if having to buy a bike on frame measurements alone, whats MOST important?

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martinn
Posts: 348
Joined: 1 Dec 2012, 8:20pm

if having to buy a bike on frame measurements alone, whats MOST important?

Postby martinn » 25 Jul 2020, 9:20pm

Hi All

Have seen that the Spa Mono is now back in stock, and have permission of finance to make a capital purchase.

I live very far away from Spa, certainly not close enough for a quick visit, and the nearest outlet for Spa to me, only has a single Spa in their shop, (Which is too small for me)

I have a few bikes, but being 5'11"(and a bit) tall i generally fall between suggested sizes.
I ride a Orro Terra C which is comfortable in a large (Top tube of 581)
I have a custom steel bike with a top tube of 575 (I think) this is also very comfortable)
And i have a ridge back speed which is a 21", which i always felt was too small (First bike for 20+ years so what did i know)

My current bike which the Mono would replace is an 1985 Dawes Galaxy, which is I believe a 22 1/2" frame.

given I cant test ride the bike I would like, which of the geometric measures would be the best approximation, (I appreciate like most things in life, "its a bit more complicated than that")

Many thanks

Martin

whoof
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Joined: 29 Apr 2014, 2:13pm

Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby whoof » 25 Jul 2020, 9:28pm

If I had to pick one place to measure it would be effective top tube length. I've had bikes ranging from about 42 to 56 cm but the effective top tube on them only ranged by about 0.5 cm

Marcus Aurelius
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby Marcus Aurelius » 25 Jul 2020, 9:32pm

Stack and reach are the ‘king’ parameters. The other geometry measurements don’t have such a big impact, per unit difference. Also take note of whether it’s a ‘compact frame’ ( curved top tube ) as this can affect things somewhat.

rotavator
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby rotavator » 25 Jul 2020, 9:40pm

As Whoof says, measure the ETT on bikes that you are comfortable on. If you are going for flat bars you will probably need a longer ETT than for a drop bar bike.

Also check stand over height for obvious reasons.

Also check front centres if toe overlap would bother you. This was an issue on a Spa Elan that I test rode.

You could also compare stack and reach between bikes that you are comfortable on versus the Mono.

Brucey
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Joined: 4 Jan 2012, 6:25pm

Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby Brucey » 25 Jul 2020, 9:54pm

seat tube angle, ETT, head tube height above BB, in that order. Then other stuff like BB drop, front centres, etc.

cheers
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bazzo
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Joined: 27 Jul 2012, 7:37am

Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby bazzo » 25 Jul 2020, 10:14pm

I like you am 5’11” my legs are perhaps very slightly short for my height. I have several bikes which are all 56 cm or there abouts. With the slopping top tube seat tube length is difficult to determine. My advice is buy the smaller frame you can raise the seat post and put a shorter or longer stem on. There are only three points of contact hands, feet and saddle.

martinn
Posts: 348
Joined: 1 Dec 2012, 8:20pm

Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby martinn » 25 Jul 2020, 10:27pm

Ok, will see if I can work out the stack and reach.
I was going to get the 56cm, but that's what I thought on the terra c, and ended up taking the large.
Seat tube angle, I can find out the mono and the terra c so can compare.
Thanks

slowster
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Joined: 7 Jul 2017, 10:37am

Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby slowster » 25 Jul 2020, 10:43pm

The top tube measurements of your current bikes cannot be compared with that of the Spa unless you also know the seat angles. As a rough rule of thumb plus or minus one degree seat angle would have the effect of moving the saddle approximately 1cm forward or backward respectively. Spa have not listed the geometry of the Audax Mono, but if it is the same as the standard Audax, it has a 72.5 degree seat angle.

You could probably set up either the 56cm or the 58cm frames to give you the position you wanted (assuming that you do not have the stem/handlebars set so low that the difference in head tube lengths would be critical, i.e. so low that you would not be able to get them that low on the longer head tube of the 58cm, which is probably unlikely).

Therefore the main handling difference would be the presumably 10mm shorter stem that you would use on the 58cm, given that its effective top tube length is 9mm more (again assuming the geometry is the same as the standard Audax). NB It also may not be so simple as just adding the ETT and stem lengths, even if you will use the same handlebars: STI hoods are long and likely to extend reach by ~20mm compared with many plain brake levers, and the reach to plain brake levers will be even less if you exploit their ability to be rotated back on the bars (whereas STI hoods are usually set so that they are horizontal).

Other than that I think the differences are likely to be largely aesthetic, i.e. how much seatpost is exposed and the amount of spacers under the stem. Even the latter is not so simple as a 20mm number (the difference between the head tube lengths of the 56cm and 58cm), because a stem with a different angle can reduce or increase the amount of spacers required.

You might find it worthwhile reading the article with accompanying photographs linked to in the OP of this topic viewtopic.php?f=5&t=138446. That poster, Samuel D, originally had a Spa Audax 54cm, and later replaced it with the 58cm detailed in the article.

In your shoes I would probably get the 58cm, because I would want a similar saddle to bars drop as Samuel D's bike, and also because I would have the brake levers rotated back like on Samuel D's bike.

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Mick F
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby Mick F » 26 Jul 2020, 9:04am

I bought my Moulton TSR without any frame size info ................... because they're all the same size!
Adjusted the saddle height and setback, adjusted the stem length and height ......... and Bob's your uncle.
Good job I'm Mr Average when it comes to bikes, clothes, shoes etc etc etc.

Had I been small, or tall, the bike would have been completely unsuitable ........... but I'd have expected that, so would have looked up the info first.

If, like me, you are Mr or Mrs Average, buy an average-sized bike.
Mick F. Cornwall

Bonefishblues
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Location: Near Bicester Oxon

Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby Bonefishblues » 26 Jul 2020, 9:09am

Mick F wrote:I bought my Moulton TSR without any frame size info ................... because they're all the same size!
Adjusted the saddle height and setback, adjusted the stem length and height ......... and Bob's your uncle.
Good job I'm Mr Average when it comes to bikes, clothes, shoes etc etc etc.

Had I been small, or tall, the bike would have been completely unsuitable ........... but I'd have expected that, so would have looked up the info first.

If, like me, you are Mr or Mrs Average, buy an average-sized bike.

What's the average average bike would you say Mick? :P

reohn2
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Joined: 26 Jun 2009, 8:21pm

Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby reohn2 » 26 Jul 2020, 9:10am

Brucey wrote:seat tube angle, ETT, head tube height above BB, in that order. Then other stuff like BB drop, front centres, etc.

cheers

We agree :)
Though I do like relaxed and stable steering geo so that would also be a consideration.
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Cyril Haearn
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby Cyril Haearn » 26 Jul 2020, 9:23am

Average, is that normal?
I should be very wary of ordering an expensive machine without being able to try it
Could you return it if it was not suitable?
Entertainer, intellectual, idealist, PoB, 30120
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PH
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby PH » 26 Jul 2020, 10:05am

Bonefishblues wrote:
Mick F wrote:If, like me, you are Mr or Mrs Average, buy an average-sized bike.

What's the average average bike would you say Mick? :P

Probably a Chopper :wink:

PH
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby PH » 26 Jul 2020, 10:12am

In a fairly recent thread Paul Smith did some Bike Cad or similar drawings to compare bikes. It's not something I know a lot about, though I had a play on it some years ago, but if I were considering a bike unseen I'd use that to make the comparison. The numbers are one thing, but seeing it can IMO give a better idea.

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Mick F
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby Mick F » 26 Jul 2020, 6:16pm

Bonefishblues wrote:What's the average average bike would you say Mick? :P
Off the top of my head .............

23" seat tube, or maybe 24".
Standard top tube length for the seat tube, 73deg parallel or 72deg parallel frame angle.

15 1/2" collar shirt.
Size 9 shoes.
6 and 7/8" hat size.
32" waist and 32" inside leg.

:D :D :D
Mick F. Cornwall