29er for dh racing

Oh man the comments section in that Dirt Mag article prove what gullible idiots people can be.

I think citing Dirt as a reference is probably even worse than wikipedia. Love the mag but they’re rarely sure what they think let alone why they think it.

(also, there could be a hidden paragraph at the end of the article refuting the whole thing, which nobody can see because they decided to print it in charcoal ink on a black background, because of Design)

Also look at the course
http://www.vitalmtb.com/videos/features/ONE-LAP-Evan-Turpen-Pro-GRT-Fontana-2013,20286/E-T,699 if you can make the rocks then the lightest bike standing is what you need. (IE anything that is not a DH bike)

The bottom bit looks well gnarly! 😆 😆 😆
Pretty impressive that Gwinn finished second riding a DH bike

All arguments aside, the technical sections of that course look great fun. The less said about the rest of the course the better.

The less said about the rest of the course the better.

Whaaaaaat????? Surely not.
Our expert (who has ridden everything) says that;

Fontana is another world

😆

More seriously, Gwinn must have been hanging at the end pedalling his DH bike over all that and finishing 2nd. He is remarkable.
The spilts are interesting too. Gwinn is faster in the first (rougher) section, Mitch smashes it in the second (pedally) section.

Gwinn is faster in the first (rougher) section

Gwinn must have been hanging at the end pedalling his DH bike

Buowharhar buowhahahahaharrrrrrrrrrrrrrrrrrrrrrrrrrr

I don’t care what size wheels anyone is riding but I’d like to correct a glaring error: The centre of gravity of a 29er is not lower than that of a similar 26er, it’s actually higher.

All other things being comparable, aye. I think people are confusing the BB-drop effect with COG (taller wheels meaning that for the same BB height you’re dropped further below the axles)

I’ve just seen the track. Congratulations to Arron Gwin for getting second on a DH bike!

All other things being comparable, aye. I think people are confusing the BB-drop effect with COG (taller wheels meaning that for the same BB height you’re dropped further below the axles)

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.

Or get some Geoff Apps specials

wat da **** are those lol

Do they work?

I must have them for the sheer lolz but they look very pedal strikey.

Sod the pedal striking, is there not a huge dead spot in their rotation around the cranks?

bwaarp – Member

wat da **** are those lol

Do they work?

I must have them for the sheer lolz but they look very pedal strikey.

No chance of pedal strike when fitted to a Geoff Apps bike with a 15″ high bottom bracket!

mikey74 – Member

Sod the pedal striking, is there not a huge dead spot in their rotation around the cranks?

In fact the effect of these swing-pedals is to give a significant dead-spot reduction. Even more so when used in combination with optimally oriented elliptical chain rings.

This bio-mechanical research from Belgium found that a 30mm drop swing pedal like those used by Apps can improve pedal-stroke power output by up to 3.2% When combined with elliptical chain rings as on Apps’ bikes a rider can improve pedal-stroke power output by up to 5.1%

http://www.noncircularchainring.be/pdf/Biomechanical%20study%20-%20Project%20002%20Vista%20Pedal.pdf

You have to realise that Geoff Apps was designing and building 29er off-road bikes in 1981 and 650b bikes in 1979. So on that basis drop-pedal/elliptical crank-sets should hit the market somewhere around 2043. Or even earlier if only the mainstream bicycle manufacturers stop ignoring his ideas.

All other things being comparable, aye. I think people are confusing the BB-drop effect with COG (taller wheels meaning that for the same BB height you’re dropped further below the axles)

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

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

–

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.[/quote]

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.