Unexpected 29? differences : #2, grip and traction
June 22nd, 2006 by Cloxxki“Rolling over obstacles” is such an obvious advantage for larger wheels, even the most fanatic 26″ hold-outs agree on it. But more is going on at the interface between dirt and rubber.
First let’s briefly correct a misunderstanding. Many think that 29″ers naturally have a LARGER contact patch. Actually, give or take some real-life room for error, 2 tires of the same pressure but different diameter will have the same contact surface under a given load. In short : weight and pressure. A 200lb rider+bike on two 25psi tires will result in two ~4inē contact patches.
The kick is here : the larger wheel gets a longer and thinner contact patch. This means the load of the rider+bike is pressing down more focussed on the center tread.
Larger wheel traits from the longer and thinner contact patch :
- Cornering stability (steering feedback damper?)
- Grip
- Traction
Cornering stability
In practice, you’ll find that larger wheels draw smoother lines through corners. In comprison, smaller wheels are wandering all through the corner, the rider constantly (over-)correcting to stay “on line”. With a larger (especially front) wheel, you pick a line, commit, hold on, and exit the corner cleanly. All coupled with a higher entry-, apex- and exit speeds. You’re simply faster, if you dare explore the large wheel limits.
The longer contact patch also makes steering feel much sluggish. Compare this to twisting small and large knifes in cold butter. I feel this effect helps the front end of the bike track more stabily. Any trail debris that will deflect a front wheel, will have a smaller effect on larger wheels, even when keeping the roll-over ability out of the equasion.
The 29″ wheels can reach higher centrifuginal peak forces, with smaller dips from corrections, thus a higher average cornering speed. See the wandering 26″ front wheel making it’s way through the corner.
Sorry for the crappy images here, Paint saves .jpg’s very pourly. Better images VERY welcome, I’ll update!
Grip
It is my belief that having a long and thin contact patch improves pure grip. Compare to ice skates or ski’s. Short wide ones carve nicely, long skinny ones (XC ski’s) won’t carve enough to make it around the Indianapolis Brickyard. But the long ones offer sufficient grip to actually skate with them over snow.
As contact patches are still quite short, and our weight low, turning the wheel of a rolling mountainbike doesn’t ask as much effort as carving with XC ski’s. The effort is actually really minute, no fatigue occurs because of it, many report less fatique with larger wheels. 26″ holdouts fear the 29″ handlebar will be so hard to turn that it hurts cornering speeds. You decide for yourself.
Traction
Just one small example which could explain the numerous reports from first 29″ rides where the test subject found him(her)self cleaning that one nasty unrideable steep hill.
The longer contact patch is better able at keeping contact with the firm surface to create required friction with to not have the rear wheel spinning. A stone, twig or pine-cone will less likely cause loss of contact with the surface on a hill, causing the forced dismount we’ve all encountered.
Another crappy pic. It should have shown more clearly how the smaller wheel loses contact with the trail when rolling over the round trail debris.
Just to round off the story : in practice 29″ riders tend to run slightly lower pressures on their 29″ bikes that with otherwise identical 26″ setups, which only increases all the above explained performance traits of larger wheels. Their bikes require even more muscle to turn the handlebar, but apparently it’s worth it to them.
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That cornering stability figure seems a little fishy. Is it based on data or just an ilustration to what you’re surmising in that seciton? I can see bashers bashing you for that. I think you’re right, but I’m not sure it does us any good w/o data.
We need someone to make a divice that will log steering inputs and perhapse torque at the steerer to prove what you’re saying. Maybe a modified steering damper that copuld be moved from bike to bike. Proving that less steering input is required with the big wheels is all you need to do.
-M
I thought of an easy way to measure it: mount a video cam to the stem aimed at the head tube. Draw a line down the middle of the lens and a series of lines vertically on the head tube. As you go through a turn you can count the corrections as the line on the screen passes the lines on the head tube, and/or you see direction change. Bike geometry won’t matter, but the rider and section of trail would have to be the same for a comparison to be valid.
-M
I agree, but lack of scientific research will always hamper bike design. Bikes sell cheap with low mark-up, and sell in relatively small numbers.
Your test sounds like it will work well. Of course the steering in/output sensor would be way cooler.
To me it’s very telling how well I can ride my 29″er one-handed if I have to, even no-handed off-road.
I’d like to add one more dramatic benefit of the longer contact patch: much straighter tracking through loose conditions: mud, snow, deep sand. Not that everyone runs into these conditions in their everyday riding, but the difference is night and day.
Typically for deep, loose conditions one chooses a wide tire and low pressure to allow for a large (and fat) contact patch. The widened contact patch helps you float on top — but also creates a “plow” effect that impairs straight-line tracking. Given the same tire width and pressure, a 29″ tire allows you to lengthen and narrow the contact patch without reducing its size, thus preserving the float, while reducing the “plow” effect to improve straight-line tracking. There is an additional reduction of the “plow” effect due to the reduced angle of attack, further improving straight line tracking.
Thanks Glowboy! That shoudl really have been int here, too. I might add it with credits to you.
In sand pits, 29″ is just cheating. Even dropoffs landing in a sand pit are no problem.
29″ does seem to lose advantage the sand/mud is thick and there are deep tracks already. At least it asks a lot of handlebar input to steer the bike. Then, cyclo-cross bikes are all about picking a track and using it to go fast.
Your sketch showing traction over loose gravel is correct for the long axis of the contact patch, but the 26er has the advantage along the short axis, meaning it has better cornering traction in loose gravel. I have noticed this some on my own 29er and I believe that is why 29er tires require larger taller side knobs to compensate. My 2 cents.
Interesting comment, thanks! Although it sure sounds logical, I must honestly say I had not thought of it, and had not noticed any negative difference. My first 29″ front tire was a Nanoraptor, and I soon learned not to use it for loose gravel. When I got the Bontrager AcX, that instantly became the best gravel cornering tire I had ever tried. At least, on par with wider 26″ tires with softer compound, such as the Schwalbe Big Jim and King Jim.
Where does a contact patch become so narrow that lateral grip over loose is lost, and where does it get so narrow that it’ll slice up the trail and not move a mm sideways?
My gut tells me the longer contact patch also helps in loose gravel for lateral grip. It may still contact firmer ground fore and aft of the critical marble?
Interesting essay, though the ski analogy is way off. Side cut and camber are what cause skis to turn or not turn. Longer cross country skis need to be long to create a greater contact patch (and flotation) to compensate for the dual camber which picks a good portion of the middle of the ski up off of the snow when both skis are equally loaded. (yes this is a massive oversimplification) In short, ski’s and wheels don’t really work the same way.
The trouble I have with this essay and many many others on the subject of 29ers is the same as an earlier comment by Wolfy, there is no data. What often seems logical is not necessarily factual, the only reliable measure of significance is a properly planned experiment.
The June 06 edition of Mountain Bike action took two identical Fishers, one a 29er and one a standard 26 wheeled bike and faced them off performing several timed mountain bike maneuvers. The 26er won the vast majority. I was surprised at some cases where the 26″ bike was rated higher until I looked more closely and discovered that both riders in the test were well below 6′ tall. The riders had a lot of complaints about the long wheel base of the 29er, which is not surprising considering their height. So, I surmised that yet again, we get another sub par comparison to go along with the power tap data from a 26″ fuel vs a Dos Niner. Though, I think they were on the right track and had they included a diversity of rider heights, they might have had something of value.
Sadly, many of us are subject to making a bike purchase on data alone, because there are so few examples of 29ers on showroom floors waiting for test rides. I for one am holding out on my next mountian bike purchase until I either test ride a bike that really grabs my attention, or somebody somewhere stops subjective testing and writing on the difference between 29ers and 26″ wheeled bikes and does some exhaustive scientific testing.
Thats my 2 pennies for what its worth…
I read the same June 06 article in Mountain Bike Action. I had picked up the mag while waiting for my flight to Punta Cana for a week of fun in the sun. I had also only 3 days ago purchased my 2005 x-caliber and hadn’t the opportunity to take it off road yet. I had read lots of reviews and opinions on 29er’s, but this article was the first wiff of any type of pseudo scientific comparison. I was stunned to read the 26″ came out on top, especially in climbing. The article had me a little discouraged about my recent purchase and a bit stupid for being too quick to buy into the subjective writing. I was rigged to get on my bike asap on my return from cervesas\steak\lobster and the inevitable Dominican Diaharrea and experience for myself what a 29er can do. The trail I ride goes by the name of Glen Major\Walker Woods and they consist of your typical short technical climbs to long rocky straight climbs as well as a downhill portion with 90mm drop offs as well as lots of single track, fireroad.. etc. I’ve been riding these trails for 6 years mostly on a 14.5 Devinci Phantom until it got ripped off. I am 165mm(5′5″) and my x-caliber is 15.5 frame. So, the bike it is alot bigger than I’m used to. The FACT of the matter is, I came back plump from a week long feast and ready for slaughter. On my break-in ride, the climbs were cleaned, the hits were manageable and the ride was outstandingly fun! There is no scientific rebuttal from me towards the article in MBAction. I just know that there is no going back when you’ve ridden the nectar of the gods and FUN is what it’s all about
Now, get out and RIDE!
@mahcus : cm’s, right?
Even I ride 90mm drop-ofs without fear!
@matt : thanks for the response!
On the MTBR forum there’s a perfect opportunity to get in touch with a rider near you with a bike your size. Most 29″ owners don’t mind sharing thei bike for a good ride, to help a fellow rider make a better choice for himself, and not have to gamble on a new wheel size sight unseen.
Many have though, bought a 29″er wihout ever riding one. I had only rolled a couple Nishiki’s in too small sizes before I bought a used Fisher.
It seems that the large mayority doesn’t regret their purchase, although some do get their frame size wrong, same as happens with 26″ers.
The first helpful 29″ rider with a bike in your size can’t be more than a couple hours travelling away, set an ad and go for a ride on new-to-you trails! Worst case, you hate the 29″er, but got to ride someone else’s favorite trails, partly on your own trusted bike.
thanks Cloxxki.. cm’s indeed.
Great advice Cloxxki, I appreciate it. And thanks to Mhacus for reminding me about the fun part. I tend to get caught up with data and forget about the aesthetic end of things.
did i read a different comparison of gary fisher mtbs? i just recently read about two paragons 26 vs. 29 where it was shown that the timed differences on the same course were negligible, though the riders liked the 29er more.