Dirt Jump Bikes for Speed? Frame geometry questions?

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DanGT86   10 kW

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Dirt Jump Bikes for Speed? Frame geometry questions?

Post by DanGT86 » Oct 23 2020 9:27am

Would a dirt jump frame make a good mid drive bike at high speed on trails?
Would a dirt jump bike be the perfect solution for a durable playful geometry but still do well on trails at high speed?

Seems like the major drawback to Dirt Jump frames for trail use is the seat tube is usually very low so pedaling efficiency sucks. If I don't plan to pedal than why not get the lowest seat possible and most nimble geometry possible?

I've been thinking about this for the last few days and figured I'd ask for some opinions. Here is my thought process:

I have a 4kw direct drive LMX motor running mid drive single speed on the LH brake flange. It's geared 12:92 teeth for just under 35mph with 24" wheels. I love trials riding and technical lower speed riding as well as playing on steep hills. I was thinking of transplanting this setup onto my downhill bike but I'm a little bit worried about the playfulness being hindered by the slack head tube and 8" of suspension travel. At the same time I don't want the bike to be unstable at higher speeds like 25-35mph because I do a lot of street cruising to run errands and commute.

So I'm looking for some bike nerd analysis here. I have never ridden a dirt jumper with suspension. What are the drawbacks of DJ frame geometry at high speed?

I really want an ebike that I can bunnyhop! :wink:

markz   100 GW

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Re: Dirt Jump Bikes for Speed? Frame geometry questions?

Post by markz » Oct 23 2020 1:46pm

mid drives arent durable but you said "durable playful geometry" which means yes mid drive weight is low, but its hanging in front of the crank so it can get in the way if your riding over logs.
dirt jumper with suspension
Jumping with a hubbie wouldnt be wise, so mid drive is a good option.
What are the drawbacks of DJ frame geometry at high speed?
I really want an ebike that I can bunnyhop!
At speed on trails, single track trails, goat trails, bush whacking trails?
Bunny hop you'd want light weight.

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DanGT86   10 kW

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Re: Dirt Jump Bikes for Speed? Frame geometry questions?

Post by DanGT86 » Oct 23 2020 4:30pm

I can handle packaging the drive in a durable fashion. I also agree that a hub is out of the question for this. I only mentioned that the bike will be powered to drive home the point that pedaling geometry and ergonomics while seated are a non issue here.

Seems to me that the majority of complaints about using DJ bikes for trail riding is based on how bad they are at climbing due to the low seat height and commonly being single speed. They also commonly use only a rear brake. So most of the youtube vids on this topic aren't too helpful. I can easily run front and rear brakes and don't need to pedal. The other complaint in some discussions and vids is that the 80-100mm of travel on most DJ bikes is inadequate for blasting through rock gardens at high speeds. I am not doing that anytime soon.

I have an all mountain/XC style hard tail that I ride on relatively mild single track. Nothing crazy, the occasional birm and maybe knee high jumps and drops. It pedals well but the low bottom bracket prevents me from being able to hop it well because even with the seat low I cant squat down low enough. The pedals are just too far away from the seat tube and the top tube is a bit in the way of my knees when flicking the bike around.

My DownHill bike just wants to go fast. It's super stable at high speed but the dual crown fork limits the steering on slow trials type stuff and its heavy. I't makes for a really nice 40-50mph street weapon with my hub motor. It does good on wide open things like fire roads too.

So all that leads me to wonder about a full suspension dirt jump/slopestyle bike with maybe a 130mm travel fork on it. If someone wasn't planning to pedal much at all and wanted to blast down a mountainbike trail with one as if it was a Freeride or DH bike, what would be the drawbacks?

Anybody on here use their DJ bike for all their MTB needs?

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Re: Dirt Jump Bikes for Speed? Frame geometry questions?

Post by markz » Oct 23 2020 4:47pm

Either way you will have to convert whichever bike you will get into something you desire and want.
What about a DH bike without a dual crown fork so you can get that turning radius.

Not sure if what you mentioned in your last post, but remember mid drives will put your pedals wider, a consideration for some. Netpronix or Chalo or someone else didnt like the wideness.
The pedals are just too far away from the seat tube and the top tube is a bit in the way of my knees when flicking the bike around.

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Re: Dirt Jump Bikes for Speed? Frame geometry questions?

Post by Manbeer » Oct 23 2020 7:51pm

I was thinking of a similar build and considered doing a left hand drive mid mounted q128 or similar turning a sprocket mounted to the disk brake part of the hub and using v's or magura hydro rim brakes. I'm not sure how it would pan out and it could be argued that due to lack of gearing I'd just be getting reduced performance due to parasitic loss, but just as someone mentioned hubs aren't a great idea for jumping, but imo neither are the cheap cranks and bb's that come on many mid drives.

At least it would allow you to keep a somewhat decent Q-Factor and run something with bb30/gxp/hollowtech 2 or whatever

Again, I have no idea how it would work out but it was something in my list of things to do over the next year or so

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Re: Dirt Jump Bikes for Speed? Frame geometry questions?

Post by MadRhino » Oct 23 2020 9:14pm

Dirt jumpers geometry suits well to build ebikes, but their nimble handling is not particularly suitable for high speed. You’d want it long, choosing a longer wheelbase or stretching it with custom, 3 or 4 inch extended dropout plates that would place the rear axle further behind the seat tube.

Then, ‘high speed’ is subjective. What you are calling high speed might very well mean lame for some others. My personal experience is that above 60 mph, the bike’s wheelbase would need to be close to 50 inches and the steer angle 62-64 degrees.
Make it fool-proof, and I will make a better fool.
Current bikes
Street:
Trek Session 10 mod. Variable geometry. 70mph
Dirt:
Santa Cruz V10. 50mph

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DanGT86   10 kW

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Re: Dirt Jump Bikes for Speed? Frame geometry questions?

Post by DanGT86 » Oct 23 2020 10:50pm

MadRhino wrote:
Oct 23 2020 9:14pm
Dirt jumpers geometry suits well to build ebikes, but their nimble handling is not particularly suitable for high speed. You’d want it long, choosing a longer wheelbase or stretching it with custom, 3 or 4 inch extended dropout plates that would place the rear axle further behind the seat tube.

Then, ‘high speed’ is subjective. What you are calling high speed might very well mean lame for some others. My personal experience is that above 60 mph, the bike’s wheelbase would need to be close to 50 inches and the steer angle 62-64 degrees.
That is mostly what I wanted to hear. When I say speed I am talking about 25-35mph like a descent on a downhill bike rather than 60mph on the road. My fear was that it might feel sketchy in the same way that bombing down a giant hill on a 20" bmx bike does. I guess I'm probably being paranoid here because it's not like slopestyle and dirt jump riders aren't going fast.

The bike will primarily be for hooligan riding around town jumping landscaping and loading docks but several times a year I go to an offroad park and ride my hub powered downhill bike on a motocross track as well as on the atv/dirtbike trails. The hub really kills all the suspension action offroad and inspired me to build this mid drive setup. There is a nice little dirt jump park by my house with a pump track and a skate park nearby as well. A full sus dirt jump bike seems like the best jack of all trades if the primary drawback is crappy pedaling geometry.

As for stretching the wheelbase I'm kinda looking to trust the engineers that figured all of that out in the design of the bike and just add my motor. I know that's probably silly because the motor is about the furthest thing from original design you can get.

My hub bike is a specialized big hit with a mxus 3kw motor and 2" of wheelbase extension in the custom dropouts. It handles really well but I don't have many other bikes around to compare it to. With the long wheelbase and long travel its a bit of a slug at the skatepark. It seems that in the mountain bike world the bikes with less travel are designed more for flat ground and pedal efficiency so they have long seat tubes and low bottom brackets. Generally just more frame in my way than I prefer. This is what got me thinking about DJ bikes.

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MadRhino   100 GW

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Re: Dirt Jump Bikes for Speed? Frame geometry questions?

Post by MadRhino » Oct 24 2020 11:58am

For the purpose and speed that you want, then it is a good choice of bike style.
Make it fool-proof, and I will make a better fool.
Current bikes
Street:
Trek Session 10 mod. Variable geometry. 70mph
Dirt:
Santa Cruz V10. 50mph

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