I’ve just been revisiting slowman’s formulas on handlebar drop and the results somewhat surprised me. I was interested to compare how much lower the handlebars get as you increase your seat angle.
First off I wanted to confirm that I agreed with Slowman’s formula for the change in drop against seat angle so I created my own little mathematical model from first principles and then compared (yes I am a boring engineer). Both of our numbers were nearly identical at all seat angles (max variation only a few mm).
I then created a spreadsheet with drop versus seat angle. I then combined it with the change in seat elevation (vertical) as the seat angle gets steeper.
The results surprised me for my seat height (79cm) (6’1"tall) according to these calculations my handlebars would only be 1.8cm lower at a 80 degree angle versus a 73 degree angle and only 1.3cm lower at a 80 versus 75.
The drop is bigger at 80 but the seat is higher end result difference is very small.
Does this result surprise anyone else?
I am not saying I don’t agree with steep angles, I just am readjusting my views on why we should or shouldn’t use them.
All calculations were based off slowman’s original formula…not too sure what assumptions he used for that.
Flexibility I don’t believe to be of any real relevance in comparing the different seat tube angles. They all assume a standard leg torso angle and are merely rotating the body about the bottom bracket. So the body position is actually completely the same it is just being rotated in space.
Hope that makes sense…
Basically what I am saying is if I was riding at 80 degrees and I wanted to move back to 73 degrees I would have to raise my handlebars 1.8cm to maintain the correct (same) torso/thigh angle.
With the formula that you set up, does it also give you the distance the stem needs to be shortened/lengthened to maintain all of the same body angles?
I hadn’t done those calculations but I have now.
This gets a little less accurate now as I had to use a number which I measured off my bike as it can’t be generated form slowman’s formulas. The distance from BB to where your elbow sits. Sticking with basic tri bike fit theory this distance like the seat height should remain constant as the rider rotates about the BB for the varius seat angles.
In going from a 80 degree to a 73 degree I would have to shorten my stem by only 3.1cm. Which is interesting as the movement in my seat setback would be a full 9cm (from being inline with the BB to 9cm behind).
Alternatively I could leave my stem the same length and shorten my aerobars by 3.1cm to have the same effect.
Not surprising since your seat is higher from the BB vertically. The BB-Seat distance is still 79, but rotating that hypoteneus to 80 from 75 or 73 decreases the horizontal vector and increases the vertical vector. SO the delta seat to armrest is changed by more than 1.3 or 1.8 cm. (I don’t have my sliderule to figure out the exact number, and this computer doesn’t have a scientific calculator, so I can’t give you the exact numbers) I would guess that your seat goes ~1-1.5 cm higher in off the ground.
I guess I didn’t define what I was saying very well.
My handlebar drop would decrease by 3cm (moving from 80degrees to 75 degrees).
But also my seat would be 1.2cm higher in the vertical(although the same distance form the BB)
So the end result would be the handlebars would be 1.8cm higher
Well your here Slowman one question…
Calculating seat height is fairly easy.
I think your formulas are extremely useful and accurate for getting a starting point for aeropad drop.
Is there any chance you could develop a formula for reach? nose of saddle to aerobar ends or BB to aerobar ends etc??
I would love to be fitted by someone fist certified but haven’t found someone in New Zealand. I have been to many fitters but most don’t understand even the basics of tri fitting (they consider a seat set back of 7to 8cm behind the BB to be a forward position!!). So my best current option seems to be your formulas…a mirror , a digital camera and trial and error.
So a formula for reach would be a great help if you consider it possible.
Yip your bang on seat goes 1.2cm higher for me when going from 75 to 80 degrees.
Yeah I guess the numbers aren’t completely surprising, I had just never fully crunched the options before.
I always thought handlebars were a lot lower on steep bikes. I guess handlebars are a lot lower on TT bikes than road bikes and a bit lower again on steep TT/Tri bikes.
Your results confused me as well. I had a rough model drawn up based on a few pictures I had taken of myself. This sentence “But also my seat would be 1.2cm higher in the vertical” had me confused. Wouldn’t your seat drop going from 80 to 75? as in the pic below? The steeper stickman is at 80 the shallow is at 75. Granted this is laid out on my size and by no means a proper or perfect fit but the results shoudl be similar. the stickmen were merely rotated about the BB. Dims are in MM.
“1.2cm higher for me when going from 75 to 80 degrees.”
I guess what surprised me is my frontal area will only be a little smaller at 75 (1.8cm less total height depending on head position)) than at 80. But I would have a much flatter back which I guess begs the question which is more important frontal area or shape for aerodynamics…but I guess that a whole other discussion.
“I guess I didn’t define what I was saying very well.”
if you’re referencing the ground, then yes, 1.8cm. if you’re referencing the saddle, then 3cm. but don’t take this to mean you’re only 1.8cm higher if you’re at 75°, and that’s the end of the story. your ARMRESTS may only be 1.8cm higher, but your aspect facing the wind is changed in a way more representative of that 3cm gap.
“Is there any chance you could develop a formula for reach?”
i suppose i could. but that would require work… and… well…
let me help you out a little here. you won’t like 75° (the way i measure, 14cm back from the nose of the saddle). and you won’t like 77°. 30% chance you’re like 79° best, 70% chance you’ll like 81° best (which would translate to about a 78° seat angle bike).
those are for biomechanical reasons, quite apart from the aero advantage gained from a lower drop.
I quite agree there are many reasons to ride steep…many far more important than being aero.
I wont ever forget wondering what was wrong when I did my first sprint tri on my QR superform 12yrs ago. The first km of the run just didn’t feel right I wasn’t tight up the backs of my legs…my legs didn’t feel like rubber. Very cool feeling as everyone else was grovelling to find there land legs again.
"Flexibility I don’t believe to be of any real relevance "
I disagree and feel that the weakness in formula based bike fitting is that it can’t take into account biomechanical individuality. Formula analysis can only be used as a general guide but not as an accurate way to fit every rider.
You can take any two people with exactly the same measurements but what if one has a hypolordotic or reversed curve in the cervical spine. What about hamstring flexibility, low back flexibility, pelvic rotation/unleveling, unequal sacro-iliac motion, foot pronation, leg length differences in tibia or femur, psoas muscle tightness or imbalance, lumbor lordosis or thoracic kyphosis, trunk rotation, etc. Not to mention medical conditions such as degenerative discs, etc. These factors will determine how low you can ride in the front, etc., not what the formula says.
There are more factors to consider than just number crunching. The bike has a degree of adjustability while the body has a degree of adaptability. Through certain strength and flexibilty exercises the body can even be trained to become more adaptable in many cases. But some people may never be able to ride comfortably in a certain position even though another identically proportioned rider can.
Sorry I should of qualified that statement a bit better.
I was only looking at the theoretical change in position with regard to slowmans formula (which maintains a constant hip and shoulder angle), and I do realise theory often dosen’t match reality so I should of more clearly stated the limitations of wht I was calculating.
I quite agree flexibility is incredible important in determining bike fit. I would only suggest using formulas as a starting point and only when a good bike fitter is not avaliable.
Its a real problem in New Zealand (and probably many other places) I don’t know of any good competent bike fitters here who fully understand tri bike fitting. So I do find formulas a useful starting point.
I’ve always thought the ideal bike fit team would be to put together an engineer and a chiropractor. I know what you do for a living. Can you guess what I do.
I guess as a medical professional you must find it quite frustrating what you see written on Tri forums sometimes. I know from an engineering perspective there is certainly a lot of miss information about.
As a chiropractor, whats your take on the whole issue of orthotics/supportive footwear? Do you think there neccesary for many people or are they a bandade solution to a muscle imbalance or weakness problem?
