Frame Weight Distribution - questions
I wonder how FWD affects climbing and descending: I wrote that the best handling is when you have 40% of your weight over the front wheel and 60% over the rear: http://www.cptips.com/bkefit.htm . But what's happen when you descending - then you want to have that distribution in that inclined position - so that means that bike with a little bit DH geometry will have FWD on the parking lot something like 30% / 70% and bike with XC will be maybe 50/50.. So, because manufacturers don't publish such info in geometry chart I wonder what is you FWD with your specific bike.
I think that when somebody advise another to buy 1 size smaller size to have better descending ability, that's because weight transfer backward, not so much because distance to the bar.
I wonder also what define FWD of mtb frame - what is more important - ETT,head angle, chainstay length, BB height, rider's height over the specific frame's size....?
I measure mine in that way: measure you with your bike's in your hand, after that place 2-3 books at the floor and put the front wheel over the digital scale and rear on the books. Stand in riding position and in attack position. Measure front weight over the scale for two positions. Move scale back and measure rear weight for two positions.
Divide front weight and rear for the two positions to the whole weight(you and your bike). Better accuracy will be with two scales, otherwise you'll have to stand in the same position when you measure front and rear weight.
rider: 6'5", inseadm 36", Specialized Sx Trail 08 XL, 180mm fork
Front wheel weight: 30.4%
Rear wheel weight: 70.3%
Front wheel weight: 37.7%
Rear wheel weight: 60.9%
rider(my wife): 4'11", inseam 28", Titus Motlite XS 120mm fork
Front wheel weight: 38.6%
Rear wheel weight: 59.4%
Front wheel weight: 44.1%
Rear wheel weight: 56.8%
I was toying with a few other factors myself which I think are related. Been advocating having a well balanced bike with as even a weight distribution as possible when building up a bike. The weight distribution of the bike itself will also affect the way one rides and how much you need to shift forward or backward
With increasing travel on the fork, (at least for me) when climbing, its not just putting weight in front but spreading it both forward and in an arc over the whole frontage ( ie with a longer bar). It requires one to go less forward (therefore also allowing the use of shorter stem which is better when going down), keeping more traction on the rear while minimizing any snaking in front at the same time.
Gone through all the TT length vs HT angle and the usual measurements on a bike.
Most important for an all round long travel bike to perform as best going up and down boils down to 2 things for me. Slacker head tube angles need steeper seat tube to climb well. Down side to it is it makes the reach shorter. More so if you need to have higher seatpost height (which I guess would apply in your case at 6'5"). the other is the bar height off the ground.
To compensate I have my seatpost height just ever so slightly lower than optimal and tilt the saddle up a little.
Works to serve both going up and down very well (even when adjustable seatpost is not lowered on the downs)
I dont measure the weight distribution when on the bike but do a after-and-before test before every round of setup changes eg swap out toa different set of bar/stem combo on a particular bike. Testing same terrain/trail on the same day, the differences in handling and traction on the same trail will kind of tell what is good and bad with each setup. No 2 setup will have the exact same body weight distribution, hence I don't actually go and phjysically measure that.
Key magical numbers for me -- front 160mm travel @ 68 degree head angle. Prefer a 73 - 73.5 degree seat tube angle but can make do with different bar stem adjustments if its lesser.
For those weight distribution you listed for your wife-- might want to observe if her bar height is a little too low or maybe the seat is a little too high. Worse case scenario maybe the bike is a little too small overall. Just an observation...I may be wrong.
Best handling position on a mountain bike is when your mass is over the pedal, that may varies from front to rear, 60/40, 70/30, or 80/20, and the other way around if the grade change. If you are riding road bike or TT, then static weight is fine.
MTB require dynamic weight shift, to put yourself in the best position for any given situations. Makers goals is to balance FWD for their given category. XC, AM, DH, FR, DJ, and Trial. XC are center to forward bias, and DH, and DJ are rearward bias. Also stem length and bar width/height also a recommended combo. Long low for XC, short high for DH/DJ, and others in between for other categories.
Users also have the way to adjust the geo to fit their riding style. You choose bikes on how it fit you and your riding style, not FWD. If you are riding flats all the time then it may be something to consider but even that is still not an issues as an upright commuters, and cruiser are just as comfortable on the boardwalk.
It just make no sense to choose a bike by it's predetermine FWD as they varies, furthermore, your body is the heavier than bike, throwing your body mass around would trumps WD of any bikes.
Originally Posted by codename47
You may have raised an interesting question in regards to the topic. I've never put much thoughts to the specific of FWD. To me it's what makes one bike more attractive to another at the same travel, and one that performs better in an all around riding. Good example is the Pivot Mach 5 and Mach 5.7, slight increase in travel but different geo.
In the pic, your friend looks like he just landed and he push the bike out to absorb landing, that's not an attack position his shoulder is almost over the seat tube If this is his normal descending position, he'd have a hard time getting the front bite in some situation, and it's his style of riding that keep him in that position not FWD, or frame size. He looks 80/20 to me. I think the pic may have taken out of context for the topic at hand. That's the position I'd be in if I'm about to manual over ruts
I have 2 pair of identical bikes, Ibis Mojo, and Intense 5.5. Both set are small and med. I ride them and my position does not change much, I can get over to the rear more with Small. At any rate, I'll give this FWD some thoughts. It's pretty cool to break it down like this.
Yeah, I agree that looks like "endo avoidance" position, and not a good position for front wheel traction.
Originally Posted by mimi1885
I've been thinking about this lately too (am currently frame-shopping) and my main question is - with two identical bikes but different frame sizes (the only real difference being a longer TT on the Med frame) - one Small with a longer stem, one Medium with a shorter stem, so the reach to the bars is identical, but the front wheel/tire will be further forward from the BB on the Medium frame, which will give it less weight on the front wheel, so effectively the smaller frame (with a longer stem) will have more-forward FWD, no?
Originally Posted by fsrxc
This depends a lot on what bike you are getting. If XC, your idea is right. AM is a bit difference, What I notice from my bikes aside the slightly longer TT and a bit higher Stand over, is on the small I feel I'm on top, and a bit more in the bike on the Med.
The rides is also as expected, a bit more playful on the small frame, and more stable on the Med. Manual is almost automatic on the small, where it takes a bit more umph! on the med. A lot of the moves of course has to do with the fact that I ride both and used to the way the small frame perform. If force to stay on med I would have adjusted the maneuver to feel more comfortable.
Another factor to consider as well is the Suspension design. For example, DW link design are known for slacker HA, lower BB, but the BB does not drop under acceleration, like some other design. Numbers alone some times does not tell all, gotta get out there and do some test ride, and see how the bike behave and performs on the trail.
Also it's not the amount of rear travel that determine how well it climb. I have a 25lbs 2.75" Klein Adept, that does not climb as well as a 29lbs 6.5" Maverick ML8.
@mimi1885, he's descending all the time like that. There are no rocks, drops or something, I have series of pics of him in that position.
My previous frame was Sx Trail but L, I rode it with the same components like I ride now XL and L was no good for me - I had feeling that all the time I'm far behind the BB with too much weight over the rear, the WB was too small for me.
@fsrxc, with two different sizes frames...I think that no matter what combination you make to bar/stem combo you'll have different FWD(when you're in neutral position on flat), if the frame is small you'll have rear weight transfer, if the frame is big you'll have more weight on the front.
Originally Posted by codename47
There's something wrong with the pic then. I hope that's not your idea of an attack position. It's a bit odd to have the weight over the rear like that. He's squatting on the bike, very rearward bias. May be the camera tilt a bit so it look much flatter than it is, but even on a steep descend you don't get into that position. I'm in that position when I'm about to do a vertical roll.
@Codename47-- a lot of ground to cover on what you have raised here.
But in short from my recent experimentation-- the recent trend of long flat bar meant mainly for DH surprising made the XCish (climb) portions on my ride much easier. Can read more in link below
Warning, its a long post mixed with other things in there with a little theory of mine on measuring the reach of my bikes.
A little outdated as I am now fine tuning the stem length since the new setup ~1 mth back.. riding too much, no time to pen
I think you are overanalyzing the frame's weight distribution.
When a bike is pointed downhill and a skilled rider is pumping the terrain, a certain magic begins to happen which has little to do with weight. If you freeze a DH bike during a race run and slide a scale under both tires, you would definitely find nearly all the weight on the front end. But a bike is a dynamic ride-not meant to be frozen and measured in that manner. At any given moment, a skilled rider is weighting and unweighting the bike (pumping) over various terrain changes-in order to do this, the rider must be centered in a decent position with neutral knees and elbows (not like your friend in that picture). The way a rider uses weight (and inertia) in this position is not necessarily in a vertical plane (like gravity), but in a direction that provides control/traction/acceleration/whatever.
Consider this:what can you do when you need more traction than your weight and tires can provide? You must force the bike into the ground with your weight plus the extra force of pumping your weight towards the ground. Doesn't matter if you are on flat ground or a hill.
A well-designed bike will put you in a position to do this, by giving the best frame geometry/fit/suspension... which will influence weight distribution.... but analysis of a static bike's weight distribution will not necessarily lead to better geometry... does that make sense?
Think about this: a pro DHer on a world cup course rides the bike on terrain he could barely stand on in spots. Anyone can sit on a bike and let gravity (weight) pull them down a hill... a rider with real skills is using his own strength/balance/coordination to do more than what weight alone can do.
So anyway, I think the bike must have the right geometry and fit to put the rider in the right position to ride the way he rides. If you focus too much attention on a certain weight dist percentage, you may get theoretically good traction, but not a necessarily good ride.
You better just go ahead and drop that seatpost down to the reflector... the trail gets pretty rough down there.
@mimi1885, great pictures. I think that he's constantly breaking during descent, that;s why he's in that position.
@anvil_den, interesting information. I tried various bars and stem too:
stems: 40mm Holzfeler/0 rise, RF Diabolus 50/10 rise, 70mm D2/10 rise
bars: Burgtec RW(9/4) 750/40 rise, Boobar(7/5) 780/30 rise, Nuke Proof(9/6) 760/38 rise, and now Reverse(9/5(according to e-mail to them)) 820/15 rise
strange that I climb better with short stem and descend with longer. I didn't figured myself why is that
@Uncle Six Pack
When I moved to XL Sx Trail with the same everything: chainstay, BB drop, HA, SA, suspension type...except longer ETT, I feel that the sag at the fork is different - I compressed it more..why is that - proper fit, different FWD or both?
Can I be better fitted on Knolly D XL with lesser reach that SXT for example because of "better" suspension and cg over BB position?
Is fit with "reach" of one brand with similar suspension travel and HA can be very different than that of other brand?
Last edited by codename47; 11-08-2010 at 05:55 AM.