[singletrackmack]

Hi all. Had a quick question regarding how increasing the BB drop will effect tire clearance if the chain-stay length stays the same. I figured this was the best place to ask.

My old frame fit a specific tire just fine. On the new frame, the same tire hits the seat tube right in the middle. Both frames have the same chain-stay length, but the new version of this frame has -5mm more BB drop.

I am pretty sure the reason why the new frame will not fit this tire is because the increase in bb drop brings the axle in closer and reduces the rear center. However, when I brought this to the attention of the manufacturer, I was told that bb drop has no relations to tire clearance.

I was pretty sure the change in BB drop between these two model frames is the reason this tire fits the old frame model and not the new one, but since the manufacture is telling me otherwise, I am not so sure now.

Any thoughts as to if/how the bb drop effects tire clearance?

Thanks!

 

[AdamR83]

www.bikecad.ca is a good place to play with this.

There's no simple yes or no answer here IMO, but if you raise the BB by 200mm it will give you loads of tyre clearance, drop it by 200 and you'll have none.

In the real world its a little more complicated than just BB drop - seat tube angle, seat tube diameter and offset at the BB will affect things too.

 

[singletrackmack]

Thanks! The seat tube diameter and bb offset are the same. The seat tube angle is .5 degrees slacker on the older model than the new, so if anything, I would think that would give more clearance for the newer model, but I think the additional bb drop has a bigger effect on tire clearance in this case.

 

[AdamR83]

Crazy stuff, I'd have thought that 0.5 seat angle would make more difference than 5mm BB height. BikeCAD definitely the place to go to play with this then.

 

[natzoo]

The best way to solve these sorts of problems is with 2D CAD (or bikecad)

This is with a 35mm diameter seat tube and 430mm chainstay:

BB drop	STA	Max Tire Diameter
55mm	74	754.5mm
60mm	74	750mm
60mm	74.5	753.5mm

Your results will vary based on your actual STA, chainstay length, etc...

But to answer your question. Yes, more BB drop = less tire clearance.

However, there is probably something else going on. Maybe different rim width or some dimension out of spec/measured differently between the two frames

 

[#/er]

Published geometry numbers can vary also - for example brands can state ST angles with fork sagged or unsagged. It’s really hard to compare two bikes without a second person measuring with you on the bike using accurate measuring tools

 

[singletrackmack]

Thanks all! I have both bikes in my shed. I am switching the same wheel/tire from one to the other. I bought the newest version of the bike and planned to sell the older one, but then discovered I can’t fit the specific tire I want on the new one, even though it is claimed to be able to this tire by the manufacturer.

With the chainstays set at 450mm on my old frame, there was only 5-6mm of clearance. Seeing how the BB drop reduced clearance by about 5mm in natzoos post above, I am thinking the manufacture not accounting for the reduced rear center that increasing the bb drop causes is why this tire doesn’t fit the new frame in the 450mm chain stay setting.

I will try to get the 2D cad going on my PC when I have some time as I could not get it to work on my phone.