[Closed] 29er for dh racing

If you run flats you can also go some of the way to having this effect by running silly thin pedals such as Point ones or crampons.

But yes the actual CoG of a 29er is higher - however being lower relative to the axles makes them harder to tip from side to side I think.

Also, as the CoG of a 29er is higher isn't this going to negate some of the extra grip from the larger tyres at high lean angles. Let's say for the same weight a 2.3 29er running 21mm internal diameter rims.... you could run 2.4 or larger diameter tyre....and you run that with a rim with a 33mm diameter internal width.....which is going to have more grip/stability and when?

I'd argue on high speed flat (non bermed turns) turns the latter is going to have more grip as the bike would have a lower centre of gravity, the increased diameter of the tire would negate some of the grip advantages, the increased rim width would add more stability by decreasing tire deformation which would in turn allow you to run lower pressures.

That might be the case if the CoG was higher but it isn't. Also you can run thin flats but then you'll need non SPD shoes with thick soles so it's a wash. You'll never get near the 30mm plus drop of a low BB 29" with a 26" unless you get a frame with a 10.5" BB and 130mm cranks.

There seams to be a received wisdom here that a high CoG is a always bad thing. Maybe this is because people are confusing the physics of stable objects like cars etc, and the physics of inherently unstable objects like bicycles.

The physics that governs the balancing of tall and top heavy objects like bicycles is pretty weird and very counter intuitive. For instance a bike with a high CoG will take considerably longer to fall over than a bike of the same mass but a low CoG. This gives a rider more time to regain control if they lose their balance.

Well, that's true but is it relevant? Bikes don't crash because they gently fall over, they crash because some larger force than the bike and rider can deal with pushes them over. And in most cases, a lower COG is beneficial when resisting that

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Northwind - Member

Well, that's true but is it relevant? Bikes don't crash because they gently fall over, they crash because some larger force than the bike and rider can deal with pushes them over. And in most cases, a lower COG is beneficial when resisting that

As the positioning of the front wheel contact patch relative to that of the CoG is the main means by which bicycles maintain balance.

Incorrect front wheel positioning is usually the cause of a failure to balance. This could be due to the front wheel slipping, following a rut, glancing off an object or simply that a correct wheel position cannot be reached in time to aid recovery. A higher CoG will buy you more recovery time whilst a high rearward CoG will also reduce the unbalancing effect of any front wheel imprecision.

mudrider - Member

A higher CoG will buy you more recovery time

But make it harder to recover, and make it easier to get in bad situations in the first place.

Mass (rider) + same BB height + higher wheel axle height = lower CoG.

JCL, your grasp of basic engineering is rubbish. The CoG of the bike and/or bike+rider system has nothing to do with where the axles are. Despite that, I wouldn't be surprised to see 29ers dominating DH racing in ten years time with the unfortunate side effect that packaging challenges will mean that the top DH racers (just like DH skiers) are all over 6' tall. But that's what happens in elite sport...

It's a unique situation as axle height is almost always static in every example available, motorsport etc. You could argue that it is effectively lower and that the actual CoG remains the same (although it does change as you go through suspension travel). I'm happy with effective centre as a description as it isn't just a lower roll centre.

Northwind - Member

mudrider - Member

A higher CoG will buy you more recovery time

But make it harder to recover, and make it easier to get in bad situations in the first place.

Wikipedia states...
"Bike designs tend to consist of a trade-off between maneuverability and stability."

If a higher CoG makes a bike more stable then a low CoG should make a bike more maneuverable because the bike will lean more quickly. So yes in certain circumstances where faster cornering is needed of a low CoG bike could make it easier to avoid trouble. However, this concept of maneuverability is based on the idea that a maneuver involves altering the direction of the CoG. Consider a situation where a rider traveling at speed needs to avoid small boulders. He or She could steer the bike around the boulders but a faster approach would be to move the wheel contact patch left or right whilst the CoG travels in a straight line. This involves temporarily unbalancing and re-balancing the bike. So in this instance a high CoG bike would be able to avoid bigger boulders than a low CoG bike traveling at the same speed. Because of this phenomenon, there has been a long tradition of Penny Farthing slalom racing where despite their height the course is completed with surprising speed and ease.