Trying not to waste more money on bike fits

Discuss light weight issues concerning road bikes & parts.
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robeambro
Posts: 286
Joined: Sat Jul 07, 2018 6:21 pm

by robeambro

Right, so.. A few months back I got a bike fit, just so that when I buy my new road bike I will have my size and position dialled in.. Or so I thought. As of today, the shop has closed down. And, I'm not 100% sure of the competence of the fitter anyway, otherwise I wouldn't be here.

Anyway, by using one of those evil machines by Shimano (basically they throw a tube up between your legs) they measured my inseam at 88cm. I was told the machine tends to overestimate the actual inseam by about 1.5cm. In fact, when I measured it myself, I found it to be around 85. So I'd say it would be somewhere around 86cm.

Anyway, I was put on a "bikefitting bike" or whatever they're called. The fitter adjusted the contact points, yada yada yada, used a laser thingy to horizontally align my saddle (arguably, a techy version of the KOPS) and finally churned out a report with my ideal bike measurements (screenshot attached).

My main concern is with saddle fore-aft. My "ideal bike" has a crazy steep seat tube angle, and seems to suggest that the horizontal distance from the centre of the saddle to the bottom bracket must be only 203mm. It seems like a very small number to me. I don't think I have a short femur, but I've never really measured it.

I had a quick look and with the great majority of bikes around my size, seat tube angles would be around 73.5*, which, at least according to my calculations, with a saddle height of 76.8cm and a standard 25mm offset seatpost, gives me more than 240mm horizontal distance from the bb to the centre of the saddle, and I'm not sure how much room for adjustment I'd have to reach 203mm. Some bikes have the option for zero offset seatposts, but as I like aero bikes, not all of them do.

Sooo, what should I do? I just want to make sure that if I splurge 6k EUR on a new bike, I won't regret it. And possibly, not waste some more hundreds on a new fit only to check this small thing.
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by Weenie


RocketRacing
Posts: 387
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

Man i oversimplify things. I must be a fitters dream... or nightmare.

For me, make sure standover is good. Make sure you can get your knee over the spindle when the crank is forward and horizontal (set saddle position for this). Set the seatpost height based on your shoes/cleats/pedals. Straight leg when heel touches peddle is close enough for now. Fine tune stem length if required, but down the road.

My goal is good fit without maxing any one element of the bike (like say having to slam the saddle for or aft).

I fall between small and medium. I opted for small as time passed.

If you trust the numbers, just make sure the bike you buy is in the ballpark. Most riders can fit a few sizes, imho the goal is to stick to the size that requires the least adjustment to meet your fit.

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C36
Posts: 464
Joined: Fri Mar 03, 2017 3:24 am

by C36

Did the fitter tried different position while you were pedalling or just one?
What is clear is that only a custom frame will have this geometry (and the builder will either confirm or not that geometry). I don’t recall any std frame so short for this height (way shorter than longer) and yes the seat angle is very very vertical, 1.5deg above what we could expect for almost a 57cm frame.

2 options
- the guy had no clue
- your body will only accommodate a custom frame.



Envoyé de mon iPhone en utilisant Tapatalk

RocketRacing
Posts: 387
Joined: Thu May 10, 2018 2:43 am

by RocketRacing

Again to oversimplify. Sit on some bikes, ride the bikes. If you are willing to pay 6k euros, a bike shop will accomidate you, and likely give you free fit consultation, let you try and compare options. If not... find another shop. If they can’t properly fit you to find you a new bike, they are useless to you, as you seem to be very fit focused... and fit related support with that shop would be crap. You have every right to want ideal fit. Bring your fit diagram for their opinion.

That being said, i am between a medium and small, but can ride my buddies large because he likes a lower seatpost... yet he has longer legs. That is his preference. Sometimes it is more than the numbers. Feel is key. To a point, Short bikes can be made long, long bikes made shorter. Get the bike that requires the least adjustments to make you comfortable, and you are probably not far off.

Also, geometry is a big part of feel. The more geometries you ride, the more you will feel the small differences, and find your “fit”.

RussellS
Posts: 837
Joined: Wed Feb 03, 2010 1:31 am

by RussellS

I'm going under the assumption that you already ride a bike. Does it fit you? Do you ride it? If so, then you should have a very good idea of what bike size fits you. All with minimal adjustments. Stem maybe 1cm shorter or longer. Saddle slid forward or back 1cm. Bar height 1-2cm higher or lower. All minimal adjustments to get the bike to fit you perfectly. Buy that size bike. If going to a shop to buy a bike, set several up and ride them for an extended period. 90+% of everyone fits a standard sized bike. There really aren't that many abnormal sized people that standard size bikes don't fit.

TobinHatesYou
Posts: 2621
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

Good news, your math is wrong.

Basically we know your ideal saddle position has a vertical of 740.7mm. So when you use 740.7mm and an opposite angle of 73.5deg instead of 74.7deg, that means the setback is 219.4mm instead of 203mm. The hypotenuse (straight distance from BB to top of saddle) is then 772.5mm instead of 768mm. You probably have enough rail left to jam your saddle forward 16.4mm. If not, then make sure to get a frame/seatpost combo that works.

Your setback is pretty average in absolute terms.

robeambro
Posts: 286
Joined: Sat Jul 07, 2018 6:21 pm

by robeambro

Thanks all for the answers. To answer Russel's question, yes I do have a bike, and no it does not fit me properly. This is why I want to try and accommodate the fitter's numbers, at least in the ballpark.

TobinHatesYou wrote:
Sun Nov 18, 2018 7:22 am
Good news, your math is wrong.

Basically we know your ideal saddle position has a vertical of 740.7mm. So when you use 740.7mm and an opposite angle of 73.5deg instead of 74.7deg, that means the setback is 219.4mm instead of 203mm. The hypotenuse (straight distance from BB to top of saddle) is then 772.5mm instead of 768mm. You probably have enough rail left to jam your saddle forward 16.4mm. If not, then make sure to get a frame/seatpost combo that works.

Your setback is pretty average in absolute terms.
Oh, I am happy to be wrong if that's the case! But I'm not sure.. Hmm..

So, what I've done seems similar to your calcs, but results are kinda different.

The "hypotenuse" (vertical BB-Top of saddle) should be equal to: 768mm * sin (73.5), which seems to result in approx. 737mm rather than your 772.5mm estimate.. And it seems to make more sense, in your estimate the vertical distance is bigger than the distance measured across the seat tube.. But I guess the latter should be larger (ie the hypotenuse).

The setback should be then calculated as: sin (16.5) * 737mm, where 16.5 is just (90-73.5). = 218mm. This is however without any seatpost setback, and I thought I would add 25mm on top. Which gives a grand total of 243mm or so.

Please prove me wrong, I have never wanted to be wrong so badly, but I can't see the error in my way. :mrgreen:

TobinHatesYou
Posts: 2621
Joined: Mon Jul 24, 2017 12:02 pm

by TobinHatesYou

You are confusing yourself. 768mm IS the hypotenuse of a right triangle with side a = 203mm, side b = 740.7mm. (and angles of 90, 74.7 and 15.3deg)

768mm is only an appropriate saddle “height” for a frame with a 74.7deg seat tube. Your actual saddle height as a vertical measurement from the BB is 740.7mm. This doesn’t change. Since the angle of the seat tube changes, so does the hypotenuse or diagonal seat height before you account for the new offset.

I suppose if you have to add a 25mm offset post it becomes 41.4mm...but due to how most saddles work, the use of 25mm setback post looks to be implied.

(Not to scale)
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real-basic-geometry.jpg
Last edited by TobinHatesYou on Sun Nov 18, 2018 1:52 pm, edited 6 times in total.

ernestkus
Posts: 13
Joined: Sat Nov 16, 2013 2:46 am

by ernestkus

Well, a bikefitting is just as good as the bikefitter..
Probably help to understand if you search 'steve hogg' on youtube

jemima
Posts: 123
Joined: Fri Apr 22, 2011 12:36 am
Location: Perth

by jemima

Alan Mito size 6 in Standard Geometry will be close enough.

Your numbers aren't that far off my own geo requirements.
Curve Grovel ti.

DJT21
Posts: 264
Joined: Sun Mar 27, 2011 7:35 pm

by DJT21

You need to work out your saddle height first; if it's too high then you'll struggle to get all the other parts in the correct place.

If you've measured your inseam at 850mm, then a 768mm saddle height is likely too high.

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tymon_tm
Posts: 2687
Joined: Tue Sep 05, 2006 4:35 pm

by tymon_tm

bike fitting is worse than curing cancer with "alternative medicine". first of all, have you ridden a road bike before? if yes - go from there, play with what bothers you, leave it be if it works. if you haven't, borrow whatever bike for a week or two and see how you feel about it.

why bike fitting won't work? you said it yourself - fitter gave you one value, told you it's not correct (!!!), then your own measurement resulted in something completely different. how can you make anything out of it? :lol:

your measurements seem pretty normal, looks like a standard size will work, so I wouldn't bother with wondering whether you'll need some funky set up - with 99% certainty you will not.
kkibbler wrote: WW remembers.

zefs
Posts: 211
Joined: Sat Aug 05, 2017 8:40 pm

by zefs

Even the Retul fits are a starting point. You are supposed to do some rides (eg. long distance endurance, hard efforts on another ride) and give feedback to the fitter/tweak from there, so no point to fixating on just the numbers they give you. If you are willing to spend that much on a bike and want the best fit possible you could find a reputable fitter that understands how everything works and not someone that just knows how to take measurements. If you are intending to race and need to optimize the fit for power output they also offer this service but basically a fit will depend on your goals as well. You can have 2 people with the same measurements/flexibility and fitness but their fit will differ if one's goal is racing up mountains and the other's long endurance rides.

As a note KOPS does not put you in an ideal position, it was initially used as a starting point to see if a bike frame "fits" someone.
You could achieve KOPS with a wrong cleat setup too, that doesn't make the position correct.

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tymon_tm
Posts: 2687
Joined: Tue Sep 05, 2006 4:35 pm

by tymon_tm

you might wanna check this topic out

https://weightweenies.starbike.com/foru ... 4&t=148120
kkibbler wrote: WW remembers.

GothicCastle
Posts: 185
Joined: Mon Jul 04, 2016 1:52 am

by GothicCastle

robeambro wrote:
Sat Nov 17, 2018 11:36 pm
Right, so.. A few months back I got a bike fit, just so that when I buy my new road bike I will have my size and position dialled in.
That might be your first mistake: assuming a bike fit is something you have done once, get it right the first time and move on.

Assuming your fitter was reasonably good (which is an assumption, anyone can call themselves a fitter), they’d want you up to change things gradually, and take time to acclimate to the changes. A first fit might require a few visits over weeks or months. Some fitters are good which physiology, some are good at looking for a neutral position, etc.

That being said, being concerned with the seat tube angle seems a little strange, unless you are doing triathlons or have very specific weight distribution requirements. Go for a neutral body position that you can comfortably hold. Some fit systems measure your current bike and use the position of the saddle on the post to express an effective seat tube angle of your current setup. This might be what you see there (meaning your saddle was jammed a bit forward).

I don’t know what your level of experience is with riding, but I’d be more concerned with figuring out stack/reach values that work for you, then using those numbers to find frames that are reasonably close. In addition to that, you’ll want to figure out neutral coordinates over the bottom bracket and a good handlebar width.

If you have a shop nearby that rents nice bikes, consider renting a number of different frames for a weekend. Spend tome on a long and low frame, an endurance frame and an all-round one. Figure out what works and compare that to the coordinates you get from your fit.

by Weenie


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