrease creating a 19mm drop.

The problem I see is that unlike changing the front tire/rim only, which makes a more direct drop or rise, changing the rear causes the front geometry to pivot around the front axle, which exaggerates the effect. It's a short wheelbase bike too, which will also exaggerate the effect.

Imagine a well handling 700c bike designed around 38mm tires. Imagine replacing only the rear with a 19mm tire. That much drop will change the head tube angle a good bit. And without increasing the fork offset the trail number grows. I'd wager that you'd feel that a lot. And that it wouldn't feel good.

Do an experiment. Measure the actual distance from the ground to the outermost edge of your rear rim. Then deflate the tire enough to drop the rear end 19mms. While you are doing it, keep an eye on how much the top tube and head tube angle changes. It aint gonna be insignificant.

A 19mm tire change at both ends, while keeping things "even", will still be very noticeable. As the tire size grows, so does the trail number. A 19mm change at only one end is going to throw the originally designed balance out of whack. A 19mm change at the rear will be profound IMO.

Darth Lefty 

It's been tried, here's a recent example New innovative suspension from Tantrum Cycles. Any thoughts... - Page 4- Mtbr.com

SquidPuppet

That's a great example of my point. Those guys are altering their forks and/or headsets and/or their dropouts to maintain the geometry.

Big wheels up front and small wheels in the rear provide many benefits for off road bikes and motorcycles. Bigger wheels can just go over bigger gnarlier stuff, its that simple. Smaller following wheels stay planted better and when their is an engine involved they hook up and drive better. But geometry is what it is, and it's REALLY easy to over tweak stuff with even small changes.

bwilli88 

I am going to call BS on much of this, I lowered the rear of 5 different bikes everything from a hybrid, high trail roadie, low trail tourer and a nice rigid 29r mt bike. I did it by letting air out of the tires so they were 1 inch lower than before. It changed the head tube angles by less than a degree in most cases. This will not be noticeable to most BF members let alone 99.7 % of the rest of the bike riders in the world. My Centurion Pro Tour changed from a 73 degree HTA to 74. I have also changed the rear tire from a nice plump 37c to a 22c and that didn't make a difference but made the ride very harsh with an almost 15mm drop in size. Squid Puppet I am going to say that your argument is bunk and that it is not going to be exaggerated nor very noticeable. In regard to the OP thinking that his bike will be smaller, maybe by a few centimeters but more likely millimeters.

SquidPuppet

I'm sorry, but that is quite literally impossible to do by lowering the rear end. Either your tools or your techniques are faulty.

You can call bunk and BS all you want. I've made the changes both in CAD and on bikes, and know the facts. One degree is a LARGE change on a 700c wheeled bike. When you changed the head tube angle, how much did the trail change? Use a frame builder's CAD program and you'll see what I am talking about.

How do you know how sensitive 99.7% of cyclists are to geometry changes? You are making a baseless and enormous assumption that everyone else will have the same level of sensitivity as you. The odds of you being correct are very low. When I changed from 45mm to 35mm front and rear on a bike the difference was night and day. A huge impact on steering. And that's a balanced 10mm change. Would it be fair for me to assume that everyone else would perceive the change exactly the same way that I perceived it?

jack pot

^^^^^^^ +1 from a a long time moto/bipsychler

bwilli88 

Oops, Sorry I flopped the numbers.

Changing the HTA 1 degree changes the trail by about 6mms. Still a negligible number. And I also stand by the fact that 99.5% of the people in this world have no clue about HTA and trail and couldn't care less. Remember that the world has 7+ billion people and how many of them ride a bike and have no idea that it has a 700b (635ERTO) rim on the front with a 26 x 1 3/8 or 650b on the rear or the opposite. I live in Cambodia and see things like this all the time.
Now if we are going to talk about Bikes nerds then they might know the difference but I doubt one degree HTA difference would be that noticeable nor will 5mm of traill.

SquidPuppet

It's not negligible. Think about this for a minute. Most modern road/urban bikes are sold with head tube angles ranging from 72.5 to 74.5 degrees. First note that is a very narrow range. And numerous .5 variations in between. Ask yourself, why are the increments .5 degree? Answer, because even .5 degree makes a noticeable difference. It matters. When you get into cruisers and hybrids you'll see 71 degrees. The difference in range is often described as quick and nimble at one end and super slow and stable, even sluggish at the other end.


So now it's a FACT eh? Regardless, that entirely misses the point. I've never claimed that the OP or anyone else knows about bike geometry (the numbers) or cares. I said they would feel the difference if a change was made. If you gave any frequent bike rider a stock bike and told them to ride it for a couple days, and then changed the head tube angle and rake, did not tell them what changes you had made, and then asked them if they noticed anything different, I'd bet that most of them would say they did notice a change. They might have trouble describing the change, but they would notice it.

What does that have to do with our discussion? Our discussion is about feeling or not feeling a geometry change.

What does that have to do with our discussion? Our discussion is about feeling or not feeling a geometry change.

You are simply wrong. No disrespect intended. Have you ever ridden a bike that had a slightly bent fork from a crash? Sometimes you can't even see the bend because it's so small, yet the bike feels really different.

A good custom bike frame builder is concerned with every aspect of the frame's design and material. He will question the heck out of his client to find out what they will do with the bike and how they want it to ride and handle. The builder will chose a head tube angle and fork rake down to .5 degrees and mere mms to achieve a recipe that he believes his client will enjoy. If he gets it wrong, by only .5 degree or a few mms, his client may be displeased with the handling and steering. Because of how it feels.

You have a long trail road bike and short trail tourer. The former is designed to be ridden unladen and the latter is designed to be ridden loaded with weight on the front. If one degree and a few mms of rake and trail were not a big deal, those two bikes would have the same geometry.