Front center questions

[cyan]

In general, I understand shorter front center means more weight towards the front wheel and vice versa. More weight on the front tends to make steering more responsive but also more prone to be over the bars (esp. when braking hard or descending). And then people like using a longer stem with a shorter front center which is correlated with shorter reach.

Q1: Between weight distribution on the front and trail (mostly determined by HTA and fork rake, assuming the same wheel/tire size), which has more effect on handling? I ask because you can have short front center and large trail at the same time.

Q2: Doesn't the shorter front center and longer stem combo distributes even more weight towards the front, hence making the steering even more responsive despite having a longer stem (which usually means more stable control)? Or the added front weight and the longer stem cancel each other out?

Q3: For climb/descent, what's the optimal front center range? Or is it better to have short or long front center assuming the overall reach incl. stem and bars are about the same? I guess the answer is relative to some other metric(s), then what should it be relative to?

Q4: How should the front center change (if at all) between a road bike and a gravel bike?

[Mark McM]

In general, I understand shorter front center means more weight towards the front wheel and vice versa. More weight on the front tends to make steering more responsive but also more prone to be over the bars (esp. when braking hard or descending). And then people like using a longer stem with a shorter front center which is correlated with shorter reach.

The front center dimension makes far more difference on off-road bikes than it does on road bikes. This is for a variety of reasons - speeds and turning radiuses tend to be much smaller off-road, hills can be steeper, and surfaces can be rougher (with more potentially wheel-stopping obstacles). For road bikes, other factors tend to be more important for handling/stability.

Q1: Between weight distribution on the front and trail (mostly determined by HTA and fork rake, assuming the same wheel/tire size), which has more effect on handling? I ask because you can have short front center and large trail at the same time.

On a road bike, trail generally has a bigger affect on handling than weight distribution. Here's an example: Normally, front center varies with bike size. But here in the US, CPSA regulations limit how short front center can be (to limit toe overlap). Consequently, below a certain size, bikes (of a given model) have similar front centers. The bike designers therefore have to make compromises in head angle/fork offset/trail dimensions in order to reduce the top tube/reach lengths on the smallest frames. This often results in poor handling in the smallest sizes (even though these bikes have the same front center and weight distributions as their larger brethren).

Q2: Doesn't the shorter front center and longer stem combo distributes even more weight towards the front, hence making the steering even more responsive despite having a longer stem (which usually means more stable control)? Or the added front weight and the longer stem cancel each other out?

It depends on what you mean by "responsive". Increasing weight on the front wheel actually increases stability at speed, by increasing the effect of trail. On the other hand, at low speed (where trail has much less effect on stability), shifting weight to the front wheel can magnify the effect of steering inputs on lateral balance and weight shifts.

At higher speeds, small steering angle inputs can result in large changes in lateral tracking, so a longer "tiller length" (stem length + handlebar reach + handlebar width) can improve stability by reducing steering angle changes. At the same time, extra weight on the front wheel will also increase the affect of trail on stability. But note here that since the tiller length is much shorter than the front center, if you change the stem length in one direction and change the top tube length by the same amount in the opposite direction (to maintain the same reach), the relative change in tiller length will be much larger than the change in weight balance.

Q3: For climb/descent, what's the optimal front center range? Or is it better to have short or long front center assuming the overall reach incl. stem and bars are about the same? I guess the answer is relative to some other metric(s), then what should it be relative to?

Sadly, the optimal steering geometry for climbing will be different than for descending. For climbing (slow speed), you'd want to a steep head angle to minimize steering flop (which affects stability when out of the saddle at low speeds), which would tend to decrease front center. For descending, you'd want a shallower head angle to increase trail and stability, which would tend to increase front center.

Q4: How should the front center change (if at all) between a road bike and a gravel bike?

Front center would typically be longer on a gravel bike, but that may be primarily be due to other factors. For example, if you keep the reach the same, then the shallower head angle typical of gravel bikes will just naturally increase the front center. A slightly longer front center may also help with toe overlap with fatter tires, and also to help "float" the front wheel over soft or bumpy terrain.