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Pedal strikes on 27.5"

Old 06-13-24, 02:13 PM
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Pedal strikes on 27.5"

Three years ago I sold my 2016 Kona Process 111 (29er) because I bought a Salsa Cutthroat (29er), figured I'd never go mountain biking again. Front travel = 120mm, rear travel 111mm.

Four days ago I bought a used 2018 Giant Trance 2 (27.5") and holy crap the pedal strikes!! Just todays 20km ride (that I rode many times on the Kona) I smacked a pedal 7 times and had to be really careful. I set up the sag to be 15% and I'll check it again, but does 3/4" of an inch make THAT much difference or is it the geometry of the suspension linkage dropping the BB more or perhaps just due to more travel? Travel on the front is 150mm and the rear 140mm.
I rarely had pedal strikes on the Kona and none on the Cutthroat.

Today was the second ride on ATV trails, so VERY rocky as the ATVs have the trail scoured of dirt and its all loose, fist-sized rock and baby heads. The first ride was on a system of mountain bike trails maintained by our provincial cycling organization and no pedal strikes that I recall.
https://www.trailforks.com/region/th...-topsail-lake/

Has anyone experienced this going from a 29" to 27.5" wheels? I can't see a solution other than check the sag again, but perhaps I'm missing something.

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Old 06-13-24, 04:59 PM
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What length cranks do you have? Do you maybe have wider pedals as well which can be a factor. I would try shorter cranks maybe or take it to a shop and let them know and they might say they same.
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Old 06-13-24, 06:49 PM
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Originally Posted by veganbikes
What length cranks do you have? Do you maybe have wider pedals as well which can be a factor. I would try shorter cranks maybe or take it to a shop and let them know and they might say they same.
175mm cranks, on both Kona and Giant. Pedals are the same; transferred from Kona.
but how short? 170mm, would 5mm matter that much?
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Old 06-13-24, 07:03 PM
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Originally Posted by digger
175mm cranks, on both Kona and Giant. Pedals are the same; transferred from Kona.
but how short? 170mm, would 5mm matter that much?
I would probably do 165 that is generally what I run on my 650b stuff I have pedal strikes with. 175 is rather long kind of an old school holdover from vintage MTB. Obviously if you are a super tall individual then potentially a long crank like that would be more helpful but I find 165 is a good length.
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Old 06-13-24, 09:48 PM
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I'm running 165s on my enduro bike (29"er 160mm full suspension.) I occasionally do get pedal strikes, but that's just par for the course on certain terrain IMO. My XC bike (120mm FS) has 175s and I definitely have to be more cognizant of potential pedal strikes on it.
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Old 06-13-24, 11:44 PM
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Pedal strikes come from low bottom bracket height and long cranks. You can have a low bb on a 29er and high one on a 27.5er. Both my bikes are 27.5 and one has a low BB and I put 165mm cranks on it which helps a lot.
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Old 06-14-24, 04:29 AM
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My only experience, after 30 years of cycling and owning a dozen bikes, is 175mm cranks with the exception of an old mid 80s Raleigh my brother gave me that had 165mm cranks, and that was 20 years ago. I remember thinking that these 165s sure do make small circles.

i went and watched a number of YouTube videos and they’ve indicated that shorter cranks might be worth a try.

I also went and checked out the BB height of the Kona Process and Giant Trance and tell me if my calculations are accurate please. Using ETRTO sizes of 622mm and 584mm respectively:

Kona Process = BB drop of 35mm and with a 29” wheel means a BB height of 276mm.
https://archive.konaworld.com/archiv...rocess_111.cfm

Giant Trance = BB drop of 15mm and with a 27.5” wheel means a BB height of 277mm..
https://geometrygeeks.bike/bike/giant-trance-2-2018/

The above does not include tire size. The Process ran 2.3” and the Trance are 2.35”, the 2.35” tire should (?) have a slightly higher sidewall height then. So, in fact, the Process and Trance are very close in BB height, with the Trance probably slightly higher BB due to tire size.

So, either the sag is off which I will check today or I’m just out of practice. Oddly, my Salsa Cutthroat has a BB drop of 70mm meaning a BB height of 241mm using 2.2” wide tires. Never get pedal strikes but I’m not on as rough terrain.
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Old 06-14-24, 08:35 AM
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You shouldn't get pedal strikes with 15% sag IMO, but my friends at the LBS are using 155mm cranks. Of course, you need to develop your own "best case", and there are manufacturers who rent adjustable length cranks to determine that.
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Old 06-14-24, 09:00 AM
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165mm cranks will help for pedal strikes and your hip and knee joints.

BMX pedals or SPDs will also help.

IIRC, the Trance can accommodate a fairly wide tire. Wider tires are also taller, and that can also help.

A combination of all 3 might make the bike rideable, or just sell it and get something that works.

Last edited by Polaris OBark; 06-14-24 at 09:04 AM.
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Old 06-14-24, 10:23 AM
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Low BB height combined with smaller tires...2 things working against you.

Its bottom bracket drop is 15 millimeters, which lowers the Trance into the pedal bashing club.

https://www.pinkbike.com/news/review...-trance-2.html

Giant doesn't want to publish BB height for some reason but I'm guessing it's pretty low. I won't buy a bike that has BB height of less than 340mm. My current trail bike has BB height of 349. Zero pedal strikes.
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Old 06-14-24, 10:31 AM
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Originally Posted by prj71
Low BB height combined with smaller tires...2 things working against you.

Its bottom bracket drop is 15 millimeters, which lowers the Trance into the pedal bashing club.

https://www.pinkbike.com/news/review...-trance-2.html

Giant doesn't want to publish BB height for some reason but I'm guessing it's pretty low. I won't buy a bike that has BB height of less than 340mm. My current trail bike has BB height of 349. Zero pedal strikes.
But the Kona Process I had has 35mm of BB drop, isn’t that less of a BB height than the Trance2?
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Old 06-14-24, 10:40 AM
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Originally Posted by prj71
Low BB height combined with smaller tires...2 things working against you.

Its bottom bracket drop is 15 millimeters, which lowers the Trance into the pedal bashing club.

https://www.pinkbike.com/news/review...-trance-2.html

Giant doesn't want to publish BB height for some reason but I'm guessing it's pretty low. I won't buy a bike that has BB height of less than 340mm. My current trail bike has BB height of 349. Zero pedal strikes.
They probably don't want to publish BB height because they get tired of the emails from people complaining when it's 5mm off from published.

How to you measure BB height anyways. No sag? 20% sag? 30% sag?
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Old 06-14-24, 10:40 AM
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BB height may be taller due to 29" tires.
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Old 06-14-24, 05:02 PM
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Originally Posted by prj71
BB height may be taller due to 29" tires.
The BB drop on the Process is 35mm, 15mm on the Trance. Despite wheel diameter it appears the BB height is the same.
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Old 06-16-24, 05:23 PM
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Assuming the frames are designed around a given wheelsize in mind, there is no reason a 27.5” bike would necessarily have a lower BB than a 29er.
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Old 06-18-24, 11:11 AM
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I used to get pedal strikes, then I learned to just not pedal at certain points of the trail if I am in a rut, a steep hill, or jagged rocks. I mostly rode a couple trails so I knew when or when not too pedal. I run 175's on a 26in bike.
If you are running wide flats, and small tires those would be negatives. If you are sitting deeper in the travel that will get ya too. I would give it 5-8 rides or so and see if you adapt, or it's time for new parts.
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Old 06-18-24, 12:35 PM
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Originally Posted by LesterOfPuppets
They probably don't want to publish BB height because they get tired of the emails from people complaining when it's 5mm off from published.

How to you measure BB height anyways. No sag? 20% sag? 30% sag?
No sag. That's common knowledge.
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Old 06-18-24, 12:37 PM
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Originally Posted by digger
The BB drop on the Process is 35mm, 15mm on the Trance. Despite wheel diameter it appears the BB height is the same.
Have you measured the BB height on both to confirm?
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Old 06-18-24, 12:41 PM
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Originally Posted by slow rollin
I used to get pedal strikes, then I learned to just not pedal at certain points of the trail if I am in a rut, a steep hill, or jagged rocks. I mostly rode a couple trails so I knew when or when not too pedal.
That's what a lot of folks do to counter what the stupid bike industry has done. They somehow thought that lower BB = more stable and they tried to sell people on that fact..."lower, longer, slacker, blah, blah blah"

The reality is the center of the gravity when you are on a bike is around your waistline and can easily be changed by standing up, sitting down or using the dropper post. Lowering the bottom bracket a few millimeters near the ground has a negligible effect on stability but has a big effect on pedal strikes vs. no pedal strikes.

MTBS are off road machines. Much like driving a car...You aren't going to take a sedan down some old logging or forest service roads. You want to be driving something with ground clearance so you don't scrape the bottom such as a truck, jeep or other high clearance vehicle.
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Old 06-19-24, 02:35 PM
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Originally Posted by prj71
Have you measured the BB height on both to confirm?
No. I no longer have the Kona Process, but I did the math by subtracting the BB drop from 1/2 the wheel diameter.
Appears to be the same.

Today I checked the sag and it seems I did not have enough air in the shock. It was 210psi but I need 250psi. THAT might be the issue.
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Old 06-19-24, 05:08 PM
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Shorter crank arm is actually a good idea if you have a shorter inseam.
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Old 06-22-24, 12:43 AM
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Originally Posted by digger
175mm cranks, on both Kona and Giant. Pedals are the same; transferred from Kona.
but how short? 170mm, would 5mm matter that much?
170 good, 165 better. Change out the air spring to increase fork length to 160mm. If you're truly desperate, as I was, put one offset bushing the wrong way 'round in the rear shock. Clean the bushing and mounting hole in shock real good with solvent or rubbing alcohol. Tooth up the outside of bushing only with abrasive or file then coat with Locktite. Red or blue you choice. Press it in and then lube the mounting bolt real good it'll stay put I promise.
The 1cm increase in fork length will translate into, I forget what mine ended up, 5~6mm in bb height, the offset bushing gives you another 3mm.
Then add 1cm for going down to 165 cranks the result is almost 2cm's higher and say bye-bye to pedal strikes. Unless you a real klutz then you'll just have to learn how to finesse the rocky sections.
Btw it's not the wheel size but frame geo that dictates bb height.

Last edited by tungsten; 06-23-24 at 12:19 AM.
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Old 06-22-24, 07:11 AM
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Originally Posted by prj71
That's what a lot of folks do to counter what the stupid bike industry has done. They somehow thought that lower BB = more stable and they tried to sell people on that fact..."lower, longer, slacker, blah, blah blah"

The reality is the center of the gravity when you are on a bike is around your waistline and can easily be changed by standing up, sitting down or using the dropper post. Lowering the bottom bracket a few millimeters near the ground has a negligible effect on stability but has a big effect on pedal strikes vs. no pedal strikes.

MTBS are off road machines. Much like driving a car...You aren't going to take a sedan down some old logging or forest service roads. You want to be driving something with ground clearance so you don't scrape the bottom such as a truck, jeep or other high clearance vehicle.
Lower BB does give more stability especially in corners. Its not an issue of where YOUR center of gravity is, but a matter of where your weight is being transferred to the bike (the bottom bracket)

And really, even the current bikes have plenty of BB height. Its just a matter of getting a sense of where your pedals are.

Every time I get an MTB with a lower BB, I get some pedal strikes early on. Than after a month or so I fully adapt and get no more than I used to back on the days of much higher BBs.

Longer Lower Slacker is mostly “blah, blah, blah” to those who have not spent some time on modern geo, or are not doing much actual trail riding. Better climbing, better descending, better cornering.
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Old 06-24-24, 01:36 PM
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Originally Posted by Kapusta
Lower BB does give more stability especially in corners. Its not an issue of where YOUR center of gravity is, but a matter of where your weight is being transferred to the bike (the bottom bracket)
That would not be true at all. Stability comes from your center of gravity (roughly around your waist area) which can be adjusted by moving around the seat or by having a dropper post. The few mm difference in BB height near the ground means squat for stability.

https://davesbikeblog.blogspot.com/2...et-height.html

My current trail bike is 349mm. The Transition Spur is 335mm. That's a difference of less than 1/2" which is negligible near the ground for stability but a big factor whether or not you get pedal strikes.



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Old 06-25-24, 03:02 PM
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Originally Posted by prj71
That would not be true at all. Stability comes from your center of gravity (roughly around your waist area) which can be adjusted by moving around the seat or by having a dropper post. The few mm difference in BB height near the ground means squat for stability.

https://davesbikeblog.blogspot.com/2...et-height.html

My current trail bike is 349mm. The Transition Spur is 335mm. That's a difference of less than 1/2" which is negligible near the ground for stability but a big factor whether or not you get pedal strikes.
In my experience, 1/2” does make a difference in stability.

Your view seems to be at odds with nearly all mountain bike designers and also with the MTB riders that have been riding bikes for many years and have found this to be true.

I have absolutely found this to be true.

That post link to has nothing to do with mountain biking. And honestly, even for road bikes arguments are pretty damn weak.

The issue is that in moutain biking you are often side loading your bike. Most commonly, when carving a corner and leaning the bike farther than your body. You’re not pushing straight down through the bike, rather you’re pushing a little sideways against the bottom bracket. The farther up from the ground the bottom bracket is, the less leverage you have over the bike’s pitch.


Last edited by Kapusta; 06-25-24 at 09:25 PM.
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