BMC - why the long trail?

[openwheelracing]

if anything the BMC should understeer...correct me if I am wrong. How is it oversteering?

[Spdntrxi]

Originally Posted by openwheelracing

if anything the BMC should understeer...correct me if I am wrong. How is it oversteering?

this is how I understand it as well.. understeer

[openwheelracing]

also, stem sizing and handlebar sizing (reach, width....etc) all come into play. So unless all variables are equal, I think OP may need to dive in further to find the culprit.

[NHAero]

Originally Posted by openwheelracing

also, stem sizing and handlebar sizing (reach, width....etc) all come into play. So unless all variables are equal, I think OP may need to dive in further to find the culprit.

That's a good point!
Do we agree that the thing that matters here is where the hoods end up, that handling is not affected specifically by stem length, but by whatever combo of bar reach/width and stem length chosen?

[Mark McM]

Originally Posted by spoonrobot

Some people prefer to lean, some prefer to steer - within the bounds of what the machine wants.



As much from the bike as it does from your inner ear and other anatomical balance mechanisms.

Well, of course it isn't as simple as just leaning vs. steering; balance and maneuvering a bike requires both - steering input affect lean, lean affects the steering input, which in turn affects lean, which in turn affects steering, etc. It's a big feedback loop, with the rider right in the middle of it all, both acting on, and reacting to, the dynamics of the bicycle. But I do agree that the rider's preferences and expectations, and how they've learned to react to feedback from the bike (all influenced by the rider's prior experience) will affect how a particular geometry feels and handles.

I've tested ridden a few BMC road bikes, and I personally didn't like how they handled. While they certainly tracked a straight line well, they felt unresponsive to steering inputs in quick maneuvers - in fact, it almost felt as if they were fighting back when I tried to turn the bars quickly. When testing one model of RoadMachine, it occurred to me like it handled nearly identically to my Redline cyclocross bike (whose stability is good on rough surfaces, but has ponderous handling on smooth surfaces). Afterward I compared geometries tables and found they sure enough, they had the same head angles and fork offsets, and nearly the same front center.

It was commented that the BMC geometry was tweaked in response to input from Cadel Evans. Evans originally competed in mountain biking (where he won many UCI races, including the World Cup Overall title twice) before switching to road racing. Mountain bikes tend to have slacker head angles, more trail, and longer front centers than road bikes, so maybe his preferences were formed during his earlier career on mountain bikes.

[Mark McM]

Originally Posted by rothwem

Something weird I've experienced on my old Kona Jake the Snake is that its tricky to hold in a straight line when climbing a slow/steep road. The only thing I can point to is the long trail--its a 71.5° HTA with a 45mm offset fork, which works out to about 70mm of trail with 35mm tires.

The tendency to wander back and forth when pedaling hard (particularly when out of the saddle) is probably not due to steering trail per se, but due to steering flop. (Steering flop is just what it sounds like - the tendency for the front wheel to turn, or "flop", in the direction the bike is leaned.) Increasing one tends to increase the other, so it can be difficult to separate the affects of trail and flop. However, flop is more affected by head angle than trail, so two bikes can have the same trail by adjusting the head angles and offsets, but the one with the shallower head angle will have more flop.

[Mark McM]

Originally Posted by openwheelracing

if anything the BMC should understeer...correct me if I am wrong. How is it oversteering?

I'm not sure understeer and oversteer are the best terms to use when describing bike handling. In motor vehicles, understeer and oversteer are caused by variations in front/rear wheel slip angles, which causes the rate of turn to vary from what would be expected by the steering angle of the wheels. Except at very low pressure, bicycle tires have little slip angle, so they don't really "understeer" or "oversteer" in the classical sense.

However, bicycle geometry can affect the interplay between lean angle, steering torque and turn radius, so it some cases it can feel like the rider has to apply more or less steering torque for a given turn radius and lean angle (and in some cases, may need to apply counter-torque). The steering torque for given turn radius or lean angle can vary not just with the bicycle geometry, but also weight distribution and rider technique.

[72gmc]

I've not ridden a BMC but I find this conversation interesting. Do BMC riders find the need for a "BMC fit"?

I ask because I had a cyclocross frame (Soma) with notably different handling than my daily driver Davidson. Trying to get the Soma as close as possible to my familiar Davidson fit didn't result in a fun ride. I only found a happy place when I experimented with a new reach/stack to suit the frame and me.

[Spdntrxi]

I just adapt and quickly.. I've never found any issue with BMC and all I really care about is straightline and downhill handling. I crit race and do the usual spirited group rides. Only climbing my BMC kinda feels dead... but then again my BMC is an aero bike and not an all rounder.

[LJohnny]

I have owned a SLR01 since 2013 and I have consistently found it to be a great handling, comfortable and streamline ride. Clean geo. In my opinion BMC is frequently overlooked, but the race geo bike hit the target right on,
Maybe because their marketing was to aggressive? Or carried by snobbish shops? Here in DC BMC was sort of found in the high end shops mostly. And not many had floor models. Who knows…


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[Spoker]

So much of our energy and attention is directed in keeping the bike upright and going straight.
A bike may feel " sluggish" but in hour 5 of a 100 mile ride you feel fresher.
Not even talking about a pro who rides daily >100 miles.
If I recall Joop Zoetemelk wanted his frames build with >60 mm trail in a time when steep HA were fashionable.

[72gmc]

Originally Posted by Spoker

Joop Zoetemelk

That guy. Winning Worlds at the ancient age of 38! Who would want to ride like him ...

[OtayBW]

Originally Posted by Mark McM

I've tested ridden a few BMC road bikes, and I personally didn't like how they handled. While they certainly tracked a straight line well, they felt unresponsive to steering inputs in quick maneuvers

Sounds like a high trail bike. No surprise. I have an SLC01 and I like the way it tracks and steers at speed. But I don't do much quick maneuver steering.

[pff]

Originally Posted by openwheelracing

if anything the BMC should understeer...correct me if I am wrong. How is it oversteering?

Accuracy of these terms aside, I would have thought you were right. The slack HTA should create a more "stable" less "twitchy" ride, and it does, but when already leaning into a turn, slight adjustments can have out-sized effects. It's like the response to inputs scales up super-linearly. Perhaps this is an effect of high "wheel flop".

Originally Posted by Dadoflam

Cadel Evans was retained by BMC on staff after he stopped racing to developand refine the geometry and handling of the road frames.
In no small part this was because was one of the very technical riders with an intimate knowledge and understanding of his bikes and geometry requirements.
What you are riding is the outcome of this input.

Unfortunately I don't have his proportions (or his lungs) so I'm not sure if this is relevant to me And in any case, I've compared to a slew of other bikes from Trek, Cannondale, Spec, etc. and found that BMC is waaaay out in left field. Mohoric, Pidcock etc have shown a knack for descending with more "normal" racy trail figures (Pinarello and Merida both 56mm).

Originally Posted by Peter P.

You mean to tell me your Parlee, with the same head angle as the BMC but a mere 2mm more fork rake, handles much better? I can't believe 2mm makes that much of a difference. It's the tire size I tell ya!

I am also skeptical that it's only because of a little rake. I controlled for tire size though by running everything between 28 and 32 slicks on both bikes. Other theories include the substantial chainstay difference of 2cm. The BMC has short chainstays compared to most other bikes with the same trail. Does this contribute to an unbalanced feel?

Originally Posted by openwheelracing

also, stem sizing and handlebar sizing (reach, width....etc) all come into play. So unless all variables are equal, I think OP may need to dive in further to find the culprit.

Touch points are pretty close, although not identical because I want my gravel bike to be more upright (also, descending on rough terrain often involves un-weighting the front wheel rather than maintaining a more equal weight distribution, so higher bars help push the weight farther back). However I don't think it's that important when descending, as I can control weight distribution independently of touch points, e.g. by bending elbows. If it were down to touch point differences, I would be able to see that pushing weight back or switching to hoods would resolve observed issues. Not to mention I have always had the same impression of the BMC even though I twiddled with the cockpit several times.