Tire Clearance Issue - Is BikeCad Wrong?

[GrantM]

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

You should measure it again.

700x25 tire should be between 672 and max 674 actual measurement, not 685.
All you have to do is take 622 and add the 2x tire width, and you'll get pretty close
to the actual number. Plus, for clearance only the radius really matters.

if there is something off about the BikeCad calculation, it's not the wheel diameter at 676mm.


Grant McLean

[Peter Polack]

I don't think the OP should be beaten up too much. Making mistakes is one way we all learn. Sure, it can be expensive but think of it as tuition for an education in frame design.

I've made similar mistakes and I'm not a framebuilder, just a customer who was able to spec his custom frame.

[edoz]

Next time, choose a builder that will not let you design your own frame, and will not let a bike shop step in between the only two parties that should be involved. (Hint, the customer and the builder)

[morrisond]

You should measure it again.

700x25 tire should be between 672 and max 674 actual measurement, not 685.
All you have to do is take 622 and add the 2x tire width, and you'll get pretty close
to the actual number. Plus, for clearance only the radius really matters.

if there is something off about the BikeCad calculation, it's not the wheel diameter at 676mm.


Grant McLean

I measured 4x times - the tire is a Conti 4000s which are known to be tall.

[e-RICHIE]

The lesson here (for others) is that you shouldn't/don't design a road bicycle rear wheelbase to a tire dimension. And, if you are juggling, always lean to the longer choice. Short rear triangles and modern drive trains don't play well with each other especially when misalignments are factored. And the friction that results from crossed-over chainlines will work against any gains you may be looking for atmo.

[Tristan]

I measured 4x times - the tire is a Conti 4000s which are known to be tall.

Out of curiosity how are you measuring?

I just measured a 25mm GP4000sII on a Rail carbon rim (18mm internal width) and it was ~668mm. I got this measurement by rolling the wheel one complete turn to measure circumference, then dividing by 3.14 to get diameter.

I find it hard to measure from one side of the wheel to the other with a ruler / tape measure because of the triangulation of the hub.

[Andrew R Stewart]

The lesson here (for others) is that you shouldn't/don't design a road bicycle rear wheelbase to a tire dimension. And, if you are juggling, always lean to the longer choice. Short rear triangles and modern drive trains don't play well with each other especially when misalignments are factored. And the friction that results from crossed-over chainlines will work against any gains you may be looking for atmo.

This is what I've been saying for years. My own frames have 43cm CSs and got me through a 68 mile ride yesterday that had over 5000 ft of climbing and I hit 48+ MPH a few times. I never missed the 2.5cm from the CS, was able to use every gear combo (3x9) and appreciated the wide 25mm tires on the uneven surfaces we traveled. Andy.

[steve garro]

IMG_4806.JPG

Doesn't everyone check their work with dummy wheels?
I sure as hell do.
- Garro.

[sam]

There are lots of bikes out there where a shop was the middle man, I.F. serrotta seven etc. etc. etc. With a good shop and a good builder it can be a good outcome. What I can not believe is that a builder built to someones design, other then there own. What if the OP was a jerk(he is not by the way) and named the builder and said look at this bike I got it wont even work with a 25mm tire, what kind of badmouthing would be going on. Hate to say it but sounds like the builder needed to pay bills and took the job, which hey I kinda get. But it is not the best thing for your client, and not the best thing for you. Just dont do it.

I have customers come in and ask to build an Indy fab, and give me some geometry chart from some old bike they had. I just nicely tell them no, they want custom so lets do custom, you want an indy fab lets do an indy fab, and following someone else design is going to get you niether. Everytime they have been happy in the end, and each one is now a lifetime customer of our shop because we looked out for them.

If I get to the point of selling my own frames in the future, to tell you the truth I would probably be more blunt. And ask them to find another builder, if they did not quickly see my point. If you want a bike from Sachs get one from Sachs, dont ask me to try to build his frame- you get the point

[Jonathan]

IMG_4806.JPG

Doesn't everyone check their work with dummy wheels?
I sure as hell do.
- Garro.

Every single one.

[Mark Kelly]

I get 674mm for a Conti GP4000s 25mm tyre on a Velocity* A23 rim. Measured using a wooden plank held in contact with the tyre and parallel to the support surface, in this case a wall because the bike is hung by its front wheel.

This suggests a possible solution: use a wheel with the widest rim you can, you should be able to get the tyre volume you want without rubbing.

*An old Australian one, not the new US made ones.

[e-RICHIE]

This suggests a possible solution: use a wheel with the widest rim you can, you should be able to get the tyre volume you want without rubbing.

I'll suggest the framebuilder considers rear wheelbase by dint of the drive train, and not pay attention to clearance. If you do the former, the latter won't be an issue atmo. PS I'm talkin' road bicycles here.

[Mark Kelly]

I'll suggest the framebuilder considers rear wheelbase by dint of the drive train, and not pay attention to clearance. If you do the former, the latter won't be an issue atmo. PS I'm talkin' road bicycles here.

To be clear, I meant a possible solution to the OP's present problem, not a possible solution to the SNAFU that caused his present problem. The second is beyond my remit.

[Mike Flanigan]

This one hits the nail on the head :)

The lesson here (for others) is that you shouldn't/don't design a road bicycle rear wheelbase to a tire dimension. And, if you are juggling, always lean to the longer choice. Short rear triangles and modern drive trains don't play well with each other especially when misalignments are factored. And the friction that results from crossed-over chainlines will work against any gains you may be looking for atmo.

[ulrich vogel]

You should measure it again.

700x25 tire should be between 672 and max 674 actual measurement, not 685.
All you have to do is take 622 and add the 2x tire width, and you'll get pretty close
to the actual number. Plus, for clearance only the radius really matters.

if there is something off about the BikeCad calculation, it's not the wheel diameter at 676mm.


Grant McLean

I´m sorry, but tire width isn´t the same as tire hight and theory and practice are different!

Ulrich Vogel

[duanedr]

I´m sorry, but tire width isn´t the same as tire hight and theory and practice are different!

Ulrich Vogel

I have this argument frequently. I would add a 'very' between 'are' and 'different'.

By the way, beautiful work you show on your site and flickr! Wow. very impressive!

[GrantM]

I´m sorry, but tire width isn´t the same as tire hight and theory and practice are different!

Ulrich Vogel

Road tires? I've measured dozens of road tires, not different more than by a mm or two.
OP is saying he got 685 and another measured 668 for the same tire.

The issue is that if you order a frame and say you want it to fit "x" size tire, how is that 17mm supposed
to interpreted by either the software or the framebuilder? A quick check of the
BikeCad program suggests they assume 676, which is within 2mm of the radius of 311+25 shortcut
method for context.

If a sized 25 tire measures over 31.5mm in height, there isn't much point of telling the builder
that you want to run 25 tires. The "width vs height" argument is secondary to the fact
the actual width varies just as much, if not more than the measured size. A '25' veloflex
roubaix is only a hair over 23mm, and a michelin 25 prorace is over 27.

As others already said, chainstay length should be considered for more than just the
clearance issue, but in these days of fatter road tires, clearances are a shit show.

-g

[Eric Estlund]

BikeCad is just a tool. Its a cold arbiter of the values one either types in, or leaves in as a default. I don't trust it, I trust in my ability to use it within my skill set. Just like I don't trust a file to do anything (lazy files).

Measure stuff, learn from mistakes, and push forward.

[Rayburn Shaun]

The issue is that if you order a frame and say you want it to fit "x" size tire, how is that 17mm supposed
to interpreted by either the software or the framebuilder?

A customer needs to be specific on the brand and size. Anything other than specified could require alternate frame dimensions. Framebuilder needs to know the real tire dimensions to design to specifics. Or just let the framebuilder do the designing around the tire. Done.

[ulrich vogel]

311+25 shortcut method

Hi Grant,

again, I´m sorry but this theory is wrong, because 311 radius or 622 diameter measures the bead seat diameter not the "total rim diameter".

From this short cut theory a wheel circumference of (622mm+25mm+25mm)x3.1416 = 2111 mm would result. But tire manufacturers specify the circumference with about 2135 mm - which is closer to the reality. Based on 2135 mm circumference, a diameter of 680 mm results.
The total rim diameter of a 622-rim is arround 631 mm. According to your theory only (622 mm + 25 mm + 25 mm - 631 mm) x 0.5 = 20.5 mm tire height would be visible - that is not the reality.


What a tire manufacturer says about tire widths:
Standard tire widths are calculated using fairly wide, standard rims. Yet in practice, narrower rims are used more often, which in turn leads to tires becoming slightly narrower as well.
In order to ensure that tires have sufficient frame clearance, tire manufacturers generally prefer to keep production closer to the lower end of the permitted tolerance (+/- 3 mm).
Carcass casing materials have become more and more sophisticated over time, that reduces the tire widening after the fitting. In order to correct this tire widening, over the past few years slightly wider carcasses have been used, so that the actual widths are now much closer to the standard widths.

And I would add that of course also affects the tire height.

So I repeat myself:
I´m sorry, but tire width isn´t the same as tire hight (and wheel diameter) and theory and practice are very different!
The shortcut theory "take 622 and add the 2x tire width" is a theory - but wrong in reality.

Ulrich