Understanding BB drop on downsizing wheelsets...
 

I have been working on a build like this: https://www.cyclingabout.com/custom-...ing-bike-2019/

I was originally going to do 29 x 2.15" Almotion wheel set. I am thinking I might want to throw on a 27.5" wheel set instead last minute. I am trying to understand BB drop and what that truly does to the setup of the bike in relation to peddle strike issues.

The bike is a Surly Ogre. The author of the article personally rides 27.5" x 2.4" with Schwalbe Super Moto-X MTB Tyre. I was thinking of doing a similar wheel build, but 2.4" is too small for BB drop. The current BB drop of the bike is 68mm. In conversation with the author, he mentioned to me:

I don't totally understand the math and just starting to now. If I understand this correctly, if I go from a 29x2.15" that means the wheel diameter is 730mm, and I want to do a 27.5 x 2.8" at 726mm, then I do 730-726mm = 4 mm.

I am not sure where the author got "2mm" from. How actually do I figure this out exactly?

So does that mean if I want to do a 27.5 x 2.4" instead, I subtract 706 from 730, and come up with 24mm difference. Does that mean that the Bottom Bracket then drops an additional 2.4 centimeters closer to the ground?

That sounds like a lot and I don't have a bike to test it on, so can folks tell me if that is a lot. That is too much correct?

So if the BB is 68mm pre-change, then I take 6.8-2.4= 4.4cn=44mm?

Am I doing this right?

The drop will be half the diameter difference, not the whole amount.

The amount that the bottom bracket height will change by half the difference in the tire diameter. The other half is at the top of the wheel, where it doesn't effect the bottom bracket height.

The real number that will effect the pedal strike potential is the bottom bracket height. Bottom bracket "drop" is a frame building spec and it stays constant regardless of tire/wheel size. The distance from the bottom bracket to the ground is the thing that changes with tire/wheel size.

In the case of the Ogre, you would take half the tire diameter(730/2=365) minus the BB drop of 68 and that gives you a bottom bracket height of 297mm. For the 706mm tire that would be 353 minus the 68mm bottom bracket drop for a bottom bracket height of 285mm. You can then subtract the crank arm length(actual length from the spindle to the end of the arm) to get the distance from the crank arm to the ground.

The BB drop remains the same regardless of wheel diameter. What changes is the BB height/ground clearance. This amount of this change will be the difference in tire radius. If your bike already has a low BB, you may wish to consider shorter crank arms to minimize pedal strike.

Your method measures to the top of the tire.
The axle will move 1/2 as much.

Another thing to look at is crank length.

You might choose from:
155mm
160mm
165mm
170mm
172.5mm
175mm
177.5mm
180mm

:foo:

The most common sizes today are from 170mm to 175mm. But, there are a few outliers. It will depend a bit on your height, leg length, personal preference, etc.

In that case, when consider pedal to ground, or toe overlap, you don't divide by two. So, a 170mm crankset will be 5mm higher off the ground than a 175mm crankset.

Although, one frequently raises the seat for shorter cranks, and lowers the seat for longer cranks (based on the lowest position of the crank arms).

170mm was already chosen for rider height specific needs.

I didn't know or realize that. Thank you for clearing that up.

That is what I thought. All the manufacturers list BB drop, but they never say the "height." And I assume that will change depending on the specific tire you have and that number will be dependent on that correct?

My understanding is the BB drop doesn't change at all. So when folks say, "you have to be careful that your BB drops more, they are not referring to the "BB drop" number, because that can't change, but they are referring to the BB being lowered in general (along with the rest of the bike), and the BB "height" from the floor is changing. Do I understand this correctly?

So technically, what I need to know is how this all effects the Bottom Bracket "height," and without a wheel set, it would be impossible for me to know correct?

That's what I am trying to figure out. A Surly Ogre typically takes a tire size of 29 x 2.25 is a good range per Surly, but the manufacturer range is 29 x 2"-2.5"; and 27.5 x 2-5"-3" max. If I could get away with it, I would like to run 27.5" in the 2.4" range, and run Schwalbe Super Moto-x HS 439 Ebike Bicycle Tire 27.5 X 2.40 if possible, but I assume that lowers BB too much.

The distance from the bottom bracket to the ground is the thing that changes with tire/wheel size.

This is what I was after. I have to soak this in for a bit. Thank you.

I didn't know or realize that. Thank you for clearing that up.

That is what I thought. All the manufacturers list BB drop, but they never say the "height." And I assume that will change depending on the specific tire you have and that number will be dependent on that correct?

My understanding is the BB drop doesn't change at all. So when folks say, "you have to be careful that your BB drops more, they are not referring to the "BB drop" number, because that can't change, but they are referring to the BB being lowered in general (along with the rest of the bike), and the BB "height" from the floor is changing. Do I understand this correctly?

So technically, what I need to know is how this all effects the Bottom Bracket "height," and without a wheel set, it would be impossible for me to know correct?

That's what I am trying to figure out. A Surly Ogre typically takes a tire size of 29 x 2.25 is a good range per Surly, but the manufacturer range is 29 x 2"-2.5"; and 27.5 x 2-5"-3" max. If I could get away with it, I would like to run 27.5" in the 2.4" range, and run Schwalbe Super Moto-x HS 439 Ebike Bicycle Tire 27.5 X 2.40 if possible, but I assume that lowers BB too much.

The distance from the bottom bracket to the ground is the thing that changes with tire/wheel size.

This is what I was after. I have to soak this in for a bit. Thank you.

You can then subtract the crank arm length(actual length from the spindle to the end of the arm) to get the distance from the crank arm to the ground.[/QUOTE]

Example 1: tire diameter (730mm/2) 365mm - 68 mm - 170 mm = 127 mm is bottom of pedal at lowest position to the surface.

Example 2: tie diameter (706mm/2) 353mm - 68 mm - 170 mm = 155 mm is bottom of pedal at lowest position to the surface.

Do I have all the correct?

So then the next question is...what is considered the "safe" zone before you have issues with pedal strike or you are just "too low" in general now?

I have no idea what the BB Height is considered to be a safe zone so to speak.

Another factor(s) for pedal strike - when cornering: Pedal width, construction and Q-factor; ie how far apart the pedals are. Low bottom bracket bikes with wide pedals or pedals that are deep at the outside will strike early and often. I had two fix gears over the years that were old Peugeot frames. I ran sewups or clinchers of similar size that replaced 27" wheels. I struck pedals on a regular basis on both bikes. (All my Leotard platforms had their left dustcaps unscrewed and lost as well as the right dustcaps scavenged off retired pedals.) They hit so early it wasn't scary because I wasn't even into a full bank. I also used to hit speed bumps, sometimes quite hard. I called those bikes "slinkies".

Ben

If you have a steel frame, and steel fork. Then the BB drop will be a measurement from the dropouts to the center of the bottom bracket. And should be fixed for that frame, and often is fixed for the model.

https://surlybikes.com/bikes/ogre

https://surlybikes.com/uploads/bikes/ogre_dimension.gif

If you had suspension (front or rear), then the droputs would move, and the drop should describe a normal range. :foo: At least Trek seems to list "Geometry Position" "High/Low" which seems to take into account the suspension on their full suspension frames.

I went over from a MTB years ago to my first road bike, took it around a corner for the first time while still pedaling, smashed the pedal so hard it bent it, never experienced a "feeling" like that on a bike before, and don't ever want to experience a feeling like that again. That scared the heck out of me. Obviously it was one of those, you do it "once" you learn kind of things, and don't recall ever hitting again once I was conscious of it. But I would think that is not something I want to be conscious of on an Ogre otherwise I am doing something wrong and got the wrong bike. I would also like to learn what is the "correct" thing to do, what is acceptable, how do I know this well enough that I can move forward figuring this stuff out on my own and know what I am talking about kind of thing.

Definitely no suspension. Steel frame and forks. Steel racks. Big wheels. Bike is getting heavy. :eek:

The answer in your example 2 should be 115mm. I assume that 155 was a typo.

In any event, 285mm of bottom bracket height should be plenty of clearance. My current gravel bike has a bottom bracket height of 279mm and I'm running 170 cranks.

This, of course, isn't limited to steel frames but applies to any non-suspended frame and fork no matter what the material of construction.

Soooooo....my question is, because I am still not clear for my own needs...

Can I run 27.5 x 2.4" tires on a Surly Ogre, comfortably, without worrying about pedal strike or any other major concerns that folks worry about in situations like this?

I don't really want to run 27.5 x 2.8-3" all the time, weighted, touring kind of thing either. Or at least I don't think I do with no experience on them. If I could, I would like to run the 27.5" x 2.4" and put SUPER MOTO-X tires on it. Those tires only come in 2.4" or 2.8".

If I don't do that, then I want to run 29 x 2.15-2.35 tire, preferably Schwalbe Marathon Almotion.

If not either of those, then I will run a 2.25-2.35 tire with a light knob on it, meant for more hard packed surface and road if possible.

But the main thing is can I run a 27.5 x 2.4" tire on a Surly Orge without worrying that the Bottom Bracket "Height" (clearance to the ground) hasn't been compromised too much?

Bump...I need an answer for the morning so I can place an order if anyone knows for sure.

I've already told you that 285mm of ground clearance is plenty. That is what the 27.5 x 2.4 tires will give you. For example, the Trek Checkpoint gravel bike only has 273mm of ground clearance in the smaller sizes that have 78mm of bottom bracket drop and 40mm tires.

[MENTION=332446]dsaul[/MENTION]

So based off everything said so far, if you are saying I can run a 27.4 x 2.4" which = 706 mm....

Then why can't I run a 700 x 40c tire on this bike, which sits at 702 mm's according to this chart? ttps://www.bikecalc.com/wheel_size_math

When I speak to Surly, they say "absolutely not" do you want to run a wheel size that small on this bike. They won't even recommend a 700 x 50c which is 722 mm's. so why can I go down that far according to you, but absolutely not to Surly?

Is that just a Manufacturer thing, removing as much liability away, and crossing their "t's" and dotting their "i's" kind of thing? Because when I speak to Surly, they say "no way Jose'" kind of thing.

If I am understanding all this correctly, then theoretically, I can run a 700c tire on there successfully, and just tour with that also...can't I?

I already have a 700c wheelset I can throw on then, and that drastically changes touring options, ideas, possibilities for the future, if this is the right bike for me long term, and on and on. So I have been lead to believe that the things you are saying are not true at all, and I simply don't understand why the conflicting stories.

That's why I would rather ask in the open because that happens to me a lot. I am utterly confused without having practical experience with jumping between wheelsets what is true, correct, precise, and accurate here. I am trying to avoid trial and error as much as possible if other's have already walked these shoes and know for sure so I don't have to.

So what say you?

I have no idea why Surly wants such a high bottom bracket on that bike. Maybe they are concerned about people pedaling through corners. If you're the type that pedals through corners, then maybe you should stick to Surly's recommendation.

All I can tell you is that I am currently riding a bike with a 279mm bottom bracket height in off road conditions. I designed and built that frame with that bottom bracket height. I sometimes ride that bike on singletrack trails with large rocks and log overs, but I'm a skilled rider who knows when to pedal and how to negotiate obstacles. If I were building a bike to use strictly as a mountain bike, I would choose a higher bottom bracket.

I've tried to be as helpful as possible, but I can't make your decision for you. You are the only one that knows your skill level and intended purpose for this bike.

Yea, I don't want pedal strike during cornering if can be avoided.

I put the 700 x 40 tires on it, and there is about 100 mm from bottom of the pedal to the ground, with pedal in lowest position. I don't know if it can be ridden like that because I don't have the rest of the parts on it just yet. Hopefully later today.

I know. I appreciate it. I just like to pick things apart until I understand it is all.

I am pretty sure I am getting 29 x 2.15" schwalbe Almotion tires. Be best for touring on that bike. I will build a plus size 27.5" later for mountain riding. If the 29'er is too much bike and/or tire, I will just move over to a Disk Trucker or a Bridge club later.

I have a funny feeling I will like my bike. Besides, now I have something to do for the next 10-20 years...build more bikes ;)

I still want to know and understand it all anyway. Learning is fun.

My main bike has 265mm bottom bracket height. No pedal strike issues with 165mm cranks. :thumb:

So I put the 700 x 40 wheelset back on my other bike I normally tool around on and measured the bottom of the pedal at it's lowest position to the ground. I don't know why I haven't thought of doing this before so I have some numbers to work with in my head that is tangible. The crank is a 175mm on this bike, which in the Ogre it's a 170mm. The measurement was 125mm (give or take a mm). I can pedal through turns just fine at this height. The bottom bracket "height" was 290mm (if it is measured to center of BB). I don't know what the measurement of that was on the Ogre because I am just figuring this stuff out now.

When I measured the Ogre with the same wheelset, and the 170mm cranks on it, it was 100mm (give or take a mm). I don't know yet because I am still waiting for one last part on the build before I can test it with the 700 wheel set, but I don't think it is safe to ride at this height without pedal strike issues through turns, but I am guessing right now. Should know soon.

My gut says it won't be safe, enjoyable, and not a splinter in my mind in corners. Don't want that feeling at all.

Isn't a Jedi supposed to stay calm and trust the Force? ;)

Easy to check if pedal strike is likely. Put one pedal at 6 o'clock, and lean the bike over 45 degrees to that side. If it doesn't touch the ground, you should be good since most people aren't pedalling when banked over that far. This doesn't account for bumpy ground, but you should have the inside pedal up at that time anyway.

Is it hard for some to develop a habit of not pedaling through corners? Not for me, but I'm only one person.

I'm only discussing freewheel bikes. Clearly, pedal strike is a big problem when riding fixed. You never want that to happen!