Answered How much longer fork can I upgrade.

[Ville86]

I want to increase both handlebar and BB height. I could go with changing stem, handlebar and shorter cranks for similar effect, but I'm looking towards changing a longer front fork. I have very little experience on adjusting bike geometry before.
My bike is GZR Rampag-e, a 27.5PLUS bike with 100mm and 51 offset Judy silver that has 490mm axle to crown length. Bike has head tube angle of 68 deg. I can try to find more specs of the bike if it helps.

Do I mess up the bikes geometry putting in as long as 532mm AtoC with 44mm offset suspension? Can I put more? Should I put less? Why? Looking at RockShox 35 gold with 130mm.

 

[RickBullotta]

42mm is a big difference - that would result in roughly a 66 degree HTA. That said, 66 is an increasingly common HTA on bikes these days - slack is definitely in style (the new Santa Cruz eMTB is a 65.5 degree head angle). A cheaper alternative is a thicker fork crown race (you'll use the same fork, but lift the bike approximately 1/2 the thickness of the crown race). Will change the geometry but not the fork travel obviously. It's a very cost effective option.

Extended Crown Race for Fat Bike Suspension Fork Clearance - Custom Mountain Bike Parts | Custom Mountain Bike Parts for MTB Riders, by MTB Riders

 

[Ville86]

42mm is a big difference - that would result in roughly a 66 degree HTA. That said, 66 is an increasingly common HTA on bikes these days - slack is definitely in style (the new Santa Cruz eMTB is a 65.5 degree head angle). A cheaper alternative is a thicker fork crown race (you'll use the same fork, but lift the bike approximately 1/2 the thickness of the crown race). Will change the geometry but not the fork travel obviously. It's a very cost effective option.

Thanks you for your reply. Very much appreciated! I know 42mm is big. Even in my original post I ask "Can I put more?" it is there as more of a conversation starter ( tho a local bike shop clerk said I could go as far as 150mm...).

If I understand right, the spacer/bigger crown race would practically move a space from above my head tube below it? I got 2 spacers, but I want handlebar up as well. So I'd get slightly higher BB and slacker HTA ( that I both want), but would still leave me with the need to lift the handlebar.

And about the 66 degree HTA, Yes I've found bikes with 66 and under, but those are basically all full suspension. I know the trend has been from on XC bikes to go from 70 being standard to 68 being more or less what everyone has. I can't find proper examples on people having 66 HTA hardtail that isn't very extreme in other ways as well.

Currently thinking it be safer to go for Gold 35 120mm to get about 66.5 degree angle, unless someone can convince me 130mm for 66 is the way to go.

Brings me to another question. Oh boy is this complicated. Since increased HTA slackens the bike, should I try to keep offset at the 51mm it is now? 35 Gold only offers it at 37 and 44mm... unles I go for the 29¨ specific fork offered with 51mm. But that has longer Axle to Crown, so I need to go down on travel.

To give some background to my situation:
I tested the bike riding it a few km both on tarmac and light trails. My posture seemed ok and it it felt nice. The problems is that I'm not ok. I got some sort of spinal injury that has been gradually worsening for 7-8 years now. The more I ride the more obvious it is that my back and neck starts to hurt with extended periods of time spend on the bike. And I'm spending more time on this bike than on my previous bikes ( since getting a drivers license) as my meds are making me feel unsafe riding a car.

 

[MattyB]

Remember as you increase fork travel you slacken the head angle AND the seat tube angle. The latter is generally bad news, especially if you started with a relatively slack SA already. You can compensate to an extent by ramming the seat forward on the rails, but that may not offer enough adjustment. Personally I would guess a 120mm travel fork should be ok, with 130mm the absolute max before you start creating new geo problems for yourself.

PS - If you fit a 120mm fork and still don’t have enough stack your next stop would be a riser bar. The advantage with that is it doesn’t really affect any of the other geo numbers (it may reduce effective reach by a mm or two, but you can get that back if needed by rolling the bar forward slightly).

PPS - Don’t sweat the fork offset, unless you are a very advanced rider you won’t notice any difference. Just choose the version that is closest in offset to your current fork.

 

[Ville86]

Remember as you increase fork travel you slacken the head angle AND the seat tube angle. The latter is generally bad news, especially if you started with a relatively slack SA already. You can compensate to an extent by ramming the seat forward on the rails, but that may not offer enough adjustment. Personally I would guess a 120mm travel fork should be ok, with 130mm the absolute max before you start creating new geo problems for yourself.

PS - If you fit a 120mm fork and still don’t have enough stack your next stop would be a riser bar. The advantage with that is it doesn’t really affect any of the other geo numbers (it may reduce effective reach by a mm or two, but you can get that back if needed by rolling the bar forward slightly).

PPS - Don’t sweat the fork offset, unless you are a very advanced rider you won’t notice any difference. Just choose the version that is closest in offset to your current fork.

Thanks. I very much agree on what you said. I'm currently running the numbers I have from my old bike (Kona Mahuna 2019 L size) I never felt bad with, against my current E-bike and placing all the specs I can find to a spreadsheet. Then running possible fork changes with geometryCalc and adding those numbers there as well.

The current brand does not give all the measurements like chainstay length or wheelbase so I got some measuring to do before I get all the numbers out.

What I've found so far is that the differences with previous and current bike are smaller than I expected. There's also difference of 29er vs. 27.5+ that plays a role that is not clear on those numbers

My SA is 73 deg, and I do feel like I constantly want to push my weight a little further back on the saddle, so I think small slack there would be good. Now... what is small. How much is ok.... when is it too much.... I got no idea.
I wish there would be someone who could see my posture and do all the thinking for me. The problem is that someone would need to be both biker-posture expert as well physiotherapist or similar for it to be worth my money. Can't find one. Tried.

 

[Gary]

That looks a pretty short bike.
If that's your saddle at riding height your bars are already very high.
Raising them more is going to result in even less weight on the front wheel (grip)
increaing travel is not always an uprgade

 

[Ville86]

That looks a pretty short bike.
If that's your saddle at riding height your bars are already very high.
Raising them more is going to result in even less weight on the front wheel (grip)
increaing travel is not always an uprgade

Yes it is somewhat short, but the angle picture is taken is makes it appear even shorter than it is. Measures as given by supplier is top tube 595mm ( I tihnk this is effective length). I'm not sure how short this really is. Subjectively I don't really feel like I need to worry about not having weight enough on front wheel, but can't really estimate how rapidly it would change if I increase axle to crown on the fork.
Short top tube could be part of the problem, as I feel like pushing away from the handlebar and backwards on the saddle. I was thinking I could fix this by the slackening Seat angle (73 to 71) and increasing height from longer fork (gaining on BB height as a positive side effect). I have no experience to estimate how badly the bike would be steering or climbing after that. Could I add a longer stem to get more weight on front, if it becomes an issue?
Any actual suggestions?
Edited: Measured probably wrong.

 

[MattyB]

What type of riding do you do? From your posts it seems like you are trying to optimise your seated pedalling position, but that is not as important as on a road bike; the rider is far more dynamic on an MTB, constantly shifting weight and position. If you are more of an XC type rider clicking off the miles over relatively non-technical terrain then I can see your seated position becomes more important. Give is this info and it will be easier for us to assess how effective your proposed changes are likely to be.

 

[Ville86]

Thanks for all the replies. So far I've gotten both new ideas (if reach is getting too short) on what to look for as well as confirmation on some things I was not sure about.

My riding is 80-90% commuting on tarmac and taking shortcuts on light trails. There I only want added comfort, not better performance. Rest 10- 20% is sort of something of a mix or between regular XC and a bit towards what could be called trail type. Whenever it gets to the rougher end of XC or towards what could be called Trail the bike feels bad. That is where I would like a little performance. So I don't need need to carry or walk my bike over or snail trough difficult parts of a trail that was 99% fine. Sort of kills the joy even if it is only 0.1% of what I ride.

These are calculations with current fork Judy silver 100mm and possible alternative RS 35 Gold with 120mm travel. Wheelbase, chainstay, reach and BB height (actually BB drop I think?) are measured by me so they could be a bit off. Stack was estimated by the app. Rest as stated by bike seller.

 

[Ville86]

Oh... And I live up north. So about 2-4 months each year it gets snow, sleet, ice. So proper MTB is preferred even on the best paved roads for riding whole year.

 

[Ville86]

A bit of an update in case anyone is genuinely interested. Got my shoulder injured and had to put the project on hold, and then all this covid19 thing started and some more hold to it again. However I did manage to test some things.
Cause of my shoulder injury I got a satori headsup2, effectively adding 50mm worth of spacers under my stem to reduce force on shoulder. Some would say going up that far would ruins the bike. Well it didn't. I can feel the difference sure, and it is not all better, but it didn't completely ruin it.
Just to play around I went ahead and got a 90mm stem from an old bike to replace original 55mm one. Again, I surely felt it and 90mm was too far but I sort of liked the way steering calmed down.
This does not simulate all the changes to bike geometry or handling a longer fork would bring, but gave enough confidence that I'm positive I should add more than 20mm, even 40mm and then just shorten the travel by 20mm if it goes too far.
Thanks for all the replies and I'll try to remember to make a final update once I'll have the new fork.

 

[Ville86]

Went with a 100m travel RockShox Sektor in 29" cause I could not find good deal of 120mm travel. It has 23mm Longer AtC without any addet sag to diminish the increase. If I'm not wrong I should be able to increase the travel by 20mm without additional parts and I will probably test it later .

Haven't ridden it enough to say much but so far I'm loving it. Can't really tell the difference in any negative way. My pedals and BB got up by just enough to make a real difference and I'm no longer hitting my pedals on obstacles, basically at all. The fork eats up bumps way better, but I can't tell how much of it is about the fork just being better and what would be from the 1dec change in head angle. I feel I can even climb better as I'm able to have higher speed when entering any uphill and clear roots and rocks easier while climbing.

While I really want to try going even further by adding that 20mm travel, I thing I have gotten everything I was looking for with the change made so far. Happy with the bike. Now I just wan't to test it to satisfy my curiosity.
Thanks for all the input everyone. While not getting definite answer, some of you really helped me consider all the aspects to make my mind.