Ask Pinkbike: eMTB Sag, Stem Length vs. Reach, Fork Noises, & Saddle Comfort
Here at Pinkbike, we get inundated with all kinds of questions, ranging from the basic "Can I have stickers?" to more in-depth, soul-searching types of queries like if you should pop the question or what to name your first child. Ask Pinkbike is an occasional column where we'll be hand-picking and answering questions that have been keeping readers up at night, although we'll likely steer clear of those last two and keep it more tech-oriented.
Do eMTBs require firmer suspension?
Question: @JudyYellow asks: "A co-worker and I were discussing eBikes, suspension spring rates, and how that affects the sag rate and an interesting question came up. Should you over spring an e-bike to accommodate for the extra weight of the bike?
Normally to achieve the correct sag rate you would shoot for approx 30% of shock stroke. Sag is mostly determined by rider weight and to set sag you jump on the bike cycle the suspension to charge the spring, add/remove air to achieve ~30% of stroke (on a shock). Go ride....
Bike weight is not usually factored into the sag rate. However... when riding an e-bike you have approx 10lbs or more of extra bike weight which should affect the suspension when landing jumps, drops etc since the shock spring has to soak up the system weight (bike + rider). Is it only the rider's weight affecting the shock spring when landing drops, etc? This is the missing piece I don't have...If total system weight (bike + rider) affects the shock should I go for a 25% sag rate instead? 20%? Same applies for coil springs as well.... Do you go 50lbs up on your coil shock spring if using an e-bike?
 | Regardless of the bike's weight, I would still aim to start by setting up the rear shock with 30% sag (the percentage of the shock stroke used by the system: rider + bike weight, on level ground). Don't forget to include your rider gear in the setup equation. When manufacturers set out to build eMTBs, the higher forces are taken into account and the kinematics are tuned to accommodate. The weight of these bikes changes the rider-to-machine ratio that mountain bikers are more accustom to, which is why you may see more opinions from motorsport suspension gurus weighing in on the topic. The important thing to keep in mind here is the front and rear balance of the suspension. Most fork manufacturers suggest 15-20% sag. These sag percentages are just guidelines and can be altered, depending on how you prefer the bike to handle. Don't be afraid to play with these numbers, although it's always good practice to keep notes on what changes you make, in case you want to go back to the original settings. In many cases, you'll end up running more air pressure in your fork than you would on a non-motorized bike - 10-20 psi more, depending on the fork. That's due to the additional loads that end up being put on the bike's front end - like you mentioned, you now have additional bike mass along with your own pushing down on that fork. |
eMTBs can undergo some large forces, but setting up the suspension is still all about front to rear balance and tuning the bottom out control.
Can you use a longer stem to increase reach?
Question: @fire-munki asks: "Not sure if this is bike fit or bike modifications! Currently I’m on a 2016 Cube Stereo 140 (18") which is a fun enough bike but over the years I’ve found myself getting less enamoured with playing in the woods doing the same run each time (mainly since I’m to scared to ever really jump/hit drops) so have started heading out further on things closer to XC jaunts.
In an effort to make the bike a bit more suitable I’ve got a newer Fox Float with working lockout, got the stem right on the admittedly tall top cap and lower than stock rise bars. Now looking to make it feel less cramped so looking for longer stems (stock 45mm 0 degree), the great ebay has a few options at 50mm and 60mn 0-degree options. What I’m worried about is wrecking the handling with such changes, my saddle is pushed fairly forward to work on keeping the front down when climbing.
Any opinions either way? The 50mm is pretty much like stock so can’t see it changing it much but the 60mm might be too much? Or just what I need!"
| The 2016 Cube Stereo 140, size 18", has a reach of 428mm, which is quite short compared to a typical size large these days. Changing the stem for something longer will increase the effective reach, essentially, the horizontal distance from your feet to your hands, but it is usually a band-aid fix for riding the incorrect size of bike. You will also notice the slower handling while turning the bike. Since climbing is a main objective of yours and a stem is a relatively inexpensive and non-labour intensive component to swap out, I suggest giving that 60mm or 65mm stem a try. It will help keep that front wheel on the ground while charging uphill and give you a bit more room in a seated position. |
Charles Murray broke onto the EWS scene last year with a sixth place. At 183cm tall, his size medium frame and 65mm stem was an uncharacteristic choice, but didn't slow him down.
Trouble with a new fork spring install
Question: @dirtflipper asks: "I installed a new Marzocchi Bomber Z1 Coil. Before installation, I changed the stroke length from 170mm to 150mm. Everything works great with the shock except two things. One, the shock makes a creaking noise as it moves through its stroke. Two, when the shock compresses/extends ever so slightly it seems like there is a couple mm of play in it. So, if you go over a bunch of very small rocks, the shock does this weird chattering thing as it moves quickly in what seems to be an undamped few mm of stroke. Are either of these normal for a coil? I have never had one before, so I do not know."
| Creaky noises from bikes can be a nuisance to chase down. If you are confident that the noise is coming from the front of the bike, disassemble the head set and make sure all the surfaces are clean, re-greased, and that the bearings are running smoothly. Any metal on metal interface with dust or dirt between them is suspect; sometimes the crown race on the steer tube, frayed cable housing, or the handlebar/stem interface can be the cause. As pointed out, the Bomber Z1 is a coil fork. During the recent work to alter the stroke length, the spring preload would have been unwound all the way to add a spacer and reduce the travel. Without any preload on the spring, there can be a few millimetres of movements even without any force applied. By turning the preload dial on the rider's left of the crown clockwise, you will start to preload the spring. There should not be any movement as you described. Once the coil has contact with the preloader, you can tune the spring rate slightly to your preferred sag. Double check that the front and rear sag is balanced in the 15-20% and 30% region, respectively. Note that preloading the spring too much will make for a harsh ride. If you require less sag and have turned the preloaded more than two full turns from the first engagement with the spring, you may need to go up a spring rate. |
Coil springs are a popular choice for their small bump sensitivity and linear nature, but they can rattle inside the stanchion if they are not preloaded enough. Over-preloading them can causing the spring to bind and bend out of shape as well.
Getting comfortable in the saddle
Question: @witterisms asks: "I got a new bike at the end of last year (YT Capra), and now the evenings are much longer, I've been able to spend more time on the bike/in the saddle - but increasingly hitting the 2-3 hour ride mark it start to get sore when I am in the saddle, and then sore the day after. As far as I can tell I have the saddle in the best position possible, and the body position is comfortable when riding.
What's best to invest in? Better padded shorts (quick look on CRC shows £30+ for seemingly good shorts) or new saddle - I have seen a lot of Enduro riders on Fabric saddles, they seem to start at £45, but obviously more on the market.
From what I know, its hard to test either. All help appreciated. "
| It is fair to say that MTB fit is less developed than in the road cycling world. We move around a lot more on a mountain bike due to the terrain we cover, but that doesn't mean you shouldn't be comfortable. Chamois shorts are designed to promote blood flow and reduce pressure on the sensitive nerves down there, however, that won't eliminate the problems caused by an incorrect saddle position. Generally, the more you pay for these shorts, the better quality the foam and lycra materials will be. And, what is the price of comfort? A quality saddle that has been researched to support your sit bones is also part of this puzzle. Specialized, SQ Labs, and Ergon are some of the brands that come to mind as leaders in this game. Retül is a brand designed to find the perfect fit using digital equipment. Their devices can measure your sit bone width and suggest a saddle based on your riding requirements and usually offer a demo saddle program too. Let's get into some fit talk. Assuming that you are on the frame size that the manufacturer recommends, start by finding your max saddle height. Your knees should have a slight bend when your feet reach the 6 o'clock position of the pedal stroke. If you feel like your are stretching or reaching for the bottom of the pedal stroke, the saddle is too high. If it's is too low, your quad muscles will quickly tire and you could feel some knee pain. The saddle angle is also critical. A level saddle is the best place to start, but if your handlebars are lower than your max saddle height, tilting the nose slightly lower than the back of the saddle can help reduce any pressure points. You'd be surprised how comfortable this can be. It might feel more like a perch rather than a seat. Keep in mind that your sit bones should be on the widest part of the saddle. I would even go as far as seeing a physiotherapist that specializes in bike fits if you are still struggling to find that perfect position. |
Saddle style, width, height, and angle are all factors in finding the most comfortable position. There are a lot of factors in play here.
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Just as an example: you have a spring with a springrate of 5 N/mm. Now you preload it with 50 N, so it already sits 10 mm in its travel. Now the force to actually deform the spring (elastic torsional deformation) any further has to exceed those 50 N. Otherwise it's more like a rigid fork at this point.
A spring with 2" of travel and a 400lb rating requires 400lbs to compress those 2", always. But, where you are in that 2" of travel determines how much weight is being supported by the spring. So, on this 2" stroke 400lb spring, 20lbs will compress the spring 1/10th of an inch, 40lbs will compress it 1/5th of an inch, 60lbs will compress it 3/10th, etc. So, when you dial in the preload ring 1/10th of an inch, you're literally "pre-loading" the spring with the equivalent of 20lbs of force to create a support platform to set your sag for positive/negative suspension travel... but that's completely independent of spring rate and every 1/10th of inch of travel still requires an additional 20lbs.
It was broadly similar for me too. On Ergon's saddle width chart I'm right on the borderline between S/M and M/L, and using saddles that wound up being around the size for the S/M saddles wasn't super comfortable for me after a few hours riding, whereas the M/L sizes were primo. Beyond that it was finding the smallest width I could get away with (purely for clearance reasons). It turns out for me that was the SDG Belair 3.0, but that was only after I'd tried a few others. It's definitely worth experimenting with, but mainly once you've got a rough idea of what width to aim for.
I had this happen to my Knolly.
I tried everything and it wouldn't move.
Luckily it had a Di2 port near the BB. I was able tap (smack) it out with a piece of copper pipe.
Obviously this destroyed the dropper post but I saved the frame.
Pull the bb, try to pour some Coke down the seat tube, let it sit like that for a couple days. You should see some weeping on the seatpost to let you know it’s worked it’s magic.
Used this truck lots on seized spark plugs in aluminium heads.
And really, if you're out buying ATF, you might as well pick up straight acetone for the mix.
Us 225lbs people who have been riding normal bikes forever: Yeah ok...
15&30 f&r when seated
If you want to charge the front wheel and have front tire grip, 30% is way too much.
Try reducing sag to 20% and reducing LSC. If you have tokens try removing some.
Steering hasn't been buggered up to much either which is good.
Oh and you consider telling Pinkbike to f*ck off as a “contribution to a discussion”? You’re laughably hypocritical.