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Help deciphering geometry and body position.

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Help deciphering geometry and body position.

Old 02-28-20, 03:16 AM
  #1  
abhishek_s
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Help deciphering geometry and body position.

Hello All,

I'm an amateur/newbie and needed some advice in deciphering body position and aerodynamics of a bike. I presently own a GT Avalanche Comp 2019 which is a pretty nice entry level HT. As I increase my fitness and skills I realise that for longer distances and more efficiency on paved roads, I need to get something which is more Aero. I was looking at all road (gravel) bikes as the roads in my region are pretty bad and I was thinking about light gravel riding too. The Merida Silex and Fuji Jari caught my eye and they look like great overall packages but many reviewers say that the Merida especially, is too upright. One even said that it's like riding an XC Mountain bike. So here's my question; by looking at the geometry figures, can one figure out if the Silex and Jari indeed has better (and more aerodynamic) body positioning than my GT Hardtail? I'm looking at something comfortable and not very racy but it has to be at least 30-40% more efficient than my existing MTB for me to even consider one. For reference, Here are the geometry figures in size Medium.

GT Avalanche Comp 2019 (My current bike)
Head Angle: 69.5 deg
Stack: 582mm
Reach: 429mm
Effective Top Tube Length: 610

Merida Silex 2020
Head Angle: 71 deg
Stack: 625mm
Reach: 400mm
Effective Top Tube Length: 579

Fuji Jari 2020
Head Angle: 71.5 deg
Stack: 572mm
Reach: 370mm
Effective Top Tube Length: 545mm

I know going to the drops may help and the thinner tires will reduce rolling resistance but I'm primarily looking for overall info on how different these bikes would be compared to each other in terms of body positioning and aerodynamics. Unfortunately, going to a shop to test it out is currently not an option. Any inputs from anyone here is appreciated. Thanks a lot for your time
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Old 02-28-20, 08:06 AM
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The GT reach is 429, but its because there is probably a 70mm stem and the bar is right there at 70mm from the end of reach. So call it 500mm total for reach to the bars.
The other 2 bikes will have 80-90mm stems and you need to add about 70mm for the handlebars, then another 20mm or so to get to the brake hoods. The Merida is probably around 570mm total reach to the bars and the Jari is probably 550mm total reach to the bars.

For the Jari, there is 10mm less stack and about 50mm more actual reach.
For the Merida, there is 40mm more stack and about 70mm more actual reach.

Note that the higher up bars are, the longer the stem can be to make the same effective reach. That sounds confusing and it probably is since I didnt phrase it well. Basically, if you go from a -7 degree stem thats 100mm long to a +17 degree stem, then you may need something like a 120mm long stem to retain the same amount of reach.


To answer your question, you will be in a different position on the drop bars for sure. It will stretch you out a bit more which will make you more aero. Both drop bar bikes have spacers you can remove and stems you could flip to further drop the bars if you want and can handle it, which will stretch you out even more. No idea if you will be 30-40% more efficient though- that seems like an individual measurement.
Also, quality tires are a huge difference for speed that isnt expensive. A quality wide tire can roll faster than a cheap narrow tire. Rolling resistance is probably second to wind resistance due to body size/positioning when it comes to speed. Just mentioning this for whatever bike you end up riding.
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Old 02-29-20, 12:55 PM
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A few observations from my personal experience that may or may not have the same results for you due to any number of factors such as age, flexibility, conditioning, skill and so on.

I have a vintage Trek 930 Shockwave MTB. Itís in excellent condition and rides as well as it did when I first got it in 1993. I have since rebuilt the front shock with steel springs to replace the rotted elastomer inserts and currently run it with a Specialized Phenom Comp saddle and 40mm Kevlar belted Bell comfort-type road tires as I primarily ride roads these days.

I also have a 2016 Specialized Roubaix Comp that is my primary road bike. After a lot of trial and error, I found a really good saddle, 2019 model EC-90; and the best tires and tubes for me, Specialized Roubaix Pro, 25-28mm with Challenge ulta-light latex tubes. i also have the stem slammed at -17 degrees.

Setup on both bikes involved a lot of trial and error as well. But I am currently very happy with both bikes. Position over the cranks is the same on both bikes and reach to the center of the stem cap is similar as well. Effective drop from saddle to stem cap is also very similar. I have found very good tire pressures for each bike which I adjust based on expected road conditions. There are real differences in riding positions and overall handling however, due to overall frame geometries, wheel size, stem rise on the MTB vs. stem drop on the Roubaix, and drop vs. straight bars. Also, configured to ride, the MTB weighs about 14 pounds more than the road bike.

Both are fun to ride. I can ride both bikes as hard as I can ride. I can average 12 - 12.9 mph on my MTB on the road. Max speed is usually between 30 - 32mph at most. After 2 hours of continuous riding Iím pretty worn out. Even with the low 3x7 gearing riding steep hills is a chore. And high gear spins out way to soon on flats and descents. On my Roubaix I generally average between 16 - 17mph. Max speed is generally in the neighborhood of 38 - 43 mph depending on headwinds, tailwinds and descents. It usually takes 3.5 - 4.5 hours to reach the same level of fatigue as on my MTB. The compact 2x11 groupset has all the low end gearing I need and the top end only spins out in a very strong tailwind on flats and steep descents.

Enter gravel and things change a bit. For one thing speed is not a significant consideration. The lower center of gravity of the MTB and the quicker handling become advantages. Also, the wider, lower pressure tires and gravel friendly tread help. Even without off-road tread, my MTB is surer on looser surfaces, but Iíve ridden hard packed gravel and dirt including such roads with loose rocks, sand and other surface debris just fine on my Roubaix, albeit carefully.

So whatís accounts for the differences in performance? Well weight, obviously. While the road bike is clearly more aerodynamically designed and built, that really isnít much of a factor at speeds under 15 mph. Mostly, itís the weight, gearing and as setup, the Roubaix rolls significantly better and rider positioning causes considerably less drag. So yes, a well setup road bike can probably run circles around an MTB on most roads, if you can take advantage of constantly cycling in more areo riding positions and have the skill and conditioning to ride the road bike hard enough.

At least that is my experience. Good luck!
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Old 03-02-20, 01:09 AM
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Thank you so much for your time mstateglfr !. You're right; the extra actual reach with the hoods and other options like riding on drops and flipping the stem and taking out spacers might make a significant difference along with slicker tires for better overall efficiency. But again, like you said, it all depends on how much I can take At least I know that the options are there when I choose to get a gravel. Right now I'm in a phase where I'll probably have to increase my overall fitness and conditioning and getting more aero on my existing GT. But I do feel limited and fatigued on paved roads after about riding an hour and a half. I'm also wondering if the wider 720mm flatbar adds to the discomfort.
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Old 03-02-20, 01:57 AM
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Thanks Sapperc for the comparison and setup info on both your bikes. I've had similar experiences of being worn out after about 1.5-2 hours of riding on my GT Hardtail. But since I'm still relatively new to this, I'm hoping that there is a lot more to be built up for me, in terms of endurance. But i'm also simultaneously looking at getting better body positioning because on my GT, I often do feel like a sail when on my bike haha! And the 720mm flat bars don't help. But on lower speeds, the handling and wide 2.5 tires are surely a huge help on the rocky patches of terribly broken roads in my city.
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Old 03-02-20, 08:16 AM
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Originally Posted by abhishek_s View Post
But I do feel limited and fatigued on paved roads after about riding an hour and a half. I'm also wondering if the wider 720mm flatbar adds to the discomfort.
A flat bar is my enemy after that long. The inability on flat bar bikes to to move your hands around and adjust back position is a huge issue for me. On a drop bar bike there 4 main hand placements that vary how your hands are positioned and also your back stretch. It helps reduce fatigue for me,
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Old 03-04-20, 11:04 PM
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Originally Posted by mstateglfr View Post
A flat bar is my enemy after that long. The inability on flat bar bikes to to move your hands around and adjust back position is a huge issue for me. On a drop bar bike there 4 main hand placements that vary how your hands are positioned and also your back stretch. It helps reduce fatigue for me,
You're right! I'm realising that the regular flat bar (especially the wider ones) can be literally and figuratively be, a pain in the neck! At least for rides longer than 1 hour.
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