Tire Clearance Issue - Is BikeCad Wrong?

[morrisond]

I just had a new ti frame built and I can't get anything larger than a 23mm tire in the back and I can't really figure out why.

In all fairness to the builder apparently they told that to my LBS - but they never passed that on to me (that I recall).

I did give the builder a BikeCad file to build from so I can't really blame them - but where did I go wrong?

The Rear CS is 405mm with a 75mm BB Drop 73.5 ST angle and 34.9 mm Seat Tube.

According to BikeCad this should have given me a 5mm Tire to ST gap with a standard 25mm tire - Which should have been plenty.

With a 25mm Conti 4000S (not series 2) on a Fulcrum Zero - which is a narrow rim the Tire Rubs.

On my old Frame (A Storck Fenomalist) with 399mm CS and about 70mm BB Drop, same ST angle and DIameter there was 2-3 mm Clearance, with the same Tire/Wheel Combo.

The lower BB height would take away 2-3mm but that should still have left me with 5-6mm gap (because of the 6mm longer CS) and possibly allow a 28mm tire.

What am I missing?

[Todd Mosley]

What am I missing?

Either your name as your screen name or listed in your signature

[Too Tall]

In my mind there is a severe disconnect going on here. You designed the bike and XYZ builder was expected to build to that spec? If that is the case you should know the answer to your question and if not ask the builder directly 'cause we don't know without hearing direct from the source. There are many small details that are not part of the basic design you've provided such as CS design blah blah blah.

Popping corn here.

PS Todd is right on. If you are posting in the frame forum we require real names or your contact will go missing. TIA, Josh

[Jonathan]

Please add your name to the sig line. It's a little rule in this section.

Bike cad can't be wrong, it just uses the inputs you add or it holds as default(i guess it could be a problem in the way it calculates). That's a really short Chainstay for that much drop. Add in the seat tube size, error in mitering or whatever and some variation in tires and there is your problem and we dont know if youre talking about actual length or the virtual number. I won't be the only one to say this, but bike cad is not a substitute for experience wrt to where stuff goes and how close to cut it.

[Eric Estlund]

The Storck Fenomalist has a 399 horizontal CS. That's a 405 actual length. Adding drop and keeping the actual stay length the same will move the tire forward.

[e-RICHIE]

What am I missing?

1) You have to name the frame brand so the conversation is balanced.
2) You have to have a first and last name in all FF posts as per the user agreement.

I'll delete this thread by 11AM today unless there's a follow up comment.

[Eric Estlund]

1) You have to name the frame brand so the conversation is balanced.

I'm not sure they need to be named- I think that a phone call to the builder to sort out any concerns might be a better first step. We don't know the full story, naming them doesn't necessarily add balance at this point.

[e-RICHIE]

I'm not sure they need to be named- I think that a phone call to the builder to sort out any concerns might be a better first step. We don't know the full story, naming them doesn't necessarily add balance at this point.

It borders on conjecture.
So I'll rephrase myself: I would like the OP to be transparent so readers can determine if possibly we are getting a complete story.

[Jonathan]

It borders on conjecture.
So I'll rephrase myself: I would like the OP to be transparent so readers can determine if possibly we are getting a complete story.

I kind of think when the OP said he sent a bike cad file to the builder it removed them from being held accountable for design flaws. If they built what the op asked for, who built the frame isn't even relavant. I'm not sure I understood the OP to think otherwise. That's my read anyway. It's why 98% of us don't build frames we don't design ourselves.

[morrisond]

Hi - I think Eric just nailed it on the head - the Storck has 399 mm Horizontal CS - So I thought I was adding 6mm while in reality it could have less due to the lower BB Drop.

That totally makes sense on why it won't fit but doesn't explain why BikeCad tells me there is 5mm Clearance?

This is not a builder issue.

Thanks

[e-RICHIE]

Thanks -

Hi - I think Eric just nailed it on the head - the Storck has 399 mm Horizontal CS - So I thought I was adding 6mm while in reality it could have less due to the lower BB Drop.

That totally makes sense on why it won't fit but doesn't explain why BikeCad tells me there is 5mm Clearance?

This is not a builder issue.

Thanks

[e-RICHIE]

It's why 98% of us don't build frames we don't design ourselves.

Anything less than 100% isn't good enough atmo.

[Jonathan]

Hi - I think Eric just nailed it on the head - the Storck has 399 mm Horizontal CS - So I thought I was adding 6mm while in reality it could have less due to the lower BB Drop.

That totally makes sense on why it won't fit but doesn't explain why BikeCad tells me there is 5mm Clearance?





Thanks

It's because you are mixing up measurements between bikes. Actually CS length and virtual length. Bike cad, my version anyway, measures the distance between the center of the bb and axle in a line parallel to the Chainstay. The stork is sounds like measures rear center which is the line parallel to the ground.

[Dave Anderson]

.......doesn't explain why BikeCad tells me there is 5mm Clearance?

Have you checked your wheel/tire's actual diameter, verses what Bike CAD prefills for a given wheel/tire size?

[Eric Estlund]

With clearances this tight, it's best to discuss with the manufacturer. BCad uses the wheel OD you set (either by typing in the number or selecting from the drop down). How this relates to the actual tire on the actual wheel is a free for all in the world of inconsistent tire measurements. Small variation in CS length, drop, seat tube/ bb offset, drop out orientation, st angle, tire size etc could eat 5mm quickly. A quick phone chat or email direct would probably clear it all up.

[morrisond]

I will have to measure the actual tire diameter.

The builder did an absolutely beautiful job - great work. Total consumer error Storck does use a Horizontal CS measurement.

I can probably squeeze a 24mm in.

It gives me an excuse to build a long distance bike that will take 28mm tires - I do want to try new Campy 2015 Mech.

[Eric Estlund]

It gives me an excuse to commissiona long distance bike that will take 28mm tires

I'm not trying to poke you here, but I'd suggest asking the builder- either this one or another, to do the full kit and caboodle from design to fabrication. Let them know your chosen tire size, and ask them to do the best they can for you.

[morrisond]

I'm not trying to poke you here, but I'd suggest asking the builder- either this one or another, to do the full kit and caboodle from design to fabrication. Let them know your chosen tire size, and ask them to do the best they can for you.

Absolutely right - the communication broke down when I let my LBS be the middle man and he wasn't telling me stuff the builder told him and he was'nt telling the builder what I wanted.

I'll talk straight to them and they will probably build this next one. I really like there work.

Lesson learned - albeit a very expensive one - at least it is rideable.

[Eric Estlund]

Thanks, Dane- sounds like a plan.

Thanks for adding/ updating your signature line, too.

[morrisond]

And after measuring said tire and wheel it comes at as 685mm in Diameter....Not the 676mm BikeCad assumes for a 700c Wheel with 25mm Tires - Plug that into BikeCad and 0mm Gap - Live and Learn