Ideal fore-aft weight distribution on drop bar bikes

General Discussion
1

Hi everyone,

I’m currently designing a drop bar touring bike, mostly for road and some gravel. I design the frame, the racks and the bags as one integrated system. Weight distribution is influenced by the placement of these bags as well as by the bike’s geometry.

I’m asking myself two questions:

(1) To what degree is fore-aft weight distribution (see exact definition below) a driving design goal for most of the self road and gravel bikes?

(2) What would be the ideal weight distribution for the various drop bar bike categories if we would not have to pay attention to design goals (such as tire clearance to seat tube, sizing considerations, steering geometry etc) that determine fore-aft weight distribution so to say “as a by product”?

By fore-aft weight distribution I mean the ratio of weight carried by the rear wheel to weight carried by the front wheel, each measured when the rider sits on the bike - or the inverse ratio. The center of mass (fore-aft) of a gravel bike with rider is typically a few centimeters behind the bottom bracket - a little less than half of that when measured in inch ;). This is based on a series of measurements I did where I sat on a gravel bike (an Orbea Terra 2023, size L) with my shoes on the pedals and a digital scale under each wheel. For a road bike the center of mass is a little bit further forward.

After reading What are your driving dimensions and why? and Chainstay Length Discussion it seems that at least people active on the forum give a lot of consideration to fore-aft weight distribution. In the following few paragraphs I will nonetheless argue that, at least to a certain degree, bikes are unlikely to have the “perfect” fore-aft weight distribution because other driving design goals produce weight distribution as a byproduct. The following paragraphs will discuss what these other driving design goals are.

Weight distribution is mostly dictated by (1) the ratio of rear center (i.e. horizontal chain stay length) to front center and (2) seat tube angle. The combination of (1) and (2) put the body of the rider somewhere between rear axle and front axle which determines the weight distribution between the wheels.

(1) Rear center to front center ratio
For road and mountain bikes front center is dictated by (a) sizing and sometimes toe overlap considerations, (b) the preferred cockpit (MTBers like the control/steering lever provided by wide handlebars, roadies like narrower handlebars for aero and ergonomic reasons and longer stems to give these narrow handlebars stability at speed and (c) steering geometry (head tube angle and fork offset).
Neither (a), (b) or (c) is driven by fore-aft weight distribution considerations but they are driving/determining weight distribution as far as front center goes. This is why I said above that “other design goals” determine weight distribution as a by product.

Rear center, i.e. horizontal chainstay length, on road race bikes is often as short as possible in order to create a stiffer, “snappier” rear end and to keep the bikes overall length short in order to be able to maneuver the bike in a bunch quickly. On mountain bikes rear center is either also kept as short as possible to be able wheelie and pull the front wheel over obstacles easily or one wants to increase the overall length of the bike which is done by increasing chainstay length (mostly on Downhill bikes for high speed stability on loose ground). A longer wheelbase provides straight line stability and is therefore sometimes a consideration on endurance road bikes and gravel bikes because they do less bunch riding. This is a somewhat crude overview of factors influencing rear center but I hope you get the point that, again, other design goals determine weight distribution as a byproduct.

(2) Seat tube angle
Seat tube angle on all types of bikes is influenced by (a) pedalling economy (steeper seat tube angle is associated with more physiological power production on road bikes) and (b) a comfortable distribution of body weight between hands and butt (not to be confused with the topic of this post, i.e. distribution of system weight between the two wheels).
On mountain bikes seat tube angle is mainly driven by the need for weight over the front wheel on steep inclines. This is the only major example in bike design where fore-aft weight distribution is the crucial factor that dictates an element of bike geometry.

I’m interested in what the ideal fore-aft weight distribution would be for race road, endurance road and gravel bikes in mixed terrain (uphill, downhill, flats) if all of these other design goals I mentioned above would not already define fore-aft weight distribution more or less. The reason why this is not only a theoretical question is because on a bike with luggage it is possible to influence the weight distribution greatly while still paying attention to the other design goals mentioned above by placing the luggage further reward or further forward.

If you share your “ideal” weight distribution you could express that as: rc:fc ratio (or the inverse of it) at STA of x for category “road endurance” or whatever other (drop bar) category you have in mind.[1] Rider position on the bike also has a large influence on weight distribution but by stating the bike category (alongside rc:fc ratio and STA) we can assume a category-specific typical rider position. As a consequence we do not need to make statements about rider position that are hard to quantity. And keep in mind that the “rc:fc ratio at STA of x” statement should not be one’s overall geometry preference given all the conflicting design goals we face in bike design but one that is free from design goals other than fore-aft weight distribution. This at least is the best way I can think of to quantify fore-aft weight distribution. Another way to express it would be to simply say: more rearward/forward than is is typical for a certain by category.

[1] rc:fc = rear centre to front center ratio; STA = seat tube angle.

2

I think it’s an important factor, but not the most important factor. To me, rider fit is paramount, component compatibility (what tires/cranks/etc fit) is second, and handling (which I think fc:rc balance would fall under) is third.

I don’t know if “of the self” is a typo for “off the shelf” but assuming it is - I think many production bikes don’t put a ton of design thought into fc:rc ratio, especially in the S/XS/XXS sizes. You’ll see some pretty wonky design decisions if you look at enough geometry charts for production bikes where the handling on smaller frames is quite different than the M/L sizes. Higher end road bikes are getting better about this with size-specific fork offsets and wheel sizes but it’s far from common.

I don’t think there is any “ideal.” It really comes down to rider preference and it is not something that is easily summarized by any rule. One person’s “compliant” or “planing” could be another person’s “mushy” or “unresponsive” and vice versa. I do think people tend to be a bit obsessive over +/-5mm of chainstay length but the fc:rc ratio is certainly something you can feel when riding.

2 Likes
3

I guess since I make a lot of bikes with bent seat tubes, I think of seat tube angle as a frame jig setting, like BB drop.

Instead I like to focus on the saddle clamp to BB X measurement because it factors in the saddle and seat post setback too.

Screenshot 2024-03-06 at 11.50.04 AM
Screenshot 2024-03-06 at 11.50.04 AM2020×1180 441 KB

100% agree. We humans can quickly adapt to new physical situations. I can switch between my two hardtails with widely different F/R ratios and within minutes feel comfortable piloting both. They do feel different and sometimes I might prefer one over the other for given terrain, but they’re both fun to ride.

Same goes with drop bar bikes. I just made myself a test gravel bike with a 685mm front center and 520mm rear center for a FC/RC ratio of 1.32. Compare that to my more normal gravel bike with 650/435 = 1.49. And guess what, they both are fun to ride. They both ride no-hands intuitively and corner as well as you’d expect for that bike style.

3 Likes
4

I agree with Eva’s take a lot and would sum up my take with the following

     use case + rider fit + rider skill = design/handling

Design/handling certainly has a bunch to do with weight distribution and FC:RC but, for me, those are driven, not drivers. And, like Eva says, I think some folks obsess over things that, for the casually talented, don’t make that much of a difference. Consider how your body/position changes over the course of a long ride or throughout the day.

Re: stock bikes. Use case, trend and what the brand identifies as their core demographic/center of bell curve determines where the design starts. Maybe at that point there’s some consideration put into fc:rc but I’d bet its more of a driven dim. From there, the design moves toward either end of the bell curve. Since there are a lot of margin challenges the designs are simplified to find mfg efficiencies and to minimize CAPEX (tooling/molds). As Eva pointed out, XXS/XS/S suffers. So does XL/XXL.

Just take a look at the Trek Madone geometry.

image
image1633×1293 163 KB

If brands were really looking to optimize design, weight distribution fc:rc then I’d think we’d see a return to 10-12 stock sizes (1cm increments) and a willingness to open more than 2 molds for a fork with flip chips built in for fine tuning.

2 Likes
5

Good to see all of your comments.

Agreed that this is subjective and it’s hard to generalize. But I’m curious what your personal subjective preference is if you can express that (for example as “rc:fc ratio at STA of x").

You can satisfy both, sizing and handling design goals and have both of them driving. You might then have to make some trade-offs but in my experience those are small. Just having sizing and component comparability as driving values leaves the geometry under defined. But I agree with you that sizing is essential.

6

I’m going to still firmly say “it depends” :grin:

Here are the drop bar bikes I currently ride, all of which I very thoughtfully designed:

They all ride great! In my opinion, the handling is impacted much more by the trail than the weight distribution. And even then, the weight distribution feels more impacted by bar height and stem length than fc:rc ratio. I don’t really think about seat tube angle much since as @manzanitacycles mentioned it ends up being abstracted away by saddle setback and curved/offset seat tubes.

1 Like
7

Although I can only speak for myself, I don’t think you’re going to get an answer for this. Some of us have already mentioned why STA isn’t a number to focus on.

Since you’re focusing on a drop bar touring bike, you can look into how to optimize handling for different load setups. Rear loaded bikes will likely need longer chainstays so your heels can clear the panniers and the seat tube can clear a full-coverage fender. Adding trail to the front can help overcome steering vagueness with rear-biased loads. You’ll also need to beef up the top tube so the rear load doesn’t twist the bike.

If you want to pack like a randonneur, you can reduce trail up front so the added weight on the fork doesn’t overpower the pilot.

If you want a more central load like a lot of bikepacking setups, then the weight doesn’t have as much impact on the handling.

1 Like
8

For me, weight distribution (position of centre of mass) is a primary part of designing the bike very closely linked to rider fit, because both of those affect how the rider interacts with the bike and how the bike will handle under the rider, and steering is a critical part of that.
All the other design parameters are based off that. The rear centre considerations that you mention only work because they are shifting the weight distribution.
Other design goals sound more important, but I would argue that it’s because your customer is going to assume that the bike will handle well so doesn’t think about it.
That’s my approach at least.

3 Likes