Tandem Tom

As a newbie here I have a question. I think I know the answer but bare with me!
Rear dropouts are available in different angles am I correct in assuming
the angle is determined based on a line drawn between front and rear dropouts that is parallel to the ground?
Thanks!

unterhausen

No, it's the angle between the rear stays.

Tandem Tom

Ah!
Thanks

epnnf

ummm, I found this

unterhausen

That's not the angle in question. The angle that dropout manufacturers spec is not actually used in Bikecad, it's all linear dimensions. The reason that the ground isn't used as a reference is shown by your link, however. The builder can slope the dropout any way they want relative to the chainstay. That's why you can change that angle if you want.

This video shows the dropout design changing right at the beginning https://www.bikecad.ca/dropouts_from_static_drawings

guy153

That's interesting actually (although not the angle people are talking about). Shimano have a spec for the angle of the derailleur hanger-- it's meant to rake back a bit by a certain amount and there's a range they give for it. But I don't think it really matters. So long as it's basically pointing downwards.

unterhausen

one of the commonly available track dropouts has a weird angle between the stays. I have seen a lot of bikes that look pretty awkward because of that. I suppose you want the axle slot to be horizontal, but I think I might fudge a little to get the interface with the stay to look better. It's interesting bikecad lets you play with that. You could also bend the stay.

I'm not sure how changing the angle with linear dimensions really works. It would be nice to know the angle though, since that's what dropout manufacturers tell you.

duanedr 

If the hanger is not pointed down and rearward, it's possible to have the hanger block you from inserting a wheel with short stays. Shimano and SRAM want it pointed down and back such that the derailleur mounting bolt center is 6-10mm behind the axle center. At the risk of speaking for someone else, I think I have seen Sachs curve the end of the chainstays to match the dropout socket angles (what the OP was talking about) and achieve the correct hanger angle (which is what we're now discussing). There are few thngs worse than having the seatstay enter the dropout at a weird angle in my mind.

I have used socketed track dropouts that didn't have an angle between the slot and the chainstay socket. This results in either having to bend the chainstay to achieve the horizontal slot while aligning to the socket OR ending up with non-horizontal slot. If the BB is high enough the slope of the slot is minimal but it's a compromise nonetheless. Bending the chainstays is the right approach - I just didn't think of it at the time. Angling the cut on the chainstay doesn't allow the plug to insert into the tube well so that's not a great option either.

79pmooney

The fix gear of my logo has custom dropouts at 11 degrees; a carefully chosen angle t\hat allows me to slide the hub anywhere inside the 2" slot, keep the brake pads on the relatively deep Velocity Areo rim and minimize frame height change. I use cogs from 12 to 24 teeth, sometimes both on the same ride. The 12 tooth slides the hub all the way back to the highest point in the slot and hence drops the bottom bracket the most. That cog is also used when I pedal around high speed mountain descent corners. I want all the clearance I can get!

And on the ease of wheel removal - the slot runs so far forward that a 25c tire nearly hits the seat tube using the biggest cog (to keep the chainstays from getting too long with the 12 tooth). Slot opens down at the front, not forward so removing the wheel is always easy. I dreamed this up, then learned after the bike was built that it was done in the 1930s (I don't know by who) and I've seen a modern day Shaw with them. A real joy for removing fix gear wheels, something I celebrate every time I climb and descend really big hills. (And I routinely leave the house in a bigger cog, warm up, then flip the wheel to do the bulk of the ride.)

Road Fan

Tandem Tom , I'm not sure what your designer's convention is, but it seems to be to achieve practical goals in how the frame will be used. Another example is shown by 79pmooney ! But there can also be practical problems, proving that design matters. For example here's an issue with my 650b.

Given the angle of the dropout slot to the horizontal (or vertical) is important, why would a builder use a vertical-style dropout and not install it in vertical orientation, or at least in consideration of wheel extraction? My 650b has them sloping with the opening toward the BB. The problem is that as the wheel with inflated tire is slid down for removal (the attachment is QR), the wheel has to move forward tending to close the gap between the tire and the chainstay bridge, and interference between the inflated tire and the bottom of the fender at the chainstay prevents the wheel from releasing from the frame. Some space is lost to the fender hang-down and the chainstay-fender attachment bolt. That space could have been conserved by making the D/O truly vertical at its front edge (and possibly lengthening the chainstays a few more millimeters), since the error is just a few millimeters. It's improved by minimizing the fender hang-down, but not enough that I can R/R the wheel without reducing air pressure. It's slightly better with 38 mm tires v. 42, but the stays and bridges could accommodate tires possibly up to 48 mm, though I'm not sure what the brake limitations are.

I can usually see where such a choice has a positive trade-off, but I just don't see it here. To be fair to the builder, this bike was designed in the 2011 time frame, before "gravelism" was pushing ever wider tires on an otherwise standard road/rando configuration. And the original owner (the custom customer) was an urban rider back then, not looking for a current gravel bike.

unterhausen

Vertical dropouts almost always have the slot angled with the bottom closer to the bb than the top. The shimano dropout standard are drawn that way, but the angle isn't called out. I'm too lazy to trig it out, but the hole for the derailleur vs. the place where the axle sits is close enough that it probably requires the axle slot to be angled this way.

duanedr 

It's this. The interface with the wheel, derailleur, and frame has to be in a certain place and orientation. The builder decides how long to cut the tubes. The builder of your bike cut them a bit too short or you're using a bigger tire than anticipated or you've added fenders that weren't planned. Bridges are very fiddly to get just right and perhaps as they were adjusting it into place they made a choice that the fit was perfect even if it was a few mm further from the BB than they designed. They decided to braze it in rather than risk adjusting it more and then having it be too short. There are a million little decisions like that. Could be a combination of all of the above - each contributing 1mm to a 4mm problem.

I think a lot of times, we try to design and build right up against the edge to achieve a particular ride or other design goal and then we risk going over that line by a couple mm. The consequences are something the rider has to deal with for a lifetime. It's a good lesson. Those extra few mm in chainstay length wouldn't matter but not having them causes you frustration for the life of the bike.

Andrew R Stewart 

+1 here. A very good explanation of the decision flow chart we go through when making a frame. Beginning builders should read this a few times, as should any custom build customer too. Andy

unterhausen

I used to always go for shorter chainstays. I have one bike I really like that probably would still be my main road bike if I had just made the stays 10mm longer. Which is nothing. It does work with the tires/fenders I designed it for though. I think the whole thing with short stays and tight clearances is just silly. We have been doing this for no good reason since the '70s.