This is probably the second most important junction on a bike frame (second only to the BB), so it deserves a separate post.

First I want to illustrate my point by showing a Scott Addict's headtube.



As you can see, the downtube's junction on the headtube is fairly high, and the fork has a crown as tall as Yao Ming.

This creates 2 issues:

1. The front end stiffness suffers, because the two headset bearings will be closer for a given frame size, and that doesn't give you great torsional stiffness.

2. The aerodynamics suffers as there is a lot of area between the downtube and the crown junction.

My approach (and this is nothing new, Cervelo is already doing it) is to spread the bearings as far apart as possible, including depressing the fork crown. My fork length will be 355mm-ish... although I'd have to find out if manufacturing will run into any issues...

Re point 2, Pinarello/Merckx are doing something interesting to smooth out the airflow around that area.



This is a good move as we have seen wind tunnel data that indicate the high air turmoil around this junction. But their "fairing" after the fork appears to be pure dead weight and does not add to the structural soundness of the area.
So a "lower the downtube" with a modified shape will fix that. The enlarged section will also contribute to the front end stiffness. In addition, this will effectively place the lower bearing near the middle of the downtube junction rather than below it, which should have some stiffness benefit (however little).

Lastly, a lower downtube junction gives you a slightly shorter downtube as well (probably 3mm). No biggie but this is WW, so there you go...

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I better catch up on 80s rap music.

I was more thinking of this dude...




Anyway, both are fairly cool IMHO, but there is a Falcon brand in the UK who sell entry level bikes, so I want it to be a bit different. Plus one less letter in the decals is more ww...

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I agree in principle with your thinking on this junction.
We have seen clear effects on this in our structural tests.
But when it comes to aero it's not. The air behind the front brake is already disturbed, you will not mend what is already broken by doing this. Also if you run standard der cabling these will interfere with the airflow.

On the trail: Personally I don't like how a bike with a trail >60 handles. I favor high speed stability when descending. Especially if you use high profile aero rims! Others tolerate highish rakes, especially light guys (not me). So I'm told... Also your weight distribution matters..

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Technical Artisans

Re aero effect, it should be more relevant if one can optimize the cabling, which I am attempting to do. Here is Pinarello's CFD simulation...



Re trail/rake, etc., agreed that one should not underestimate the impact of deep aero wheels on handling. That said, would it make sense to have a high rake fork for aero wheels and a low rake fork to go with low prof wheels? Talk about a slow wheel change... ).

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Not sure if you're developing your own fork too, but the obvious solution to some of the issues in this area is to use something like the THM Scapula approach and integrate the brake, or a tri fork with rear mounted brakes (although not too sure how these desgins would ride on an all round bike, and the maintenance is a PITA)

Love this post by the way, looking forward to seeing the end product!

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So true (that the ISP can define the ride quality). When Look launched the 595 with the ISP, that's exactly what we said about it....there's too many variations in seatposts, so when a company designs a frame around a certain seatpost and then a user changes to a lighter/flexier/etc post, then the ride qualities have now been changed for that bike.

With an ISP, the frame manufacturer controls the ride quality with no variance. Now the bike rides exactly as they intended it to.

Too many people see the ISP as a gimmick/marketing spin, but the benefits are real.....

+100
Most people I know who have had the experience of an ISP frame (including myself) love it.

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I love my ISP and I also love the way that ROB set up the stubbie inside the tube instead of using a topper which I think just look ugly. Not sure how easy this would be to do with a Carbon frame but it is the cleanest set up I have ever seen.

There are a lot of other things you could build into a nice aero road bike. If I had enough money to build another custom bike, there are a few things I would do differently for sure adn they would be all aero related.

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BIG DADDY B FLOW
AERO & LIGHT is RIGHT

Cervelo SLC 5890g/12.98 lbs

I would also consider looking at a brake like the Tektro T726R on the front with a cable hanger which would allow you to run a cable down the front of the head tube which is more aero than a housing and cable running down the side of the head tube with a conventional brake. If you are designing the bike you could use this brake front and rear and keep the cables completely inline with the frame and out of the wind.

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BIG DADDY B FLOW
AERO & LIGHT is RIGHT

Cervelo SLC 5890g/12.98 lbs

Is that glued or welded? Or does it offer any adjustment? BTW, the rear brake cable solution is really similar to what I have in mind. I like the way it's in the middle rather on the side (eg, the C'dale EVO). Aerodynamically slightly better and for smaller sizes gives you a bit more leg room for the braking action.

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I have been hearing above argument on some other places too but not sure what is more aero. Judging the design of the Tektro brake for me a side pull DA 7900 brake design appears to be more aero. By the way I like the 7900 brake design best from all others out there.
Not sure if a center pull brake cable and housing is not disturbing the clean airflow in front of the head tube that much that the drag of a side pull housing should be acceptable. Unless it is all hidden like the brake of the Scapula fork but with center pull and with some brake housing going down inside the fork and headtube entering from the headset cap.WW issues?

@elviento - I wish you good luck with your project.

I was just about to write that
It might be worth it to take a look at EE brakes too, they are very nice and compact and would go well behind the fork.

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I have EE brakes and once the aero brakes arrive I will be able to compare them better. These are tests done on Slowtwitch that show that the center pull brake set up (modified version of this and not as neat) is as good as a pair of the Simkins aero brakes. There is now housing at all when you are using these aero center pull brakes as only the cable is exposed. A cable is just way better than housing and cable.

There brakes are not light at 149g each but I am sure I can lighten them up a fair bit when I get them.

My tri bike will not be the most aero but the goal is it get it under 6.8

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BIG DADDY B FLOW
AERO & LIGHT is RIGHT

Cervelo SLC 5890g/12.98 lbs

from slowtwitch classifieds but it gives you an idea of the size and how clean this can look.

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BIG DADDY B FLOW
AERO & LIGHT is RIGHT

Cervelo SLC 5890g/12.98 lbs

Never mind the cable, but between that cable hanger and those brakes most aero benefit of anything north of the rim is lost. The straddle cable mounting fi is just plain horrible. Sort of like having a bicycle mounted on the front of your aero car.....

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