I have been searching the net about handling effects, but haven't found my answer yet.

I'm looking to build a fixie with stable handling, but also with a good weight balance between wheels. It is intended for crits, with a possibility of using on tri in the future, so it has a steep seat tube and low front.

The thing is that for the same fit, when you build a slacker headtube, the wheel is further forward, so more weight is on the hear wheel, unless you make longer chainstays too.

So, if you make an steeper headtube, but less fork offset, so that you get the same trail, and is on the limit of toe overlap, is it just as stable as an slacker headtube with more offset and the same trail? What are the effects of varying the angle while keeping the trail?

The other way I could do it is keeping the slack angle, making a shorter top tube with a longer handlebar stem. Maybe thats the most stable one? Or then would it get too hard to turn? Is the 73 degree angle the well stabilished sweetspot, period?

I know tri and crits are very different applications, but I can't get both, I don't want a quick-turning bike for a crit anyway, and both would have the same seat tube angle, hence why both in one. My road bike already feels unstable enough.

I appreciate any thoughts on this, and links for detailed info about that! Thanks

The steerer axis Line thru the fork crosses the axis line Down from the Hub, above the ground-plane ..

To really Know, draw it out on a Big sheet of paper .. Old Fashioned draftsmanship.
If you have an old Triangular scale conversion ruler, then you can do a Scale Drawing.. (or use the Inch=a CM conversion)

The same HTA and Rake has a smaller Trail As the wheel-tire size is smaller ..




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Dave Moulton's Blog - Dave Moulton's Bike Blog - Shimmy Re-Visited

Dave Moulton's Blog - Dave Moulton's Bike Blog - Head Angles and Steering

Dave Moulton's Blog - Dave Moulton's Bike Blog - Trail, fork rake, and a little bit of history

Dave Moulton's Blog - Dave Moulton's Bike Blog - The ideal handling bicycle

there is a reason why most bikes are 73 degree with 45-55mm rake. It is the sweet spot. Low trail road bikes usually have 73 degree head tubes too. I don't think you want a tri geometry on a road bike. Those bikes are really pretty unstable with the intent that they aren't going to be ridden in packs or on twisty roads. You can get most of the benefit without the forward seat tube angle. Just swap seat posts and add clip on aero bars. For most people, just adding the bars is more than enough.

If you want to play with geometry, there is a web version of bikecad. Also, there is a freeware program called rattlecad. Not sure if either one of those will do toe overlap, the paid version of bikecad does. I really don't care much about toe overlap on my bikes. I think most people that race can handle a toe overlap situation when it happens once every 5 years

A few comments- First is riding a fixie in a crit... being able to coast while cornering at one's limit is a nice thing. I agree with using common road bike geometry. Like Eric has said, there's a reason why millions of road bikes are designed so similarly. How many bikes have you ridden and also documented their geometry (as opposed to relying on published charts)? Will this bike ever have fenders? If so then add about 15mm to the front center goal if toe overlap is needed to be avoided. I use 58cm as the minimum front center with 170 cranks, 700x25 tires and no fender. Handling is not about any one design aspect but the blending of many WRT the rider's position and body make up. Andy.

My thoughts on frame design (starting from scatch like yo were have a custom designed and built): Pick BB height and seat tube angle. BB height for desired pedal clearance and stability. Seat tube angle for seat placement (and occasionally for other reasons - I pick steep seat tube angles to allow a more forward rear wheel, larger tire and fender and just live with large setback seatposts). Chainstay length, again for the usage of the bike; tires, fenders, panniers and heel clearance. Chainstay length locates the rear wheel. Now math tells me where the front wheel needs to be for good weight balance. Next I look at the location of the headtube toptube intersection. This will locate the stem and dictate its length. Now the fork is closely defined. I have a narrow balance between angle and offset. (I like fairly high offset/low trail because I like quick steering and because my stems are always 120 mm or longer.)

The time in this process to check toe clearance is when picking the chainstays and resultant front wheel location. (Moving the front wheel 1.5 cms forward means moving the rear wheel 1 cm back - both relative to the BB - to preserve the same weight balance over the wheels for "normal" bikes.)

Of course, none of this works when looking at exisiting or stock bikes. But it can be a useful exercise to do before you start looking. And this is my priority sequence as reach to the handlebars and weight balance dictate for me what bikes fit. Your bodily features may require a different sequence.

Ben

I had read 3 of those, except for the "ideal handling" one. What I understood from it is that, given that "ideal" 67mm trail, if you get slacker, it feels more stable and harder to turn, albeit harder to ride hands off? That link doens't consider stem lengh though.

My choice for an steeper seat tube is because i already have my saddle slided to the limit forward, and the seat post has no curvature. I have seen some that curve backwards, but not forward yet (not in Brazil). I also have good flexibility, i can wake up and touch the whole palms of my hands in the ground while kissing my knees, so staying aero is not a big problem to me.

I have tried to clip on my road bike. I feel the saddle is too far behind and the front could very well be some 15cm lower, or more... I can keep my torso much lower without the clips on that bike.

I still have a month to think about those answers here before i get to start building the thing, so I'll be messing with those softwares! Thanks!

Oh, about the toe overlap, it happens in my road bike (gts r3 pro frameset), it is not a problem when you can coast, but it is an issue on a fixie.

Front center you mean front axle to botton bracket?

The fixie thing is that, here in Brazil, every crit I have heard of so far are for brakeless fixed gear. Some have a "road bike cat" but the main race is for the fixies...

The fixie frameset i have just sold had 290mm from the ground with 28mm tires, enough so that the bike would loose traction in a corner before getting to the point I would hit a pedal on the ground.

Perhaps your crits are more like our track events... But don't confuse the likelihood of loosing traction when heeled over in a corner with BB height, pedal strike excepted. Rider skill is still the biggest factor in these situations, and being able to coast at that perfect moment can be all the difference.


It's obvious that your needs and goals are not those of the vast majority of us here. So I repeat my suggestion of building your own data base wit your own measurements. Anything else will be assumptions or excuses. Andy.

There is some history in the U.S. of building bikes with extreme steep angles and high bottom brackets. People came to realize that it wasn't that great of an idea. I know tri bikes are built like that, but watching people riding them, they are scary. I don't think it's just because they are runners riding a bike. Putting that into a pack of riders is asking for trouble. I wonder if your current bike isn't a little unstable because you have a forward position, moving the center of gravity forward and messing up the balance. It's possible that putting the front wheel further out will help re-balance the bike. I have no experience with that though.

To the OP: By triathlon, do you mean draft-legal ITU-style triathlon, or US-style (or Ironman style if you prefer) non-drafting triathlon?

that's a really good question. Do the pack tri's use TT bikes? I almost don't think so since the tri fail videos all have people crashing in ironman style tri's, and there is nothing more prone to crashing than a pack full of TT bikes

Pack Of TT, Time Trial , bikes ? How Time trials are started one bike at a time* .. wheel sucking in a Time trial
gets you a DQ, ... disQualified.

*even IOM TT with Motos.