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Suspension set up and its effects on handling


Jono Waddell
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To all the seasoned MTB riders out there i have a question.

 

I recently built up a full sus MTB (GT Sensor) from my old hartail. Now the sensor is designed to have 120mm travel, but my old fork from the hardtail only has 80-100mm.

 

I have always ridden it this way, and its what ive become used to, but lately i have though that perhaps its not right. Yesterday riding down some rocky decents, the bike felt terrible, twitchy and all over the show. I had a mate take it down and they said that it was one of their scariest bike experiences.

 

Now the question, will the mismatched suspension travel make a huge difference? (Perhaps the poor handling will explain my dislike for anything remotely technical).

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The twitchiness is explained quite simply: a short fork will result in the head angle being much steeper than it was designed to be, which is the nervousness you're feeling. A longer fork will without any doubt fix that.

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What MH said. Also, a longer shorter stem can also add to the twitchiness, so can a narrower bar.

Edited by Capricorn
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A shorter stem would make it more twitchy, a longer one will make the turning in a bit 'slower'.

 

Get the proper length fork for the bike, it will handle much better.

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Nope, still not on the same page. " Twitchy" is a negative word in this sense and to say that without knowing 1) what length stem and width bar he is running or 2) as a act on it's own doesn't make sense. Shorter stem will sharpen up steering and bike control when paired with a wider bar. (Roughly add 20mm to bar width for every 10mm of stem length dropped - NB: pending on starting point of both), but by no means make it "twitchy".

Twitchiness will come most likely stem (see what I did therewink.gif) from:

1. Fork being the wrong travel

2. Narrow bar paired with long stem

3. Handlebar height (could be HB Rise, steerer spacers, stem rise, HT length)

 

Moving kit over from old HT to sensor it could even be all of them. Could also be that your fork's sag and/or compression has not been set properly.

 

THEN. What you are feeling are not because of mismatch suspension in numbers. With too little travel up front you have reduced your bike's head angle, shorten it's wheelbase, dropped BB height & steepen ST angle. If the bike was designed around a 80/100mm fork with 120 rear wheel travel, the designers would've compensated. Slacker head angle and slacker seat tube angle.

 

To better explain. My bike is designed to have 67 degree head angle with 160mm fork. With a 140mm fork on there (dependent on fork's axle to crown length) head angle will drop to 70 degree head angle. This is race bike territory and definitely not something you'd wanna throw down the mountain. 100mm fork with headset cup spacers will sort the head angle, but I'll lose travel. So I'll get rid of the twitchy feeling and the feeling that I wanna go over the bar, but lose control over rougher stuff due to travel.

 

* I have a super short stem on my bike and the handling is by no means twitchy. Razor-sharp, yes.

Edited by The Crow
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)

THEN. What you are feeling are not because of mismatch suspension in numbers. With too little travel up front you have reduced your bike's head angle, shorten it's wheelbase, dropped BB height & steepen ST angle. If the bike was designed around a 80/100mm fork with 120 rear wheel travel, the designers would've compensated. Slacker head angle and slacker seat tube angle.

 

To better explain. My bike is designed to have 67 degree head angle with 160mm fork. With a 140mm fork on there (dependent on fork's axle to crown length) head angle will drop to 70 degree head angle. This is race bike territory and definitely not something you'd wanna throw down the mountain.

 

That's what I was referring to yes, just not in so much nice detail (was typing on a tablet ;) )

 

On my 140mm bike, dropping the U-turn on the fork from 140 to 110mm has a drastic effect and brings the head angle to over 71deg (it's steeper to start off with than most AM bikes, but you get the point). Awesome for slicing tight singletrack when climbing, but not so great for bombing down stuff, as you've said...

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My bike is designed to have 67 degree head angle with 160mm fork. With a 140mm fork on there (dependent on fork's axle to crown length) head angle will drop to 70 degree head angle.

 

Crow, is it not for every ±20mm that HA is changed by 1 degree?

 

I read that in quite a few places... the general rule of thumb..

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Ok, so it seems that the general idea should be to start with the fork? Is it possible to modify a fork (I have a Rock Shox Recon) to the 120mm.

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I am seeing a lot of results for a Recon 120mm, even a few for 130mm.

 

Give Ian at Cape Cycle Systems a call, he should be able to guide you.. 021 761 3528

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It all depends on the fork manufacturer and model as they don't all have the same A2C and a lot of effort goes into designing the new breed of 160/170/180 single crown forks with an as short a A2C as possible to keep the front end down.

 

Yeti SB-66

150mm Fork: 67

160mm Fork: 65.9

http://yeticycles.com/#/bikes/SB66/1/

 

Ibis Mojo HD

150mm Fork: 68.5

160mm Fork: 67

180mm Fork: 66

 

Reason the Ibis is more than the Yeti when going from 160 to 150 is because the guy went from Fox to RS and the Yeti is Fox / Fox. 160 to 180 is not that much because the 36 180mm Fox has a very low A2C for the amount of travel. Going 140 to 120 a couple of years ago would give you a drop of 2 degrees.

Edited by The Crow
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Ok, so it seems that the general idea should be to start with the fork? Is it possible to modify a fork (I have a Rock Shox Recon) to the 120mm.

 

Possible. Depends how old it is. Back in the day they were to be a certain length. Now they most recons are 120mm and then just spaced down to the amount of travel needed. Like POS said, give Ian a call. He will need details on the fork though.

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Nope, still not on the same page. " Twitchy" is a negative word in this sense and to say that without knowing 1) what length stem and width bar he is running or 2) as a act on it's own doesn't make sense. Shorter stem will sharpen up steering and bike control when paired with a wider bar. (Roughly add 20mm to bar width for every 10mm of stem length dropped - NB: pending on starting point of both), but by no means make it "twitchy".

Twitchiness will come most likely stem (see what I did therewink.gif) from:

1. Fork being the wrong travel

2. Narrow bar paired with long stem

3. Handlebar height (could be HB Rise, steerer spacers, stem rise, HT length)

 

Moving kit over from old HT to sensor it could even be all of them.

 

Could also be that your fork's sag and/or compression has not been set properly.

 

THEN. What you are feeling are not because of mismatch suspension in numbers. With too little travel up front you have reduced your bike's head angle, shorten it's wheelbase, dropped BB height & steepen ST angle. If the bike was designed around a 80/100mm fork with 120 rear wheel travel, the designers would've compensated. Slacker head angle and slacker seat tube angle.

 

To better explain. My bike is designed to have 67 degree head angle with 160mm fork. With a 140mm fork on there (dependent on fork's axle to crown length) head angle will drop to 70 degree head angle. This is race bike territory and definitely not something you'd wanna throw down the mountain. 100mm fork with headset cup spacers will sort the head angle, but I'll lose travel. So I'll get rid of the twitchy feeling and the feeling that I wanna go over the bar, but lose control over rougher stuff due to travel.

 

* I have a super short stem on my bike and the handling is by no means twitchy. Razor-sharp, yes.

 

Agreed. Also running a wide bar (710mm) with a short (50mm) stem. Not twitchy at all. Sharp, yes.

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